Fitting encoders to a drafting machine

Has anyone made a drafting machine like this one
http://images1.americanlisted.com/nl...re_9028622.jpg
...into a digitizer by attaching encoders to the x and y axes?

I regularly need to measure the areas of drawn shapes. If I could click a button when the crosshairs cover the appropriate corner points to capture the x-y coordinates into a .csv file, it would simplify my work.

Better yet, the inputs would take the place of my mouse, like a big digitizing tablet.

2 linear encoder will make the job but 5µ accuracy is a bit too good so a rotary encoder attach with a drum and a wire and it's done !