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Siegerswoude: survey at night

In the Middenwei area of Siegerswoude, located in the municipality of Opsterland in Friesland, distinct linear patterns were observed in the grasslands near the Neef family farm (Middenwei 9) (figure 1). These patterns are also visible in images from the Digital Elevation Model of the Netherlands (AHN). Further investigation using geophysical precision agriculture techniques and drone-based remote sensing has revealed additional traces.

Figure 1: The Siegerswoude site in Friesland. Bottom left: visible light image; right: digital terrain model (DTM) of the location (source: 4D Research Lab).

Objective/Research Question

The primary aim of the research in Siegerswoude was to explore the applicability of electromagnetic induction measurements for identifying and mapping archaeological features. Additionally, drone imagery was captured using both optical and thermal infrared cameras to determine whether these images could reveal phenomena not visible in the electromagnetic induction measurements or on satellite and AHN images.

An extensive study was therefore conducted in Siegerswoude using precision agriculture techniques, drone sensors, and trial trenching. For detailed results, reference is made to the underlying reports; this description focuses on the applied sensor techniques.

Methodology

The research was carried out using electromagnetic induction (EMI) to survey the entire site, with measurements taken on a 1 m grid of transects. While a 5 m spacing is standard in precision agriculture, this smaller spacing provides more detailed images, allowing for better data analysis in archaeological applications. To assess the effectiveness of the data for archaeological purposes, we compared the results at different resolutions.

Thermal infrared imaging with a drone was conducted at various times throughout the night. Flights were carried out shortly after sunset, at midnight, and just before sunrise. Additionally, we flew at different altitudes over the same terrain, helping us identify the best times and altitudes for capturing optimal thermal images. During the day, the entire site was photographed using visible light imaging, creating a photogrammetric surface reconstruction that resulted in a high-resolution orthophoto and a highly accurate elevation model.

Finally, the findings were validated through excavation, focusing on locations where the most prominent anomalies were detected.

 

Results

The precision agriculture techniques used in the study have proven to be effective in mapping archaeological features. The orthophotos and detailed elevation models, created from drone imagery with both optical and thermal infrared cameras, add significant value to the data collected through traditional remote sensing methods. These innovative techniques not only provide a clearer view of the structures identified with conventional methods but also uncover additional details and phenomena that were not visible in the other images.

Lessons learned

  • Thermal images proved to be most effective around midnight, after a period of cooling, when the differences in relative cooling were more clearly visible (figure 2).
  • Thermal images taken just before sunrise were less effective, likely because the contrasts in relative temperature decreased during the second half of the night.
  • September offers ideal conditions for drone thermography in this particular landscape and local climate.
  • A flight altitude of 100 meters for thermal imaging was found to be efficient, striking a balance between sufficient detail and contrast on one hand, and broader coverage on the other.
  • Drone  photogrammetry has also proven effective, providing valuable supplementary data to the Digital Elevation Model of the Netherlands (AHN) through its higher level of detail.
  • Precision agriculture techniques are capable of detecting archaeological phenomena, but their effectiveness is highly dependent on the transect spacing used, which is crucial for achieving a usable resolution.

 

Figure 2: Comparison of the thermograms, captured at different times during the night (from left to right: 20:10, 01:30, and 06:00) (source: 4D Research Lab).

References to reports

  • Van Doesburg, J., van der Heiden, M., Waagen, J., van Os, B., & van der Meer, W. (2022). Op zoek naar lijnen. De waarde van elektromagnetische inductie en optische en thermische infraroodbeelden in Siegerswoude (Friesland). Rapportage Archeologische Monumentenzorg, 273. Rijksdienst voor het Cultureel Erfgoed. https://www.cultureelerfgoed.nl/publicaties/publicaties/2022/01/01/op-zoek-naar-lijnen
  • Waagen, J., García Sánchez, J., van der Heiden, M., Kuiters, A., & Lulof, P. (2022). In the heat of the night: Comparative assessment of drone thermography at the archaeological sites of Acquarossa, Italy, and Siegerswoude, The Netherlands. Drones, 6(7), 165. https://doi.org/10.3390/drones6070165

Colophon

In samenwerking met de Rijksdienst voor het Cultureel Erfgoed (RCE).