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GRASS Help System |
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.