Scaling a point cloud using known distances on the ground

[timothy&j]

I am helping develop a classroom exercise for monitoring erosion losses.

The goal is to have students use VisualSFM to process photos they take in the field and monitor sites over time with CloudCompare. The main obstacle I have right now is that I want to convert a dense cloud from an arbitrary scale/datum by using known distances between static points on the ground so that I can get a real volume (say cubic meters) when I compare the clouds.

All of the research papers and forum questions I've been reading mention georeferencing clouds with GPS methods. I don't really care about the precise location of the erosional features on an established coordinate system, I just want to assign unit values. I hope I'm not missing something blindingly obvious but I'm banging my head on the wall here.

Perhaps I still don't have the CloudCompare/SFM vernacular down to search for the correct subject. Any help or direction to useful sources is appreciated.

Thank you

[daniel]

Well, if you want to scale a cloud, you need a reference. If it's not the GPS, then you'll need either another correctly scaled cloud (TLS, etc.) or something in the scene for which you now the scale precisely.

In the first case, you can use the alignment tool and release the scale so that it's automatically determined.

In the second case, you can only do manual measurements between two points with the 'Point picking' tool. Measure the distance between two points / features in your cloud, and deduce the scale thanks to the knowledge of the real distance between these two points. It's better if you do this on multiple couples of points to get a more robust estimation. Eventually you can apply this scale thanks to 'Edit > Multiply / Scale'.