

control points - Maps in a \GIS\ must be registered to some
coordinate system.  If GIS analysis and display functions are 
to be performed on multiple maps, these maps must be registered
to the same coordinate system.  On each map, the user registers
the locations of between 4-10 points ("control points") having 
known geographic coordinates, and inputs this information to
GRASS.  Given these control points, and a coordinate system 
(e.g., \UTM\), GRASS calculates the positions of all other points
on the map.
