Photogrammetry

Simon getting an aerial view from a tree.

Following on last year, I've been using photogrammetric models to obtain orthogonal aerial views (taken at a 180º angle looking down onto the trenches). Photogrammetric software estimates the three-dimensional coordinates of an object (in this case an excavation unit), by making measurements between objects present in multiple photographs. From a cloud of photographs taken at different angles and heights (the model below uses 47 different views), I am able to obtain a perfect aerial shot—though there are other applications as well.

Photogrammetric model of Trench F, Stensö. Although only the aerial view is visible here, the model is fully 3D and can be viewed from any angle. 

Archaeologists traditionally had to resort to all kinds of methods for aerial photography—from balloons to ladders to heavy gantries for lifting up a camera. This method, which uses the Russian software Agisoft, is quite simple and only requires a camera on site. Both high and low angle views are desirable, though not strictly necessary—at Stensö we can stand on the tower to get high angle shots, and at Landsjö we can use the trees.  These shots do not have to be directly overhead. Back at the dig house, the photos need to be carefully cropped to remove areas outside the area of interest (trees, for instance) and any elements that are not in focus.

Trench F, Stensö, drawn.

From there, I am able to use the models later on for drawing or for direct documentation in the report. The drawing above was made using a series of photogrammetric models documenting the excavation of the first 20cm of Trench F at Stensö. Although drawings like this can also be done by making what is basically a downward facing photographic panorama, this actually takes less time. I also think it represents shapes better than the panoramic method. Photogrammetry is a great digital method for archaeology.