Webinar: Introduction to Using Aerial Sources for Heritage

Speakers: Helen Winton, Matthew Oakey (referred to as ‘Matt’), Matt

Helen: Thank you very much, Matt. Welcome everybody. Just to introduce ourselves. Matt and I are the managers in the Aerial Investigation Team at Historic England. I’ve got a national remit and Matt covers the north. So, as some of you may be aware, this is the first of two webinars where we hope to introduce you to aspects of using aerial sources for heritage. We’ll mostly be talking through slides but we will be asking you a couple of questions as we go through and I hope you’ll also use the chat function to ask us questions. I do have one for you first: Does anyone recognise where our little Cessna is flying over? Can you see? Here’s the Cessna and we’re flying over quite a well-known site. If you know, pop it in the chat and we’ll come back to that later.

So, in aerial investigation we are hoping to develop our online training offer, so we’d be very pleased to receive any feedback on this webinar and if there are any aspects you feel that would benefit from a longer session perhaps, either as webinars or e-learning modules, please let us know. So, as I said, this is an introduction so we’re not going to go into things in a great amount of detail. But also, if things occur to you in retrospect, you can always email myself or Matt after the webinar. So, I’ll answer the question about the photo in a minute, but first of all we have a question for you which is: “Who uses aerial sources for heritage?”. So, if you could just click the option.

Matt: OK. Votes coming in thick and fast there. That’s excellent. Thank you very much, everyone. So, you can all see the results now. Looks like 50% of you have used both. That’s really good.

Helen: That’s encouraging to hear.

Matt: Fantastic. OK. Shall we move on?

Helen: Yes. That’s great. Thank you everyone for participating in that. So, we can see that we’ve got quite a varied range of experience, some of you very experienced, some of you not. So, let’s just move on to what is the webinar about and what is it not about. So, we’re going to be covering approaches to using aerial photographs and airborne laser scanning data, so LIDAR, and how to make the most of them. And as we have seen from the poll, we’re expecting that you will have varying levels of experience and so, I hope there will be something for everyone here and at the very least, I hope that we can make you look or think a little differently about how you’re using aerial sources. The first section of the webinar will review how we look at aerial sources and then I’m going to hand over to Matt who’s going to talk about how to get the most out of the sources. And he’s going to finish with a couple of case studies in an urban and in a rural area. So, what we won’t be covering are the technical aspects of using aerial survey and neither will be covering how to assess or specify aerial surveys. These will be covered in Webinar Two which is at the same time next Thursday.

OK. So, let’s have a look at how aerial photographs can be used. So, as I expect many of you know, aerial photography is used in a range of contexts for heritage. And the images you are looking at here are linked to the Historic England aerial reconnaissance programme. We take oblique and vertical aerial photographs. The oblique photographs, in particular, provide an engaging illustration of sites and landscapes and these can be used also to record, say, as a record, of a scheduled monument, but we can also use them to communicate with land managers. So, for example, top right we can see an example of a tractor clipping the earthwork of a scheduled round barrow. And we’ve been able to use this in publicity for the Heritage at Risk Programme but also, if you’ve got an example like this, you can show it to a landowner, and this is a good illustration of what you’re talking about. And obviously, one of our unique selling points is the discovery of buried sites visible as cropmarks and in the top left you can see a diagram showing how buried features affect the growth of the crop above by giving differences in colour, height etc. So, in terms of Historic England, the discovery aspect takes up most of our summer flying time, but we can cover multiple topics in a single flight. And we range widely across the counties of England and we can do that in a single flight as well. So, we fly in leased light aircraft, usually a Cessna 172 which you saw briefly over, that was Avebury for everybody. That original slide, the first slide, that was a picture of Avebury, so well done to anyone who spotted that. So, we fly in a Cessna 172. This gives us the flexibility to cover large parts of the country and actually it’s amazing how quickly you can get around in a plane about the size of a classic Mini car. And we plan flights depending on all sorts of different factors including the weather and current Historic England priorities, so we do thematic work as well. And when we’re prospecting for buried sites visible as cropmarks, we also take into account things like soil moisture deficit, crop growing cycles, geology soils and of course, our previous knowledge of any area. So, lots of applications.

So, how can you get hold of the aerial photographs? So, all of our aerial photographs are available from the Historic England archive, and this includes a large volume of hardcopy prints that you have to visit Swindon to see. But, we’ve also been developing over the last few years, access to our digital and digitised collections. What you’re looking at here is an example, this is a GIS portal. So, it’s a map-based application and this is publicly available data, so if you want to explore this, you can have a look at it yourselves. As you can see, it was created to celebrate the tricentenary of “Capability” Brown. We’ve also added in photos of Humphry Repton’s landscapes as well, so these mainly 18th century designed landscapes. And the idea is that you click on a dot and it brings up an image for you and you get a bit of background information as well.

So, moving on, we’re also developing a similar platform, but this will have all of our digital aerial photographs. So, that’s all of the digital aerial photographs in the Historic England archive. This would include our own photography, but also the Aerofilms collection and any other scanned aerial images we have in the Historic England archive. This is currently only available to Historic England staff, but we plan to develop an online version as soon as we can. We’ve been slightly held up by the COVID-19 situation.

OK. So, that’s how to get hold of them. Matt is going to come back to looking at sources in a little bit more detail in the second half. So, what I want to do now is move on to what do we do with the aerial photographs.

So, as well as taking aerial photographs, we have a team of staff and we also provide grants to people to do mapping projects using aerial sources as the main source. This uses methods we have developed over the past 30 years and if you’re interested in that aspect of things, then please attend Webinar Two. And you can also read all about it in a recent publication and this is the Standards and Guidance document, bottom right of screen, and this is by Sally Evans. And this goes into the development of our methods, where they come from and what our current ones are. So, for us, the key parts of this process are that you look as many sources as you can, both digital and hardcopies, so that means making an effort to visit those archives.

So, we use lots of different sources. We analyse and record all archaeological features so we don’t do thematic recording which you might find in the academic field. We look at everything from potentially the Neolithic all the way through to the Cold War. And as we now work in urban areas, we’re expanding into looking at even more recent features. So, we analyse it, look at all archaeological features and then we create mapping and monument records. And one of the main parts of our standards is that these should be suitable for use in the Historic Environment Records and we try as best as we can to incorporate all our data into the Historic Environment Records, either a national database or in local authorities. And so far, we’ve covered about half the country with projects using these standards and at the moment, we have a project aiming to get our digital data, so the digital mapping, online and available.

So, moving on. Yeah, shout-out to the HERs. They are our number one customer. We see that as being the ultimate place for our data. It’s the place that local people and nationally people can get hold of it.

Matt: And it’s good to see so many HERs in the webinar today.

Helen: It’s great, yes. Good to see you.

So, I’m just going to briefly go through… This is something that we often spend half a day or a whole day on which is photo reading and analysis. And so, I’m sure many of you have experience of interpreting aerial photographs, but even if you have, it’s always worth taking a step backwards sometimes so that you don’t jump to conclusions. We all get excited when we see something on an aerial photograph. Well, I hope you do if you’re tuned into this webinar! And as I said, what we’re just trying to promote is a structured approach, taking all these factors into account, so, the things I’ve got written down the side of the screen. One of key ones here for me is… well scale and shadow and shape and pattern are the key ones. So, in terms of scale, you just need to look for something familiar in the photos to get a sense of scale. And this will help you understand if you’re looking at something, say, the size of a Roman villa or the size of a football pitch. And this might seem a very obvious thing, but a lot of times aerial photographs might be extreme close-ups and you might not have a good sense of scale and I’ll come back to that in a minute. And of course, shape and pattern are the archaeologist’s friend. That’s what we work with. We work with site types etc, so, as with any other site types, monument types, material types, so, interpretation of photos also depends on a good, broad knowledge of archaeological site types, as I said, from potentially the Neolithic all the way through to the 21st century even. And as well as that knowledge of archeologically site types, you’ve also got to think about what you’re looking at. So, you might be looking at buried features. What might they look like if you were looking at them as a buried feature or if you were looking at them as archaeological earthworks?

So, just moving on. So, as I was saying, I have a sort of plea, or I want to encourage people to think about archaeological cropmarks as indications of buried archaeological features. They’re not just abstract shapes like the ones we’re looking at here. This may seem an obvious point to make, but we have tendency to say “cropmark enclosure”, “cropmark landscape”. So, I would encourage you to please try to describe what you’re seeing in terms of archaeological remains themselves. So, the example that we’re looking at here, I would suggest that it is an Iron Age, or later prehistoric or Roman settlement, probably of several phases. They could be contiguous and perhaps overlapping because we’ve got what looks like potentially overlapping features but also it looks like the settlement might have developed and people built an enclosure onto another enclosure. And the eagle-eyed among you will have spotted a circular feature here and what we tend to do is call these “ring ditches”. That’s fine, if you’re an archaeologist you understand what a ring ditch is. But actually what we’re looking at is probably a wall slot or a drip gully from a roundhouse. Or unusually, it could be a very small round barrel, you never know, the ditch that went around a round barrel. My plea is interpret the photographs, describe them, think about what they might be. Don’t be scared. You can put “possible” perhaps. So, it’s worth giving it a go.
Matt: So, Helen, sorry, I’m going to interject there. You can see I’ve put a little green pointer on your slide. If you click on the slide where you want it to point to, the arrow will move. So, could you point out the circular structure?

Helen: That’s it there.

Matt: That was it? I got it right. Fantastic.

Helen: Excellent. So, as I say, we’re going to have a look at this site in a bit more detail. So, what I would like you to do is… This site has been photographed on four occasions. As you can see, we’re in Bedfordshire. Each of these photographs was taken in a different year. And so what I would like you to think about is whether you can tell the sequence in which these photographs were taken from the appearance of the cropmark. So, for example, which one was taken first? When was the next one? So, I’ve numbered them. I’ll give you a little time to look at them, so you’ve got 1, 2, 3, 4, and I’m going to ask you to put them in the order they were taken in the poll. Or, the other option is actually you can’t tell the difference, you cannot tell the sequence from the cropmarks. So, I’ll give you a little bit of time to think about it. We’re looking at the site from more or less the same angle. If you’re worried about scale, can you see we’ve got tramlines? So that’s where the tractor or agricultural machinery goes up and down. Those might be two to three metres apart depending on the size of the machinery and we’ve got the distance between the tramlines might be 20 to 30. So, we’re now going to ask you the question. So, over to Matt, I think. So, if you want to vote…

Matt: So, thank you to everyone posting in the chatroom there. These images were taken in different years but what order were they taken in? 60-ish% saying you can’t tell and 30% saying 4, 3, 2, 1. OK, I’ll broadcast the results for you. There you go. What do you reckon, Helen?

Helen: Thank you very much for taking part in this. And I can apologise in advance because of course, this was a trick question because the answer is that you cannot tell from the cropmarks alone. So, if we just… Can I flip back, Matt?

Matt: Of course. There you go.

Helen: So, the reason I’ve brought this up is that we often hear statements about how the appearance of a cropmark is linked to the preservation of underlying archaeological features. So, people sometimes say that a poor cropmark means that the preservation of the buried archaeology is poor, but sometimes people say the exact opposite, that the stronger the cropmark, the worse the preservation. And sometimes people also assume that an indistinct cropmark indicates that the archaeological features are very deeply buried, and this might be true in some cases, but there are really far too many factors that influence the appearance of a cropmark. Soil moisture levels being a key one. So, there really is no correlation between the clarity of the cropmark and preservation of the buried remains.
So, the first one was taken in 2008.

[They discuss an issue with the slide presentation]

Helen: OK, so, they were in the order, 1, 2, 3, 4. But you really couldn’t tell that because if you follow that assumption that things don’t appear, that the cropmark might degrade over time, it’s just simply not true. The type of crop, the time of year, the weather that year, the soil moisture deficits etc. All of these will affect things. As I say at the bottom, there’s no correlation between the quality of the cropmark and the preservation of the buried archaeology. There’s no direct correlation. You might find when you excavate that you can make assumptions but you really need to excavate to find out what the preservation is. So, don’t let anyone tell you that just because a cropmark doesn’t appear or it doesn’t appear very well, that you can make assumptions about the buried archaeology.

OK. So having dropped that bombshell, I shall move on to archaeological earthworks. So, again, we would normally spend quite a lot of time looking at these. So, I’ll just go over the key things here. As I expect a lot of you know, shadow is really the key factor here. Scale is as well. We’ve got a small house here. So, it’ll give you an idea of the scale of these archaeological earthworks. And what we’re looking for here is shadows and quite often it’ll be the trees that cast the long shadows, and this will help your brain understand less familiar objects, whether they’re upstanding earthworks or whether they’re cut features. So, we can see we’ve got shadows along the edge; we’ve got shadows of even the bushes on top of these mounds. And again, this might seem like a very obvious point to make, but in some photographs, deciding where the shadows are falling isn’t always straightforward. So, again it’s a case of looking for something familiar that will help you understand where the light is coming from.

And Matt is going to talk about archaeological earthworks in the context of LIDAR a little bit later and give you an example. So, whether you are looking at archaeological earthworks on a LIDAR image or on a conventional aerial photograph, you need to really think about where the light source is coming from, whether that’s one direction or multiple. Anyway, I could talk for ages about the archaeology in this slide. I hope you can all see at the bottom, these are actually what used to be called “bell pits”, but are now generally called “shaft mounds” and it’s basically an upcast for mining. So, you can just about see that they’re actually little doughnuts, they’ve got a little hole and that’s the shaft and what you’re looking at is the upcast from the mining operations.

OK. So, moving on swiftly… I’m just going to… As a penultimate example in this section, I thought we could look at this scheduled Iron Age hillfort in Oxfordshire. This is at Idbury. And, again, it’s just to make the very basic point that things can look very different. The same area or site can look very different depending on the time of year, the crop regime, the quality of the photography, the quality of the light etc. So, if we look at Photo 1, top left, so, this is this one, taken in 1996. You may be able to just make out that there are tramlines in the crop. Can you see that? You can see where the track has gone up and down and left these distinctive curving marks and lines. The site is in arable and buried in surface remains are affecting the growth of the crop above. So therefore, you can see the outer ditch as a dark mark and you can also see the inner bank as a pale mark as well.

So, if we fast forward three years, so top right, it’s 1999. Oh, I forgot to say you can orientate yourself from this little triangular wood. We’re looking at the site from slightly different viewpoints each time so if you just think about where the triangular wood is. So, we’re looking at it from a slightly different angle. So, we’re looking at it from this angle, from here. Now, we’ve got the very distinctive brown colour which is indicating that it’s soil. So, the site is still being ploughed, it’s still in arable, but it’s the wintertime because it’s December. The site has been ploughed, the crops come off, it’s been ploughed and it’s been prepared for the next crop. And again, note the shadows from the trees. Can you see the shadows from these trees here? I hope everybody can see my cursor. And you can also see the shadows and actually what we’re looking at are earthworks of the ramparts of the Iron Age hillfort which survive as chalky banks in the plough soil.

So, moving on almost over a decade, it’s now July 2012, so we’re in the bottom right. And we can tell by the texture… Can you see this field here? You can just about make out we’ve got tramlines, so this field is in arable, but we’ve got a different colour, we’re got a lighter texture and so we’ve got pasture in this field. And so, can you see that it’s not the whole of the hillfort that’s in pasture, there’s a tiny little bit here. So, you could be forgiven for thinking, well the bit in the arable might have been ploughed level, but again, you shouldn’t be making assumptions. But we’re also seeing the ditch of the hillfort showing up as lusher grass and again, we’ve got parching over the bank, the chalky banks of the rampart. So, well done who’s looking at Google Earth Pro.

And then, to Photo 4, taken a year later. We can see it’s still in the same regime. We’ve still got this field in pasture. It looks like it’s been harvested for hay. But, more importantly, this field on the left, can you see it’s brown, it’s a different colour, and actually we can see shadows being cast. And so, we know that the rampart is still surviving in that arable field even though it’s being ploughed and is in arable. So, just a basic point there, that you can have one site and you can’t make assumptions about the preservation of the earthworks. So, even if you’re seeing it in crop, there could be earthworks surviving there. And that’s where LIDAR can be a fantastic source. If you’ve got LIDAR to look at and you’ve only got photographs in that site in crop, then you can look at the LIDAR.

So, OK. Moving on. OK, so, I hope you’re all still with me. I’m just going to finish by talking about potentially confusing features. Now, this is a vast topic and there are lots of things than can confuse you on an aerial photograph and I can tell you that after 30 years of looking at aerial photographs, there are still things that confuse me as well. So, the most common ones are natural features, so you might find geology, drainage, vegetation, fungus rings or fairy rings. I’m just going to go back briefly. I don’t know if anyone can see it but there’s a tiny… Or can see my cursor, I don’t know how to make it… I’ve just spotted it, there we go. So, just there, there is a fungus ring. Can you see there’s a ring? You could get excited and think that was a hut circle, but I can assure you it’s a fungus ring.

OK. So, confusing features. So, geology, drainage, farming activities, spraying of crops can cause lovely circular patterns and of course, us archaeologists like circular things. But also, photo blemishes as well and again, it’s about taking a step back and having a look at, you know, is it too good to be true? So, here we are, we’re in Cambridgeshire and we were looking at a lovely hillfort before. We’ve got another lovely circular feature on a hill but if we just look here. Can you see this bit here? Can you see there’s a disjoint? I hope you can make that out on your photographs. And actually what we’re looking at here is geological banding and that is a fault line in the geology. And so it’s just looking in detail. Also, you might want to think, you know, I know you do get features where you get multiple banks and ditches, but these are very narrow and very close together. There’s something that’s not quite right and they have wobbly edges, well done Rachel. So, there we go.

So, that’s been a bit of a romp through interpretations, so you don’t need to worry because help is at hand. If you need some help I would suggest the first place to start is to get yourself a copy of David Wilson’s “Air Photo Interpretation for Archaeologists”. This has got a fantastic section explaining all the basics of cropmarks, earthworks and a brilliant section on the potentially confusing features. So, he’s got a big section on how geology shows. So, definitely worth getting in touch. I’m enjoying the chat here. I’m liking the fact that people are discussing fungus rings. You’re all completely correct, yes. Wobbly edges. So, without further ado, having given a plug for the bible, “Air Photo Interpretation for Archaeologists”, I shall hand over to Matt and shut up now. Thank you very much.

Matt: Thank you, Helen. Over to you, Matt.

Matthew: Fantastic. Right, I’m just going to have a look through some of the different kinds of sources that will find in the archives of aerial imagery. We’ll also be touching on LIDAR and then I’m going to finish off by looking at a couple of different case studies for a rural and an urban area that look at how we bring all these sources together to better understand a landscape. We’re obviously in a position now where we’ve got more sources of aerial imagery than we ever have done in the past. So, we still use traditional aerial photography, we’ve also got things like airborne laser scanning or LIDAR data and also newer technologies such as drone survey and satellite imagery, all of which add to the range of sources that we can use for studying the historic environment.

Now, we’re very fortunate in the United Kingdom to have some incredibly extensive and rich archives of aerial photography. Each of the nations within the UK tend to have their own national archives of historic and recent aerial imagery. So, as Helen has already mentioned, the Historic England, which is down in Swindon, is our archive for England. But there are also equivalent archives curated by Historic Environment Scotland and also the Royal Commission for Wales and the Public Record office of Northern Ireland also holds some aerial photography as well. Another really important collection to mention is the Cambridge University Collection of Aerial Photography, or CUCAP. This is another collection that has national coverage. It even has some coverage outside the UK as well. Taken for a variety of reasons, part of it is archaeological aerial reconnaissance but also for various other reasons as well, but a very important collection. Unfortunately at the moment, the collection is actually closed so this is inaccessible but you can still search the catalogue online. Most Historic Environment records tend to hold their own collections of aerial photographs and these can be a really good first port of call if you’re studying a landscape. There’s quite often a degree of crossover between what is held by a Historic Environment record and what is held in the Historic England Archives, so if you can actually get details of those in advance, it’s a really good idea to get hold of those and to cross-reference, because this could save you a little bit of time visiting the two different archives. Some museums hold their own small collections of aerial photographs and even some local societies such as the Yorkshire Archaeology Society which has the collection of Tony Pacitto which has recently begun to be made available online. We’re all more familiar with the online sources of aerial photography that are available nowadays. So, the common ones like Google Earth, Google Maps and Bing Maps, we’re all very familiar with. There are also some other quite significant collections online, in particular the Britain from Above collection, which is freely available and this hosts over 97,000 images taken by Aerofilms Limited during the early and later parts of the last century and we’ll see some examples of some of these in a few moments. There is also access to seamless data sets of very high quality, very accurate orthophotography. For those working within the public sector, this is the Aerial Photography for Great Britain or the APGB agreement. And of course, we have tremendous access to the Environment Agency LIDAR data nowadays as well.
The kind of photographs that we have in our archives can be broadly split into two categories. So, the first of these are specialist photographs. When we say specialist photographs, what we mean is observer-led photography that is taken specifically to record aspects of the historic environment. And this means that they’re usually taken in the most appropriate conditions to record that particular site. So, in this case, in the top right-hand side of the screen, we’ve got some more cropmarks that Helen was talking about taken West Yorkshire showing the remains of Iron Age or early Roman settlements. Most of the archives are still physical prints and a large proportion of this is actually still in black and white. Specialist photographs tend to form the majority of the collections held by the HERs.

The other type of photograph we refer to as non-specialist photographs. So, these were photographs that were taken for different survey purposes but they might coincidentally record historic features or archaeological features, many of which may have actually been substantially changed or even lost in the intervening years. So, this is blanket coverage. Again, these are mostly physical prints and almost always, in this case, in black and white. Sometimes you’ll come across these in the HER but it’s quite often the case that they may be held by different parts of the local authority. And these form a very significant part of the national archives.

Now, I’ve mentioned the fact that most of the archives, even in the digital age, are still actually physical print archives and so it’s worth pointing out here, just a word of warning about using these archives and in particular the value of looking at original source material if you can do. In this case, we’re looking at a picture of Grimsby Docks taken in the early 1940s by the Royal Air Force. This is an example of a vertical aerial photo, so taken looking directly down at the ground. On the top of the screen, you can see a laser copy of the photograph and on the bottom of the screen, a high-resolution digital copy of the photographic print. So, the laser copy is the kind of thing that you might be able to order remotely from the archive. If we zoom in on these, you can see the level of detail that you’re getting is really considerably different for the high-resolution digital scan to the laser copy. So, it’s always just something to be aware of, when you are accessing photography, the best port of call is to go and visit the original archive material if at all you can do. It’s also worth noting that of course, with vertical photography and some oblique photography as well, you’re also able to use a stereoscope and see these sites in 3D which is a very important aspect of properly understanding the landscape.

LIDAR is very extensively used for studying the historic environment nowadays and I’m sure that many, if not all of you will be very familiar with this. Now, there are a couple of sites that you can go to to look at LIDAR data that already have visualisations of the LIDAR that you can just pan around and view. But, LIDAR is also available in its un-visualised state, so the actual original data is available from the Environmental Agency’s download website. Just a point of caution about this. So, on the left-hand side of the screen we have the LIDAR data as it’s shown on the National Library of Scotland’s maps website which is an absolutely tremendous resource. Now in this case, what we’re looking at is what we would call a single-lit hill-shade model. So, this is very similar to really looking at an aerial photograph. So, in this case, we have a virtual sun lighting it from the top left-hand corner. Just like archaeological earthworks, this means that linear features that are running along the direction of the sun will actually be very difficult to spot. So, you can see the areas that I have circled in there where you actually have some field boundaries. So, on the right-hand side of the screen, we have some of the raw data downloaded from the Environment Agency website. This has been processed using a 16-direction hill-shade. So, this is essentially a model that’s lit simultaneous from 16 different directions so you get a lot more information out of it. The other point to note here is that you’re sometimes limited in terms of the resolution of the data. So, on the NLS website, you can see the one metre or the two metre LIDAR data whereas this particular site, Grassington up in the Yorkshire Dales, actually has 25 centimetre LIDAR data if you go directly to the Environment Agency website. So, looking at original data, visualising that original data is a very good thing to do if at all possible.
And there’s plenty of guidance on this. So, we have our own guidance on using airborne laser scanning data and also there’s a fantastic publication on looking at different ways of visualisation that data. So, it’s worth looking at both of those and we can pop links to that in the chat.

So, as well as the actual source data itself, we have the mapping. So, this is the spatial and textural data that is derived from the systematic analysis of all of these sources. All of the data that we produce in Historic England and the data from the projects that we actually fund within Historic England is deposited with the local Historic Environment Records. So, this is a really good place to access the data and it’s also available from the Historic England Archive as well. Some Historic Environment Records also host the spatial data that has been created from air photo interpretation and mapping projects on their online viewers. So, the example that we see here is Devon County Council. So, if you’re lucky, we’ll be able to access the data this week. A very exciting development that we’re just in the tail end of developing at the moment that Helen mentioned is the online GIS portal. So, the idea is that, by the time we get to the end of next year, we will have launched the online portal which will give free online access to 30 years’ worth of mapping projects covering over 50% of England.

So, bringing all of these sources together, and creating these maps that can be used for research and for better managing and understanding the historic environment. Here we’re just on the western end of Hadrian’s Wall, not far away from Carlisle. And this is a really good indication of how you never really get the full picture of the historic environment just from a single source. So, in this case, we have an Iron Age or Roman enclosure, in this case seen as a cropmark on a specialist aerial photograph, so somebody specifically took a photograph of this feature when they were flying, discovering new sites. Here we have a section of the vallum. In this case, it was seen on a 1940s RAF photograph but has now been levelled, so that historic photo is the only evidence of this that we have. We have some post-medieval ploughing, in this case seen as a soil mark in the 1980s and once again, our historic photography was tremendously useful in showing the Second World War airfield that was visible, in this case actually still in use when it was photographed.

So, we’re just going to look at a couple of brief case studies here. The first one of these is looking at the estate of Belsay Hall up in Northumberland, which is a property that is managed by the English Heritage Trust. In this case, they were acting as our client for this piece of research. And the kind of approach here is an approach you might do if you were incorporating aerial photographs into something like an estate management plan, for example. The area around Belsay is host to really good earthwork survival. So, LIDAR data is a very important element to understanding the historic environment in this area. However, for certain elements of the historic environment, particularly the pre-medieval elements of the landscape, aerial photography is very important. We do actually get small pockets of cropmarks forming, so, on the top left-hand side of the screen you can see an example of the specialist oblique photograph just showing an irregular polygonal enclosure. That’s also visible on Google Earth down at the bottom. What’s very interesting though, is that when you look at the Environmental Agency LIDAR, in this case visualised in two different ways which is why they’re looking quite different. We can actually see that this reveals that you do actually have some earthwork survival of this feature and this was the first time that this had ever been recognised.

However, as I said, LIDAR was an incredibly important aspect of this. This is one of the more unusual projects where LIDAR was specifically commissioned, in this case by English Heritage Trust, for the project rather than just using the Environmental Agency LIDAR which at the time was quite gappy around this area. This is vital in understanding the landscape. We actually had some really interesting new discoveries coming from the LIDAR. So, I’m just highlighting a long, curving, linear feature that you can see there and this is thought to represent the medieval park pale to the original Belsay Estate. So, this was something that was long known about to have existed via the documentary sources, but we’d never actually identified any earthwork survival of it. Many other interesting aspects to the landscape were revealed by the LIDAR, not least of which is the tremendous survival of the medieval and post-medieval ridge and furrow. So, we were to conscript large parts of these medieval open field systems using the LIDAR data.

However, as with many other areas of the country, the farming regimes have changed an awful lot in this part of Northumberland, so now around 50% of what was previously encompassed by the historic township of Belsay is actually under arable cultivation. So, we can see more ridge and furrow visible on this 10cm LIDAR on the left-hand side on the screen, but on the right-hand side of the screen, this is actually a historic photo from 1959 showing an area of ridge and furrow that has actually long since been ploughed away, so the value of combining these historic photos which are showing features which no longer exist alongside the LIDAR data.

This is a landscape whose character is very much a product of it being managed by an estate, an estate managed landscape. And so, in this case, because the ridge and furrow played such an important role, we actually took the time to individually map each of the furrows. So, this is not something we would generally do for these large landscape projects. And so this gives the really detailed mapping that you can see here today and that’s really instrumental in understanding the evolution of that landscape. In this case, we’re actually looking at an area of very early post-medieval enclosure which really stands out. One of the reasons this survives is because it was so long in pasture and actually, using the supporting field name from the OS maps, we can see word “pasture” recurring in a lot of these fields, indicating its historical use for grading. We were also able to combine documentary sources to help us far better understand the information that we were getting from the aerial sources. In this case, we were very fortunate to have the Belsay Cropping Book which is held in the Northumberland Archives. So, this gave plans of all of the farmsteads as well as listing what was grown in all of the fields over a couple of decades or so. And from this, we were actually able to reconstruct all of the land holdings of the original tenant farms.

The historic photography also showed us several other aspects of the historic landscape. So, farmstead character is something that is very interesting. In this case, you have a couple of the farms which were tenant farms, so owned by the estate. Very distinctive of that kind of e-plan 18th/19th century farmstead that you get in this part of Northumberland. These photos are really capturing them in their last iteration before many of them were really quite substantially modified by the post-war improvements in agriculture. We were also seeing some slightly more ephemeral aspects to the landscape. So, we were able to have access to some of the papers of the Middleton family and so, one of the references during the war was to them felling some of the trees on the estate to sell for pit props in the Northumberland coal mines and we were actually able to see physical evidence of these woodlands disappearing during the 1940s.

So, moving on to a more urban landscape here, in this case this is the Sunderland Heritage Action Zone. So, as many of you will know, the Heritage Action Zones are focusing research and actual money on heritage-led regeneration of certain areas of the country. So, this one was focusing on Sunderland. In this case, this piece of work was done to support the Historic Area Assessment which was commissioned out to an external consultancy. But the kind of ways that we were using aerial photographs within this project could easily be applied to somebody undertaking something like a Historic Area Assessment.

In Sunderland, we were very fortunate in having some fantastic aerial photography dating right back to the early 1920s and this was some of the photography taken by the company of Aerofilms Limited and is available via the Britain from Above website. So, we have quite a bit of pre-war photography. So, this is photographing landscapes and buildings. This was before some very substantial changes that took place during and after the Second World War. This photography is also very evocative. You may be able to pick out… you’re even spotting people in this landscape. That’s something that’s often missed with these smaller scale aerial photographs. In particular, these kinds of images are a really fantastic way of engaging people, particularly the general public in a landscape.

There’s also photography that we had during the war, in particular the early war years around 1941. This was very important for understanding the impact of the early Luftwaffe raids on Sunderland. And some of this actually captured once again, some quite ephemeral elements of the landscape. So, I’ve circled a couple of features that you may just be able to make out and these are actually barrage balloons. So, there’s one barrage balloon flying above the docks and another barrage balloon tethered on the ground. And you can a vertical photograph of that barrage balloon site on the right-hand side of the screen, photographed a couple of years later.

We were very fortunate in Sunderland of having some really high quality largescale photographs that were taken just a few months end of hostilities. On the left-hand side of the screen, we can see some of the public air raid shelters that were constructed Mowbray Extension Park and things like air raid shelters and emergency water supplies are the kind of quite ephemeral features that relate to the Second World War that were constructed shortly before these photographs were taken but very often demolished very soon afterwards. On the right-hand side of the screen we can see some of the bomb damage near the docks. So, this was incredibly useful photography for assessing the bomb damage, in particular mapping the bomb damage to assess its impact on the city which it turned out was considerably less that the post-war development.

The aerial photographs also record the early stages of post-war developments in the towns. So, on the left-hand side of the screen, we can see one of the pre-fab housing estates that began to spring up in Sunderland in the early to mid-1940s. On the right-hand side of the screen, we can see an oblique photograph that was taken by the Royal Air Force, in this case in 1948. And this is actually adding considerable detail to the plan that we can see on the left-hand side. So, we’re picking out details like the small sheds that were provided with the pre-fabs, the fact that people have got vegetable gardens in front of their houses and even down to a pony and trap just delivering milk on the pre-fab estate, which is a wonderful ephemeral detail.

I had to put these in just because they’re my favourite buildings that I found in Sunderland. So, these were the Sunderland Garths, large social housing projects that had begun to be constructed in the late 1930s. All but one of these has now been demolished. Most of them were torn down in the 1990s, so the historic photography is one of the few and very important visual sources that we have of these buildings.

Aerial photographs were also able to add interesting detail to the kind of information that you get from other sources like map evidence. On the top left-hand side of the screen, you can see one the early post-war estates being constructed. Once again, very typical of the kind of social housing projects that you were getting at the time. That mixed-use development of tenement blocks of flats and terraced houses. A very similar angle of the same estate taken in 2017. You may be able to pick out that we’ve actually got quite a different estate here. So, in this case, just two-storey terraced houses. Looking at the maps though, you can see that the footprint of these buildings is completely unaltered. So, if were just trying to assess the development of this post-war landscape from the OS mapping alone, you would assume that that estate still existed in its original form, but the photograph is indicating that it doesn’t.

We were very lucky in having quite a lot of photography in those post-war years which recorded in some considerable detail the development, in this case, the east end of Sunderland. And so we were able, along with documentary sources such as books and maps, to pinpoint really quite accurately the dates of these developments, many of which have actually since disappeared themselves.

So, I hope that gives you a bit of an idea of the kind of things that using modern historic photography can achieve in both an urban and a rural landscape. If you are interested in knowing more, you can see the research reports, freely available online, although a word of warning, we are having a little bit of an issue with our research reports at the moment, but hopefully they will be back up and running again this afternoon. So, that’s me done. We can now hand over to you. So, thank you very much for listening. It’s been a real pleasure talking to you and it’s absolutely fantastic to see so many people attending. I’m sure Helen will unmute as well at this point. So, we do have a few moments. If anybody has any pressing questions, just pop them in the chat bar.

Matt: Matt, thank you so much. I’m actually going to bring in this Q&A window for you, so please feel free to populate that with any questions that you have. You can see there’s a little text box at the bottom there and we will field the questions as they come through. I’ve also got this little resource…oh dear. It’s doing its own thing there. Not sure quite what’s going on. Let’s bring that over here so we can do that. There we go.

Helen: Somebody’s asked about sources for LIDAR in Ireland. I’m afraid I don’t know the answer to that but we could find out for you or Rebecca Bennett who’s in the audience might know. I don’t know.

Matt: Indeed. So, also, links there on the right-hand side, web links. For a copy of today’s slides, click on the word “Slides”. You’ll see the URL appear at the bottom of that window. “Webinars” will take you to the Historic England webpage for all our webinars. You’ll finding listings for future webinars and the recordings for previous webinars and all our guidance as well. Click on that guidance link and you’ll find all our guidance documents as well. So, we’ve got a few questions coming in now. It’s good to see. “What are the plans for gaining access to…?” Oh you see, now that’s what’s going on. We can’t see it all, can we? Don’t you love technology?

Helen: In terms of access to the Swindon Archive, that’s still on hold at the moment. We’re still trying to work out how we can make the environment safe for our staff first of all, and then we’ll be looking at plans for opening at some point, but really, I couldn’t say. I don’t work in the archive, so all I would say is keep an eye on the Historic England website.

Somebody has asked for the earliest nationwide aerial photographs. That would be the Aerofilms archive, I think, so that’s on the Britain from Above website and those date back to the early 1920s, and you’ve see a few examples.
“Can LIDAR really see through trees and shrub cover?”. Absolutely and we will go through that in Webinar Two, so we’ll have a bit more details on that.

Matt: There was a wonderful documentary some while ago where they did a LIDAR scan of the whole of the Angkor Wat temple complex in Cambodia and realised that they’d actually only uncovered 12% of it because the rest of it was still hidden under jungle and the LIDAR stripped away the foliage and they could see everything. It was amazing.

Helen: You can’t see Angkor Wat in England, but you can see just as good stuff.

Matt: You can indeed. Absolutely.

Matthew: It’s worth noting that there are a few words of warning with the effectiveness of LIDAR in different trees and shrub cover. We will touch on that in Webinar Two. In the meantime, I would definitely recommend having a look at our LIDAR guidance which is available as a free download from the website and that will go into a little bit more detail for you there.

Helen: Somebody’s asked how to get hold of the military sources. A lot of HERs hold them, so for example, Devon HER and Norwich HERs both have RAF photos. Some of them have them online, Somerset HER has them online. But if you want to look at hard copies, the national collection is at the Historic England Archive.

Matthew: So, we’ve got one on tips on distinguishing between crop types and agricultural land use. Once again, we do teach this and it takes quite a long time to go through in detail. I’m not sure what your thoughts are, Helen, but one of the objectives today is that we are very interested in seeing what potential demand for other online training there might be. So, with things like this, it would be quite interesting to see if there is a particular call for certain aspects of specialist interpretation that we might be able to touch in future. Have you got anything further on that, Helen?

Helen: Yes, definitely. I mean, there’s been quite a lot of research of this. So, the COSMIC project, which was the Conservation of Scheduled Monuments in Cultivation looked at the effects of different types of crops and different kinds of ploughing on the preservation of buried remains. So, there is quite a lot of material, but not much of it has been published so this is something that we’re looking at exploring and providing as an online resource, hopefully at some point.

Matthew: So, Andy’s question on the planned GIS portal for 2021. So, essentially what this will be is it will host all of our spatial detail, and behind that will sit some attribute data as well. So, we’ve been undertaking projects such as the kinds that we’ve been looking at today for around about 30 years now. So, this has developed a huge amount of spatial data. Some of the earlier projects were actually hand-drawn projects. We’ll look at in a bit more detail in Webinar Two through to the more digital projects. So, the idea is that all of that data will be hosted so you’ll be able to see all of the maps that have been made as part of those photo interpretation and mapping projects. And with the digital nature as well, you’ll be able to click on individual features and get the basic data from that and hopefully we’ll be able to include links to HERs where you can actually get the more detail monument data for example.

Helen: Are we OK to carry on, Matt?

Matt: Absolutely. Yes. I know we’ve run over time and I am aware that people have other commitments but if we carry on answering the questions, that’s being record, and so people can refer back to the recording if they want to revisit the Q&A session.

Helen: Great. OK. So, we’ve been asked have we used aerial photographs to track changes in the cliff line. We did talk about this. We carried out a very extensive coastal survey, so the Rapid Coastal Zone Assessment Surveys funded by Historic England and our predecessors. But this mainly concentrated on… So, the aerial component of that mainly concentrated on mapping archaeological remains and it was only where there was significant changes in the cliff that we might comment on that. But we always felt that that data probably existed elsewhere. And I guess, with cliffs it’s not so difficult but where you’re looking at coastal erosion, sometimes the tides can make it difficult to assess things. So, yeah, I haven’t really answered that terribly well but if you look at the Rapid Coastal Zone Assessment Surveys, they’re all on the Archaeology Data Service website, the ADS. So, you could explore that a bit further if you want some information, if you haven’t looked at it before. Do you want to take the next one, Matt?

Matthew: Yeah. So, Denise, you’ve been asking about if there are ways to map underground tunnels in former mines. So, in certain circumstances, yes. What you can sometimes find, particularly with the very early shallow mining, is that you will occasionally actually get slumping when the underground tunnels are actually collapsing. So, sometimes this will have a surface expression, particularly on LIDAR as well as on photography, so you will actually see the depressions of where the slumping is occurring. Sometimes you’ll even be able to pick out the actual pattern, the very linear pattern of the mining. If there are deeper tunnels, then this isn’t really something that you would see on aerial photographs, but you can pick out the surface features related to them, so things like adits or entrances to mines or even airshafts, then you will be able to pick these out. So, there are sometimes ways to trace some of the underground workings just from the surface features associated with them. A really good site to look on if you haven’t already, is the Coal Authority which has a fantastic GIS website where they’ve digitised a lot of their paper archives and so they’ve got really useful GIS maps where you can go on and do searches for historic mining and various kinds.

Matt: I’ll chip in if I may. Also, if any of our guests have any suggestions for other areas of aerial photography, or technical advice generally to be perfectly honest, that you’d like to see us cover in future webinars, just please make a note in either the Q&A or in the chatroom. We’re very happy to receive all suggestions for future webinars. Back to you guys.

Matthew: It was indeed. It was all Environment Agency LIDAR. I can’t remember how extensive the survey is in that area. That was the kind of field system that lied just kind of northwest-ish of the village itself. But yeah, have a look on the portal and we’ve uploaded a link to that in the chat, and you’ll be able to see what they’ve got.

Helen: OK. So, the next one is: “Any plans to push out the AP Archive to the HERs?” I think dividing that up might be a bit tricky because of changing borders and when you’ve got runs of photographs, how do you decide on the county boundaries? That might be a tricky. And again, that’s beyond my purview anyway. So, that would be one issue I can think of.

Matthew: Yeah. I suppose there is also the issue that the Historic England Archive is actually a climate-controlled, temperature-controlled archive, so there’s considerable cost to actually storing the original material in proper archival facilities. So, this is something that would probably be prohibitively expensive for any local authority to cover as well.

Helen: I think the solution is digitisation which I’m hoping will come in the future.
Matt: Are there any plans to upscale the digitisation projects?

Helen: Not that I’m aware of, no. Again, it all comes down to funding.

Matt: Indeed. As always.

Helen: We do explore it periodically, but at the moment we’re just looking at getting our born-digital images available online and publically available, so that’s what we’re working with the archive to do first.

Matthew: So, Andrew was asking about the hyperspectral imagery. So, this isn’t something that we’re going to cover in any particular detail at all. One of the reasons for that is that hyperspectral/multispectral imagery is something that we’re exploring within Historic England. Also, a lot of partners elsewhere, particularly elsewhere in Europe, have done quite a lot of detailed scientific analysis on this, but hyperspectral imagery, at the moment isn’t something that we really tend to routinely use, so we would be only mentioning it in the most basic of terms. However, if you are interested, drop us a line and we can certainly put you in touch with some really useful resources on using hyperspectral imagery for archaeological aerial reconnaissance.

Helen: Yeah, I mean, for us, it’s the availability of data like this. So, we’re exploring satellite imagery at the moment but if anything’s behind a paywall, generally we can’t afford to use it. So, that’s why I think the Environment Agency LIDAR is such a great example of government open data. And somebody’s asked: “Is LIDAR being incorporated into NMT data?”. Yes, where it’s available we use LIDAR. It’s just another one of our routine sources now.

Matt: Someone has just said that the Environment Agency has hyperspectral available as well.

Helen: Interesting. We shall have to look into that. Again, the issue might be resolution because that is sometimes an issue for us. You do need a certain resolution for it to be effective for recording archaeological features and the stuff we’ve seen so far has not been sufficiently…

Matt: They’re saying limited coverage, mostly coastal.

Helen: Yeah. But that’s something we could have a look at definitely.

Matthew: So, Holly’s asked about seeing damage on aerial photographs. I’m making the assumption here that you’re possible referring to damage to archaeological sites. Yes is the short answer. As Helen was kind of outlining, one of the very important aspects, particularly during the winter months of our aerial reconnaissance programme, is actually monitoring of schedules monuments. So, we’re taking really high quality, very zoomed-in photography of archaeological monuments, essentially to help the people that manage the monuments on the ground go through almost a bit of a triage sifting process because you know, we can photograph 100 monuments over the course of the flight whereas they may only be able to see two or three on the ground in the course of a day. And yeah, with the modern cameras that we’ve got and resolution of those, you can actually pick out even down to individual rabbit holes and things like that. Where you’ve got burrowing on archaeological sites, you can pick out that level of damage. So, yeah, using aerial photography for assessing damage and conservation of archaeological sites, it’s a really key resource.

Helen: So, moving on to the next one. Ben –hi Ben!- is asking: “Has any research been done on automated processing of aerial photos?”. Some work has been done on analysing LIDAR data by colleagues in Europe and Historic Environment Scotland have got a partnership project I think with the University of Glasgow and they are looking at trying to further develop the automation of identification of features. And as you can imagine, the numbers of variables involved in terms of trying to distinguish an archaeological feature from a natural feature or even a hedge or a building, are considerable. But we’re making slow steps because I think this is going to be the next big development for us because we’ve got large volumes of digital data now, so repeat satellite imagery, Google Earth, and if we can run algorithms that will go through and highlight where there is high potential for historic environment features then that would be great.

Matthew: So, yeah, John’s been asking about a good feature identification or reference source for LIDAR working in rural Cornwall. Once again, I think a really good first port of call will be to look at our Historic England guidance, the Light Fantastic guidance that we’ve posted a link too. So, this talks in quite a bit of detail about understanding LIDAR but also some of those interpretation elements, in particular those kinds of things that can really trip you up when you’re interpreting LIDAR. In many respects actually, the kind of skills that you’re using when you’re interpreting aerial photographs are perfectly directly relatable to interpreting LIDAR. So, for looking at an earthwork site on a vertical photograph, you’re using all of those same analytical processes that you are when you are looking at LIDAR data. One of the most important things I think, if I was just going to give one piece of advice, is actually using as many supporting sources of information as you can when you’re using LIDAR data. So never just using LIDAR in isolation and taking it at face value. So, even if it’s just looking at the site on Google Earth as well, or certainly you know, if you do have access to some other sources of mapping or historic aerial photographs, using them all together when you’re interpreting LIDAR, that’s actually a really important aspect of interpreting LIDAR. But as I say, going to the Light Fantastic guidance is a really good first port of call for that. In terms of courses, Helen, I know we’ve done things in the past, Simon has run stuff. Is anything planned?

Helen: Yeah, so Simon Crutchley and Peter Crow from the Forestry Commission -so Simon works for us- are looking at planning an online source with Oxford University Department for Continuing Education. I know that Rebecca Bennett, who has been on the call, does very good training. I don’t know if she still does that but you could always contact Rebecca. I’m sure she’s out there on the internet. It’s worth having a look at case studies as well, Best Practice, just have a look through our research reports as well.

Further information on use of drones and point cloud surveys against the cost of planes and LIDAR, etc. Yeah, this is something that we’ve been looking into and it’s a bit of an apples and pears situation -we’re going to go a bit more into this in Webinar Two, I think- in the sense that it’s different courses for different sources, as it were. So, you would use a drone in a different situation, site-based, where you need detailed work, where you’re focusing intensely on one site whereas I would use the conventional plane if we were looking at landscapes, if you need to get around lots of different sites. So, for example, we’ve got some work on Dartmoor coming up where we’re going to use a combination of the two. So, when we’re looking at large areas, we’ll use our aeroplane, but when we need to fill in details that aren’t picked up from the aerial surveys, then we’ll use drones.

Matt: There was one question actually in the chat at the beginning of your presentation. Our guest says: “One of the low points in my career was seeing a listed building and curtilage being surveyed by a drone without the owner’s position. What is the law on this? Or is this something being covered next week?”.

Matthew: Yeah I did. I dropped a quick reply to that in the chat. It’s because it was a few further down by the time I’d spotted it. The best place to go to is the Civil Aviation Authority’s website and they have a site called “Dronesafe” which is aimed at drone operators and amateur drone users as well. So, this has some really useful material on all of the advisory safety aspects as well as the legal aspects to operating drones. So, any question such as this, that’s a really good first port of call to go to that website. There are lots of different pieces of guidance and rules that cover operating drones to do with where you’re flying them, how far are you away from people, how far are you away from buildings, maintaining line of sight and things like that, so that’s all covered by that guidance. I’d definitely recommend going and looking at the CAA’s “Dronesafe” site.

Matt: Is that the right link I’ve put in the chat there.

Matthew: I think it probably is, yes.

Matt: Good. OK. I think we’ll draw the session to a close. Thank you so much everybody for attending. I can see that there are possibly one or two questions that we haven’t been able to answer for you today, but I’m sure Helen or Matt will be more than happy to respond to those questions directly after the session. So, to Helen and Matt, thank you so much for today’s presentation. Absolutely fascinating. You had us all in your thrall for the duration. I think you actually now have the record for the geographically furthest dispersed group of guests, if that’s the correct way of saying it. You have a guest from Brisbane, Australia, so go you guys. That’s absolutely fantastic. So, for our participants, thank you very much for your time today. We hope to see you all again next week for Part Two. You’ll all be sent an invite either this afternoon or tomorrow for that webinar, so I hope you can all make that. It’ll be the same time next week as well. Please take a few minutes to cut and paste or copy those links that we’ve put in the chat and indeed the web links on the screen there as well at which point we will mute our microphones now. Helen and Matt, if you could just stay in the session just for a debrief afterwards but at this point we will mute our microphones and then I’ll close the session in a couple of minutes for you. So, once again, thank you Helen and Matt.