I am trying to set up a scale of a known distance on a photgraph that I have put in autocad. I know that this distance is 100mm, and have tried to work out how to mark this known distance, and then be able to work out the measurements of other dimensions in the photograph, including non-linear measurements. Is this possible?
Using the Scale command, and the reference option (watch the command line) you can easily to this.
Scale, select, pick any point as a base point, R for reference, pick the two points you know are 100mm, now type in 100.
From there you can just use the DIST, MEASUREGEOM, or DIMENSION commands to pull other lengths.
Draw a line at a known direction and length that corresponds to something in your image. Then insert the image in the drawing. Now ALIGN the image to the line and say YES to scale objects. It will now be scaled and oriented properly.
The ALIGN command is great anytime you bring something like an image or a scanned drawing in as a reference.