A national spatial data system framework: continuously operating GPS reference stations

The key to a successful Geographic Information system (GIS) is the ability to interrelate, combine, and compare spatially referenced data sets. This requires that all data sets be positioned relative to a common coordinate system. Positional accuracies required can vary widely from data set to data set, but for a GIS to be successful the required accuracies for all data sets must be achieved relative to a common coordinate system.