Detection of burial mounds in high-resolution satellite images of agricultural land

Many archaeological sites are discovered during building and road construction work, prompting full excavations and delay in construction. In order to detect more cultural heritage sites in advance of construction work, the Norwegian Directorate for Cultural Heritage has taken an initiative to develop tools for early detection of potential cultural heritage sites in satellite images. The present work focuses on detecting the remains of burial mounds in agricultural fields. The remains of destroyed burial mounds are sometimes visible in satellite images as circular soil marks or crop marks. At best, the marks are distinct, but tend to have less contrast to the background than many other patterns in the images. Consequently, reliable automated detection based on pattern recognition is challenging. We have developed an approach with the following steps. First, the image is contrast enhanced, in order to make weak marks more distinct. The image is then convolved with ring templates of varying sizes, giving high absolute values at candidate ring locations. Finally, the ring candidates are presented to an operator, who may reject some of them. We tested our method on Quickbird images from south-east Norway. Of the circular marks that were clearly visible in the images, 73% was detected, and of the ones that were fairly visible, 50% was detected. In addition, seven times as many false positives as true positives were detected. The number of false positives can be reduced, at the cost of reducing the number of true positives as well. For example, by reducing the number of false positives from 7 times to 0.5 times the number of true positives, the number of detected rings decreased from 64% to 32%. Archaeologists state that the software tool will be helpful for locating potential cultural heritage sites. Although it makes many false detections, the interactive method to delete false detections is so efficient that, say, ten times as many false detections as true detections is not considered a problem by archaeologists.