Most of what we do as artists is copy. We sketch what we see, we use reference to create new works, and sometimes we have to copy- straight up. And the key to copying is registration. So how can you build a 3D model or draw an accurate picture of a complex 3D object? How do you get accurate surface measurements?
You need to model a complex curvy object, like a smooth stone.
The first thing I do in a situation like this is look for a way to find registration. The best thing you could have is a 90 degree angle with two straight edges. If I had the cross section of the front and the outline of the top then I could easily draw a box around my outlines and measure anywhere I wanted. So I will employ two low-tech tactics to get my outlines.
For the cross section I’ll bury it halfway in some stiff modeling clay. When I remove the stone I’m left with a cross section, but it isn’t done yet.
I now take the stone and the clay over to the copy machine and print it out 1:1 scale. Now it’s easy to draw my boxes around the outlines.
You can use many other low tech methods to measure too, like using string to measure a curvy edge, or building a box around an object.
If you need to copy something, do anything you can to give yourself a box to measure from. Sketch artists draw a quick grid to position the eyes, nose, and mouth on a character’s face. Fabricators use the 0,0 position on a table to cut materials on a CNC or laser engraver. Accuracy is as simple as good registration and good measurements.