been doing this for a long time, even easier if you put a reference tape measure in the photo and you can scale photo to the size of tape measure. almost every time I have carved a rock, since they are irregular in shape there is not a part to get the scale correct, add in the tape measure and I can scale photo to size and do my design work on the rock at scale.
For a flat piece like that, can't you just hold it on a flatbed scanner and scan it? After that you should be able to use ImageJ or Illustrator to trace it and you are done. What I wonder about are objects that have no flat surfaces. Do the 3D scanners do a good job with those (for relatively low tolerances)?
