Postby mrdic » Sat Nov 21, 2020 9:41 pm
Photogrammetry is better for establishing proportion than absolute measurement if the camera isn't fixed, so it helps to place reference objects that you can derive edges and sizes from in the scenes that you're recording. That's especially true for organic objects like the tree stump that you show. A cardboard box in that view would give a size reference, and its perspective in the scene gives the focal distance of the camera lens. Some commercial CAD apps have tools to find "analytic" surfaces for man-made/non-organic objects, these are planes and conical/cylindrical/ellipsoidal surfaces. They usually require the user to identify the mesh facets that compose the analytic surface, and the program refines the parameters of the analytic surface to best fit the facets/points, where the surface is parametrised to be within as close an average proximity to the scan's points/vertices as can be found.
FreeCAD