Fort Totten Ground-water Model

To facilitate development of a ground-water flow model for the Fort Totten Indian Reservation, accurate determination of the locations and the elevations of several data-collection sites was required. During the fall of 1993, GPS technology was used to obtain the coordinates and elevations of 187 data collection sites (wells and wetlands) on the reservation. Wells used to provide input data for the ground-water model were constructed during several different studies of area aquifers. Initial evaluation indicated that the historical well coordinates generally were incongruent as a result of the wells being constructed and their coordinates established by different people at various times.

The "RAPID STATIC" GPS mode was used for this survey. Using this method required placing a GPS receiver on a monument whose location had been thoroughly determined and documented (U.S. National Geodetic Survey first order monument) which was designated a "reference monument". This receiver continuously collected data at this location during data collection sessions. A "roving" receiver was then moved from data collection site to data collection site and GPS data was recorded for several minutes at each site during the sessions. A maximum distance of 6 miles between the receiver at the reference monument and the roving receiver at the various data-collection sites resulted in the short baselines (distance between reference and roving receivers) necessary to obtain high accuracy three-dimensional positioning.

The suspect inconsistencies in the historical coordinates were confirmed by comparison to the GPS-generated coordinates. Because all data sites were positioned in reference to high-accuracy monuments, a significantly more reliable ground-water model was constructed. The cost of conducting a conventional survey to position the data sites would have been prohibitive in this application and poor positioning would have resulted in a suspect interpretation of the ground-water model. The GPS data was readily integrated into the geographic information system (ArcInfo software) that was used to prepare data for input to the ground-water flow model and process the model's output data.