One of the core methods of VLAP is photogrammetry or structure from motion (SFM). Through photogrammetry, it is possible create high resolution 3D models from photographs taken with regular digital cameras. It is just a question of taking as many pictures as possible…

…and that is exactly what we did with our drone.

Showing off his drone skills, Derek flew the tiny drone up the hillside about 20-30 metres above the large terraced road that winds from the bottom of the hill towards the top. At the same time, the drone took about 1500 vertical images of the ground below, giving us an archive of pictures of rocks and shrubs.

These pictures were then loaded into the photogrammetry software – in this case Capturing Reality. This software then searched all the pictures for common features, creating a kind of three-dimensional mosaic of photographs.

The alignment of photographs in the software.

The alignment of photographs in the photogrammetry software.

The method works splendidly, and we now have a rough 3D model of the terraced road. This can be inserted into a GIS program, allowing us to draw high precision plans of large-scale features.

The software can also compile a true orthomosaic, giving us some kind of an aerial photograph consisting of several hundreds of detail photographs. With this method, we get more detail than from a conventional aerial photograph and that with less distortion.

The finished orthomosaic of the north road

The finished orthomosaic of the north road. Click for higher detail.

It is also possible to extract topographical data from the 3D model, as in the example below. Here, we have a slope model, which is a way of displaying the steepness of various topographical locations. Lighter areas indicate steeper places while darker areas are more flat. As you can see, the terraced road appears as a darker inverted ‘Σ’ on the slope. With this model, we can see the actual width of the road, and not just the outer sides of it.

Slope model of the northern road.

Slope model of the northern road.

The only problem with this kind of photogrammetry is that the files tend to get very large, sometimes above 30 gb. Only the most powerful of computers are able to open such files. Still, the method allows the VLAP team to get very accurate measurements of the antiquities at Vlochós with very little time spent in the field. 10 hectares is easily done in half an hour. The processing, however, takes many hours which will be done in Gothenburg during the long Swedish winter nights.