Is there any free software which will write to a file the x,y values of a selected position on an image?

I know this can be done using professional GIS software, and I know that there are a few freeware GIS systems, but
a. I don't know if any of the free ones include digitization
b. GIS systems and their installation procedures tend to be rather complicated and would seem to be overkill for this specific problem.

I was a little surprised that, although the forum has lists of software for creating maps, there don't seem to be any lists of software for analyzing maps. Have I overlooked them?

My specific problem is that I have a drawn 2D starmap that I want to convert into a 3D database. X and Y positions can be determined by measuring the stars' positions on the map. (I'll have to come up with reasonable Z distances, constrained by trade routes and the maximum ftl jump distances the traders can manage, but that's a separate issue. )

There are only about 50 or so stars involved, so one option would be to use a drawing program like to determine XY coordinates, typing in the values by hand from the on-screen display. I've probably already spent more time searching for a solution than all the typing would have taken, but it'd be nice to have a general solution for the future.

Thanks for whatever help anyone can provide!