Structure-from-Motion: Photogrammetry at CA

At Cotswold Archaeology we use photogrammetry to create measured, textured 3D models at a wide range of scales, from recording small objects such as grave goods to entire landscapes using drone technology. This technique provides powerful visual results for our clients and for you, the public. Many of our models are uploaded to Sketchfab where anybody can take a virtual tour of the artefact or site and learn more about it by clicking on the interactive information points. Take a look at the Boxford Mosaic below.

But what is Photogrammetry…?

Photogrammetry, often referred to as Structure-from-Motion, is a technique that uses a series of overlapping two-dimensional photographs to reconstruct a three-dimensional object or structure. The images are processed using powerful software that identifies common points across hundreds of photographs, producing thousands of ‘control’ points (or, common points). These common points, along with known values from the camera, are used to compute the position of the camera in 3D space. This ‘sparse cloud’ of points is then processed to produce a dense cloud of points. You can see the difference between the sparse point cloud and the dense point cloud in these images of the Boxford mosaic.

The software then creates a 3D polygon mesh between the points, and provides texture by draping the 2D images over the model. As an example, here’s what the mesh looks like for Boxford!

3D polygon mesh of the Boxford mosaic
3D polygon mesh of the Boxford mosaic

We can use these models to produce accurate, measured, two-dimensional ‘ortho’ images, which have been used to great effect for recording burials and historic buildings. In the image below, you can see how the smaller plan of this trench provides a basic location for the deposits and structures (the purple, red, black, and blue shapes), while the annotated ortho image gives more meaningful information on the colour and texture of the deposits, and the stone and brick detail for the wall footings.

2d ortho photograph of an excavation area in Bristol

When we use this technique for recording historic buildings, we are able to provide good quality visual information on materials and condition for our inhouse specialists and for clients – just look at the example below.

For more of our photogrammetry models, head over to our Sketchfab page!

Rebecca Havard

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