Geodata reveals forgotten history: the mystery of the old ditch

How aerial photos and modern data uncover a hidden World War II story

On January 2, 2025, the Central Archive for Special Judicial Proceedings (CABR) opened its doors. This archive contains information on some 425,000 people who were suspected of collaborating with the German occupiers during World War II, as well as the stories of their victims. NOS reporter Lex Runderkamp delved into this unique archive to investigate what happened during the war. In the first episode, titled “Resistance,” he follows the trail of a woman who was executed during the war. Rumor has it that she is buried at the site of an old ditch—a ditch that has long since disappeared. Harald Gortz, a geodata specialist, helps to make this ditch visible again using historical aerial photos and modern data. In this blog, you can read step by step how he does it.

A hidden story in the backyard

How it all began. In the backyard of a vacation home in the Alblasserwaard, a mystery is circulating: a woman who was executed during World War II is said to be buried here. The clue? “Don’t dig where the old ditch used to be.” But that ditch has long since disappeared.

Determined to find out what happened in his backyard, history teacher Niels de Bruin teams up with documentary filmmaker Lex Runderkamp to delve into the CABR archives.

They discover that an execution did indeed take place in March 1945 and that the woman is said to have been buried in the backyard, where a ditch used to run. But how do you pinpoint the exact location of a ditch that no longer exists?

Step 1: Georeferencing the RAF images

During the war years, the Royal Air Force (RAF) took aerial photographs of the Netherlands. These images, originally intended for military intelligence purposes, offer a unique glimpse into the Netherlands of that time. Harald has selected images from 1944 and 1945, taken around the time of the execution. 

Figure 1: The old RAF images.

Figure 2: In this 1944 photograph, the old network of ditches is still clearly visible.

Figure 3: In this photograph from April 1945, the ditches are no longer visible.

To overlay the RAF images above onto current maps, Harald uses georeferencing: a technique in which historical images are aligned with modern coordinates.

Using software such as ERDAS , Harald identifies corresponding points, such as the corners of houses, and precisely positions the images (Figure 04). This is easier than you might think. The computer assists in the process; once Harald has marked more than four points, the computer “helps” by searching for corresponding points (Figure 05). This makes it possible to seamlessly overlay the historical situation onto the current map.

In Figure 06, you can see how the historical image from 77 years ago overlaps with the current image, highlighting the differences between then and now. 

Figure 4: Positioning images.

Figure 5 The computer searches for matching points.

Figure 6: The differences between then and now become clear.

Step 2: Digitizing watercourses using the BGT

Using the georeferenced images, Harald will begin working with the Large-Scale Topography Database (BGT). This modern dataset shows all current ditches, roads, and houses in the Netherlands.

Using the aerial photograph from 1944, Harald is able to accurately digitize the location of the old ditch. He can then combine these locations with current data. He also compares this location with the images from 1945 (Figure 07) and discovers that the ditch became less visible during that period, possibly due to earthworks. 

Figure 7: Comparing images.

Step 3: Check weather statistics

Were the ditches possibly frozen at the time the aerial photos were taken?

To confirm his findings, Harald checks the historical weather data (Figure 08). The weather statistics show that temperatures were well above freezing. This allows Harald to rule out the possibility that ice obstructed the view of the ditch. The changes in the terrain around 1945 suggest human activity instead.

Figure 8: Checking weather data.

Step 4: Analyzing Elevation Differences Using AHN

Using the Current Elevation Database of the Netherlands (AHN), Harald maps subtle differences in elevation. These differences often provide clues about old ditches and other historical features.

By creating a cross-section (Figure 10), he discovers that the old ditch is still visible as a slight depression. Using point clouds, Harald creates a 3D model of the terrain, which allows him to reconstruct the contours of the 1944 ditch.

You can see that the ditch from 1944 on the right side of the lot is now partially buried beneath the driveway of the current buildings. There are now trees in the middle of the property. Only the section of the ditch that ran parallel to the waterway remains in open terrain.

The first step is to create a cross-section. By drawing a line across the plot, a cross-section of the terrain is created. The cross-section does reveal minimal differences in elevation. The ditch that ran parallel to the watercourse (and thus perpendicular to the cross-section) is not clearly visible in the profile. Clearly visible are the depressions in the terrain that can be seen in the relief (Figure 11).

When we visualize this as a colored elevation map (Figure 12), we see a distinct depression (shown in blue) appear on the image. These correspond to the ditches from the years 1933–1945. This also explains the current damp spots on the property (the areas where water pools first), according to the resident.

Figure 9: Scatter plot.

Figure 10: Cross-section.

Figure 11: Relief of an old ditch.

Step 5: Deploying ground-penetrating radar

Figure 12: Ground-penetrating radar in action.

Now that we know for certain that there were indeed ditches on the site in 1945, we can focus our search on those specific areas. Together with Benedicte van der Roest of GT Eclipse BV, Harald is using ground-penetrating radar to analyze the old ditches.

A ground-penetrating radar transmits a signal. You may have seen this in the TV series *Crime Scene Investigation* (CSI). A ground-penetrating radar sends the signal into the ground. This signal reflects off “layers” in the soil. The reflected signal is captured, creating an image that allows us to distinguish the composition of the soil.

Unlike what you see on CSI, you don’t immediately see a “skeleton” appear on the radar screen. Because the ground-penetrating radar signal reacts differently to various materials, we can distinguish between sand, clay, stones, tree roots, and, for example, skeletons, which appear on the screen as “wavy” lines.

Figure 13: Elevation differences.

Step 6: Dig into the ground

So what now? We dig into the ground to see what’s there and find some unusual material. Unfortunately, we have to wait for drier weather to continue digging, so we fill in the hole. In the summer of 2024, the time has finally come: we dig again and find bones. The police are notified and take over the excavation.

Has the mystery surrounding the old ditch been solved?

The bones appear to be animal skeletal remains, and we have not (yet) found the couple. It is believed that the site was once used as a burial ground for an illegal slaughterhouse. And since we have not yet been able to examine all areas, the investigation is still ongoing.

Thanks to Harald’s analysis using historical aerial photographs, modern datasets, and advanced technology, we were able to pinpoint the location of the old ditch on a recent map. This not only provides new insights for the research but also demonstrates the power of geodata to bring the past to life.

More information

Harald is a Business Consultant at IMAGEM. He bridges the gap between business, sales, and technology, and focuses on delivering value to the client. He collaborates with developers to create solutions for the public safety and security sector, educational institutions, and digital twins.

Would you like to learn more about this topic? Send Harald an email or send a message via our contact form.

Harald Görtz

Business Consultant

+31 (0)6 48 40 97 66 imagem

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