Electronic – How to measure small scale movement with an electronic component

I'm working on a project which involves measuring the distance between two pins which are nominally 3-5 cm apart. These pins can move up to 1cm in either direction. I need to take a measurement of the distance between the pins which is accurate to about 500 μm.

Currently we take a measurement once a day using calipers and record the distance manually, but I need to automate the process.

Update (2012-02-21)

Okay, I figured the implementation details were unimportant, but as a lot of people are asking for more detail, here goes. We want to measure the opening and closing of a crack in a brick wall over the course of a year or more. We currently drill holes and insert two metal pins into the wall either side of the crack and take measurements between the two pins. So long as we can measure the opening and closing of the crack over time, then the implementation is unimportant.