Many parks depend on surveying to do a range of things from locating boundaries and cultural resources to monitoring geomorphological change. A main component of such surveying is having precise starting points or “survey control points.” It is difficult to know where you are if you do not know where you started from.
Assateague Island National Seashore and Cape Cod National Seashore now know where they are as they recently completed GPS height modernizations to define precise locations within a few centimeters of their primary survey control point markers.
In a GPS height modernization the purpose is to connect survey markers through simultaneous GPS occupations. At Cape Cod, ten survey markers were simultaneously occupied at three different times for four hours each time. Logistics proved to be a challenge, as various GPS receivers were rotated through the process to ensure that each receiver was not on the same survey control marker twice.
Assateague has more survey control markers and could not follow the same protocol but used the same concept by having multiple receivers operating at the same time on different days.
GPS receivers used for these surveys are not typical handhelds such as those used for geocaching or vehicle navigation. Survey GPS receivers use multiple signals and frequencies and require a process known as differential correction to provide centimeter-level accuracy.
GPS survey field work planning and execution requires a lot of work and, afterword the survey network needs to be processed. Initial processing for the height modernizations at both parks involved consideration of satellite geometry, atmospheric effects, and geometry of the receivers relative to each other. Though most of these considerations were controlled by the survey software, the survey network had to be tied into a national spatial reference frame. This included occupation of known survey control markers, or the brass disks often seen while hiking or doing other outdoor activities.
In addition to these survey control markers. which are referred to as “passive control,” NOAA’s National Geodetic Survey operate GPS Continuously Operating Reference Stations (CORS) which are referred to as “active control”. The CORS data are available to the public via the Internet. By using local survey control markers and CORS, the two parks were able to produce final locations, including latitude, longitude, and elevation relative to a national datum. We use the North American Datum of 1983 recently adjusted to account for tectonic plate motion (NAD83 2011 (2010.0). For more information, click on the link below.
Consider the complex variables associated with calculating locations using GPS:
GPS satellites are moving in orbit
Earth is rotating
The continental crust is shifting
Subsidence or uplift due to aquifer removal or post-glacial rebound may be changing elevation
Yet we are able to provide locations of survey control markers at the centimeter level. The National Park Service is truly on the cutting edge.
Special thanks go out to the Northeast Coast and Barrier Network for travel funding and personnel support and to the National Geodetic Survey for technical support.