Scattered around the United Kingdom are a series of defensive bunkers known as pillboxes. Built during far darker times they serve as reminders of history and whilst some have been removed over the years many survive. Most stand empty and unused while a few serve as man-caves or apple stores. One is listed as being used as a wine cellar. They make great subjects to digitally preserve with photogrammetry.
These curious structures are not so large it takes thousands of images but always enough to present a challenge somewhere. Any whose entrances remain open are good test for photogrammetry skills as incorporating the internal space always needs thought and care.
So when you find yourself presented with a subject and camera but no scale bar what can you do?
Whilst out for a bike ride I came across a pillbox built into the side of a bridge and overlooking a canal. The camera was in the rucksack but I carried no scale bar.
The solution? the bicycle frame makes a rough and ready scale bar. Its geometry is very much fixed and could be used later to constrain the model.
Including a known object – a bicycle is just one example – in the scene allows us to include scale and add value later. This is true even if just Metashape Standard is available as the project files can be handed over later for additional processing in the Professional edition. This is something we teach and encourage in the Metashape Standard for Forensics online training course and will build upon in the forthcoming Metashape Professional for Surveying and Forensics course – watch this space for more news.
With the frame measure and points included the model scaled. Next time I’m passing I will take a few real world measurements of the pillbox, just to prove out the process.