A few posts ago, we described to you how a section of a field becomes a trench: we excavate layer by layer and designate a new planum whenever a striking change in the ground is noticeable. Each of the plana is documented by a photo and a drawing in scale of 1:20, but on top of that with every new planum the trench is recorded as a virtual 3D-model.

The first step on the way to a virtual trench is to take photos of every detail of the trench’s surface/content, ideally from every angle possible. These photos are then imported into a software that is capable of understanding the three-dimensional structure of the trench by analysing the images (photogrammetric processing).

Step by step, the software generates a three-dimensional point cloud, whose points are then linked to each other to create a meshed surface. Finally, the software adds a texture based on the photos to simulate the actual appearance of the trench. The result is a virtual 3D-model of the trench that was photographed in the beginning.

In order to use the model for documentation purposes we want to be able to locate the virtual trench in our excavation area (georeferencing). Therefore, we add markers to the trench before taking photos of it and establish the coordinates of every marker with the help of a tacheometer. Since the markers appear on the photos, they are also visible in the textured model, and so we can easily assign the established coordinates to them. The virtual trench is now embedded into a coordinate system which means it can be placed on a virtual planet earth at the exact same spot where it belongs.

After georeferencing the virtual trench, we use the software to create an orthophoto: an aerial photo that shows the trench directly from above without the distortions a normal aerial photo would include (orthorectified image). The orthophoto is then printed in scale of 1:20 and given to the colleagues in the field. While checking with the actual trench, they draw the planum on a semi-transparent drawing paper that lies on top of the printed orthophoto. This procedure simplifies and speeds up the process of drawing the planum to scale.

by Julia Arnkens

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