Many parks depend on surveying for activities ranging from locating boundaries and cultural resources to monitoring geomorphological change. Assateague Island National Seashore and Cape Cod National Seashore recently completed GPS height modernizations to define precise locations and elevations of their primary survey control point markers to within a few centimeters. The height modernization required simultaneous GPS occupations of many survey markers. After field data collection the survey network is processed using specialized software that considers satellite geometry, atmospheric effects, and geometry of the receivers relative to each other. The resulting survey network has to be tied to a national spatial reference frame to produce final locations, including latitude, longitude, and elevation relative to a national datum.