Bigger And Better: Photogrammetry Of Buildings

Photogrammetry is definitely the “new” thing in archaeology, slowly clawing its way into how we treat and record archaeological sites. As far as its potentials go though, there is still a lot of research to be done, in order to assess the uses that this technology has, but also the limits that we will have to deal with on a day to day basis.

One of the aspects that has always interested me is a question of scale. We’ve seen before that Photogrammetry deals well with anything ranging from an arrowhead to a small trench (4×6 is the maximum I have managed before, but there is much larger stuff out there). Smaller objects are hard to do cheaply, as the right lenses become essential, but larger stuff should be entirely possible to do with the right considerations in mind.

123D Catch’s website shows quite a few examples relating to Photogrammetric reconstruction of buildings, so I tried reconstructing my own, taking as a subject the front façade of Saint Paul’s cathedral in London.Given that this was mostly for experimental purposes, I just attempted a few things and went through the results looking for patterns and suggestions.

The results can be viewed here: 


As we can see, the results are of course not marvellous, but I am less interested in the results than the actual process. I took 31 photographs of the building, standing in a single spot, taking as many pictures as necessary to include all parts of the façade and then moving to a slightly different angle. I tried to make sure that I got as much of the building in a single shot as possible, but the size of it and the short distance at which I was taking the photographs meant that I had to take more than one shot in some cases.

The lighting was of course not something I could control, but the fact that it was late afternoon meant that it was bright enough to be  visible, yet not too light that would bland the textures and cause problems with contrast. I then used 123D Catch to process the photographs, and to my surprise all of them were included in the final mesh.

The thing that surprises me the most is that given the photographs I had, the results are actually as good as my most hopeful prediction. There is a clear issue with the top of surfaces i.e. top of ledges that come out as slopes. This is totally expected, due to the fact that usually Photogrammetry works with images taken from different heights, while in this case the height couldn’t change. This however can be solved by taking images from the surrounding buildings.

The other major problem is the columns, that are in no way column shape, and that seem to mess up the structure behind them as well. Given the complexity of the structure this is also expected. 123D Catch imagines the mesh as a single flat surface, and tries to simplify whatever it finds as much as possible. In this case the columns are not flat, so the solution 123D Catch has come up with, given the limited data, is to bring forward the murky back and treat it as if it was the space between the columns. This is due to a large lack of data. Next time the solution will be to concentrate more on these trouble areas and take more numerous and calculated shots that can aid 123D Catch. It does require some more work and some careful assessment of the situation, but it is entirely possible to achieve.

Apart from these problems, the results are very promising. More work needs to be carried out, but it shows that it is possible to reconstruct structures of a certain size, hence pushing once again the limits of Photogrammetry.