Should the device be automatic or manually operated?

There are digitizers where you have a "pen" mounted on an arm like a desk lamp and a number of sensors that measures the angles of all joints.

So someone who wants to digitize something holds the "pen" and then points it at different positions on the surface of the object to be digitized.

If the "pen" has a piezo or other force sensor at the tip, it could send a trig signal back when the configured pressure have been reached - a computer could then pick up the angles and compute the actual location of the "pen tip".

Another variant don't have an arm - but using a magnetic polhemus device to track the location.

Personally, I would probably consider a pen with optical tracking. IR-diodes on the pen and 3 video cameras to track the pen location. A cable to the pen to power the LEDs and pick up the pressure readings from the tip. Lots of rather cheap cameras have good resolution and if they don't have too strong internal IR-blocking filter, they can be adapted quite well with an external IR-accepting filter that makes the IR diode light stand out greatly in the image.