Ballscrew Mapping is a software feature of CNC control software (i.e. Mach 3) that maps the errors of a ball screw axis and automatically compensates form them. Any screw wears unevenly due to the fact that certain parts of the table (i.e., the middle) are used far more than other parts of the table (the extreme ends). Couple this with imperfections in manufacturing and there can be noticable errors in the screw.
Ballscrew mapping can be a very effective way to increase accuracy in the machine, but it requires extremely accurate methods to measure the true position of the axis along the length of the screw. This is typically done using gage blocks and a dial indicator or even better by using linear scales and a DRO. The most precise method of calibrating a ballscrew map is laser interferometry. Laser interferometry offers the ultimate in high accuracy, repeatable and traceable measurement, using externally mounted optical components. It is prohibitively expensive but provides insane accuracy.