As you might have noticed, we experienced a parched summer, and although most news agencies have elaborately covered the many negative effects, some also reported an interesting side-effect of the drought; somewhere in the Netherlands, a farmer spotted color differences in his fields that indicated where former ditches had been in the past. More interestingly, the shape of a medieval castle was visible in the grass near Noordlaren, a small town close to Groningen. Quickly after one of the first of these news items surfaced and was shared with me, I decided to do some drone archaeology myself. 

Now, out of personal interest, I had already done some research about the history of the area in which I grew up (Peebos, Doezum), so I knew that a house had once stood in the meadow next to my parents’ house. This house was completely leveled roughly half a century ago, and I wondered whether any pattern at all would still be discernible from the sky. So, without having too high expectations, I launched my drone into the air, flew it over the tree line that separates my parents’ plot of land and the meadow of interest, and glanced at my tablet for the live feed from the drone camera. Below a picture of what I saw.

An example of the features that drone archaeology can uncover: clear drawings in the grass that indicate the location of what still used to be a house somewhere around the 1950's in Peebos (Doezum) the Netherlands. The colour of the lines suggest that the foundation is still present in the subsoil.
Location of a former dwelling in Peebos. Source: Own work.

Conditions for drone archaeology

So, where did this first ‘drone archaeology’ experience leave me? For one thing, I found out the exact location of a house that has been gone for quite some time. I also got the impression that the people who broke it down did not bother too much with removing its foundations from the soil. I suspect this because, as you might have noticed when looking at the picture, there are some clear and straight human-made lines visible in the grass, which have a color that is indicative of what you could call suboptimal plant health. This deficient plant health usually manifests itself when something like the remains of stone foundations lie buried in the soil. Of course, when materials as such are blocking the way, plant roots cannot optimally reach nutrient rich soil and water.

Another example of the features that drone archaeology can uncover. Again: (really) clear drawings in the grass that indicate the location of what used to be a building in Peebos (Doezum) the Netherlands. Also in this case, the colour of the lines suggest that the foundation is still present in the subsoil.
Clear lines indicating a former dwelling near Peebos. Source: Own work

But, if lines or forms like these are greener than the surrounding vegetation — and the concerning plants are thus healthier than the plants in their environment — it usually means that somewhere in time, something like a ditch or other depression was present. When such a low terrain feature has filled up with sediments, the resulting soil can often hold moisture better and is sometimes more fertile. Plant roots can usually grow better in this kind of earth as well.

An example of the features that drone archaeology can uncover: clear lines in a cornfield (Peebos, Doezum) that indicate former ditches and former plot boundaries. The relatively yellow (thus unhealthy) corn plants mark the line along which big oak trees used to stand until a few years ago. A couple of areas with a difference in soil composition and thus a difference in fertility are easily recognizable.
Clear lines indicating former ditches in a field of corn in Peebos. Source: Own work.

In short, local differences in soil composition can become visible through differences in plant health and these differences are amplified when plants are having a hard time, such as was the case during the dry period of last summer. Contrariwise, precipitation can also be an indicator; former ditches were visible on drone pictures that I made of snowy fields last winter. In such conditions, local differences in the ability of soil to hold heat cause some snow to melt away faster, but differences in height can also be a factor here. Anyway, my interest was sparked, and I decided to carry on collecting data.

The collection process

In the somewhat limited time that I had this summer, I was able to capture about 5000 drone photos of dry Dutch landscapes. These include all the images required to make a few detailed drone maps of the area around the ‘river’ the Lauwers (the border between Groningen and Friesland) in and near Peebos, and some more random aerial photographs at a few other places, most of them on indication of dr. ir. Erik Meijles. The drone maps, of which an interactive web map can be studied below, were produced by letting the drone autonomously fly over a preselected area. In this process, the drone will take photos at a specified interval and fixed altitude. These photos can later be stitched together with photogrammetry software that creates the actual drone maps and can output large image files with geolocations, which are effortlessly imported in GIS-software.

Drone maps of Peebos (Doezum) and environment. Source: Own work.

Special thanks to the Geodienst, that made the interactive publication of the drone imagery on our website possible. The Geodienst offers free support with GIS analysis, grant applications and making maps. Contact the Geodienst at geodienst@rug.nl

Note: This web service is interactive, which means you can zoom around, turn layers on and off and select different basemaps, for example. Give it a go and try some amateur drone archaeology and historical landscape research yourself! To make things a little easier, Girugten has also pre-selected some interesting features as well.

Findings

In total, I found six figures in and around Peebos that indicate the former presence of buildings, all of which I could later confirm. Finding evident marks in the soil at places where these dwellings had once stood was the rule, not the exception. In these cases, the drone pictures do not add to the knowledge about the general location of these former dwellings, as those locations have been well documented in the Netherlands for about 200 years now, but for me at least, the photographs do enhance the historical experience. In addition to these vanished buildings, a significant number of filled-in ditches were visible in a lot of the meadows and corn fields that I mapped, giving an excellent insight into former plot boundaries and the cultural (reclamation) history of the area in general. You can track these results down yourself in the map above.

In addition to human-made traces, some of the earth’s natural processes also leave their marks, and I was able to capture a few interesting ones on camera. Directly to the southeast of the gas extraction site between Zuidhorn and Faan for example, a relic from the last ice age was visible. What you see on the picture below is almost certainly what is called a pingo ruin, which is what remains of a collapsed small hill formed by water and ice. Erik Meijles hinted this one, and I might have captured a few possible pingo ruins around Peebos as well.

Drone archaeology can also visualize the history of the natural landcape of areas! In this picture a pingo ruin is clearly visible across multiple fields. In the bottom left of the pingo ruins' edges, a few potentially interesting (or at least deviant) shapes are visible. The picture was taken directly south of the N980 between Niekerk and Zuidhorn.
Pingo ruin directly south of the N980 between Niekerk and Zuidhorn. Also notice the little hexagon-like shapes on the edge of the pingo ruin in the field on the bottom left corner of the picture. Do you know what caused them? We don’t. Source: Own work.

Mysterious shapes

While reviewing a series of images directly after one of the flights this summer, I noticed a lot of human-made lines that belonged to one meadow. Not being able to stitch those images together directly, I flew the drone over to that meadow again and took a picture from a bit higher altitude to see if and how the lines were connected. There it was, quite a large and square-like figure in the middle of the field, with some more lines all over the place, and some of those seemingly connecting to the shape in the middle. The image below is a cutout of the high resolution stitched pictures from the first flight job. Notice that the orientation of the main square-figure is not perpendicular to the borders of the field. Also notice that the lines are greener than the surrounding grass, so it is not likely that something impenetrable is hidden just below the surface.

A true discovery in drone archeology! Visible is a square like figure with three lengthy arms. No one knows what the figure caused yet, so it could be very unimportant, but the figure can not be explained from any (relatively) easy obtainable records. Aliens?
Figure of uknown origin in Peebos. What could it be? Source: Own work.

So, what is the origin of this figure? I had no straightforward answer, but there are multiple possibilities. One of the first things that came to my mind was the possibility of dug in cables or pipes, but that did not seem logical. Why would those be placed in such a formation with sharp 90-degree corners? To be sure, I requested information from the Dutch Cadastre, which reported that — to the best of their knowledge — there are no pipes or cables from any provider at any place on the lot. Maybe a modern water drainage system is hidden below the surface then? Well, the field is located on relatively high grounds, and that is why I am unsure about its need for water drainage. Moreover, the layout of the lines does not seem logical for water drainage purposes. Having pretty much eliminated these options and myself, and others — including the current owner of the plot — having no good alternative explanations, I got puzzled and engaged in some explorative desk research.

Into the realm of amateur-historical landscape research

It is good to understand that my family needs to travel past the field of interest to reach my parent’s home ever since we moved there in 2000. So, whatever could have been done to the plot between then and now, we will most probably know of it. The lot was grassland when we moved over and had stayed grassland up until it was ploughed over by the current owner and transformed into a cornfield a few years ago. It continued serving as a cornfield for one or two years and was then converted to grassland again. I looked at as many satellite images and aerial pictures I could find from 2000 onwards and could not see any clues about the origin of the figure there either. Just more grass. Therefore, unless I missed something, I think it is likely the shape originates from before 2000.

Moving on, I compared the first usable height model of the Netherlands with height data from Q1 2008 (AHN2) and the successor with data from Q1 2014, the AHN3. The AHN3 shows a very equalized surface, consistent with the surface one would expect to be the result of modern ploughing/equalizing machines. This explains the recent ‘‘grass-corn-grass’’ transformation of the field. The AHN2 model shows a lot more relief: most of the lines that are visible near the edges of the drone picture look like the remnants of old ditches in this height model. Consequently, I hypothesized that the lot had to have been divided into smaller areas somewhere in the past. Not surprisingly, this is in line with aerial pictures from WWII onwards. Although the borders of the plot have always stayed the same, the interior was at different times, and in various configurations split up by ditches, appearing to be serving as borders between different agricultural production areas.

With the height data and old aerial pictures in mind, I was able to isolate the figure in the center of the field. Its origin does not seem to be shared with that of the rest of the lines. Interestingly, the center figure was not recognizable in either of the two AHN datasets. If there ever was any height difference concerning this figure, it was gone by the time 2008 arrived. Moving further back in time, the very first topographic maps from the beginning of the 19th century show the field as being part of a forest, but the edges are the same every time, they have not changed since the early 1800’s. This makes the orientation of the shape in relation to the borders of the field interesting, because it looks like it has nothing in particular to do with the current boundaries. While this is likely to mean nothing, it does not lower the chance that the shape originates from before the beginning of the 19th century as well.

So, what then is the origin of the figure?

All in all, apart from gathering a few clues, I am probably not yet close to an answer. Could the figure show a courtyard that once was a part of the forest? I do not know. Might the place be a forgotten graveyard? Well, the history of the area might allow such a possibility, and the shape literally resembles some current cemetery layouts in the region, but again: I do not know. There are many more possible explanations, but I have not found one that satisfied me. Of course, there is always also the possibility that the figure is relatively recent, or of such unimportant origin that it is not common knowledge. Anyhow, I have not yet exhausted all research options and will continue my search for answers, but if you recognize the figure or have any information, ideas, questions or comments: please let it know in the comments below or contact us!

To conclude

In any event, these and other archeological findings during last summer’s drought are illustrative for that fact that landscapes change, maybe more so than we realize. In this light you could state that these houses, castles, ditches, pingos and other ‘former’ landscape features have lost their struggle against time, they are not part of the landscape anymore. And you would be right; while standing at their former locations, you would probably not notice they ever existed. However, hidden from our senses, their footprints are still there, scarring the soil and patiently waiting for the right circumstances that will inevitably reveal them again.

Make sure to check out the interactive web service with the drone maps here if you haven’t done so already!

Special thanks to everyone who helped and contributed!

This article was first published in Girugten (Year 49 of Girugten – issue 02 – november 2018).

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