The GPS antenna was mounted on a modified 2-meter measuring wheel because of its high mobility and to maintain a constant instrument height above the ground. The GPS receiver recorded data points every 1.5 meters along traverses of the sites. The traverses were initially spaced evenly across the sites. Traverses were then added where ground features, such as rapid slope changes, needed to be mapped. Approximately 8,600 data points were logged in 30 hours.

The data points were incorporated in a digital elevation model (DEM) using the ARC/INFO® GIS software. The software uses an interpolation method specifically designed for the creation of DEMs for comparatively small areas that have well defined elevation. The software assumes a surface drainage system model on the area. The software uses an iterative finite difference interpolation technique that has been modified to allow the fitted DEM to follow abrupt changes in terrain. This model of an assumed drainage condition produces more accurate surfaces with less input data. The program acts conservatively in removing closed low areas and will not impose the model of an assumed drainage in locations that would contradict the input elevation data.

The DEM grid-cell elevations were compared to known ground-level data across the sites for quality assurance and quality control. Sixty-nine grid cells were near control points. The mean difference between the grid cells and the control points was -2.4 centimeters and the interquartile range was -6.4 to +1.1 centimeters.

The DEMs were computed with a cell size of 0.6 meter. A smaller cell size did not illustrate closed depressions in the smaller pits at the B-site. The DEMs were used to produce topographic contour maps of the sites with a contour interval of 10 centimeters. These topographic maps will be used in contaminant-flow and sediment-erosion modeling at the research sites.