A TOPOGRAPHIC MODEL OF UPPER TERLINGUA CREEK IN WESTERN TEXAS, USA DEVELOPED USING PHOTOGRAMMETRIC TECHNIQUES

The Rio Grande in western Texas is an environmentally stressed river. It suffers from poor water quality, upstream diversions, the encroachment of invasive species and an overabundance of sediment. Much of the sediment comes from local tributaries that drain the arid Chihuahuan desert landscape. In order to monitor the influx of sediment from one of the local tributaries, we have established a topographic monitoring site in the upper portion of Terlingua creek. The Terlingua creek watershed is 2,851 km² and enters the Rio Grande near Santa Elena canyon in Big Bend National Park (BBNP). The general condition of the watershed is degraded due to land use practices. Recent restoration projects have been initiated on privately held land in the upper portion of the watershed and in BBNP in the lower portion of the watershed. As part of this study, a photogrammetry survey technique was used to create a Digital Elevation Model (DEM) and an orthophoto of the Terlingua creek study area. We used a helium filled balloon and a basic digital camera to take overlapping photographs (~70% overlap) of the creek channel in the study area. The balloon was maintained at an approximate height of 50 m above the ground surface. Multiple visible targets were distributed over the study area and RTK GPS was used to obtain georeferenced coordinates for these Ground Control Points (GCPs). The resulting images were processed using Agisoft Photoscan, which created point clouds, DEMs and orthophotos. The resultant DEM has a ground cell resolution of 0.03 m and the orthophoto has a resolution of 0.015 m. Results include GCP errors ranging from -0.8 to +0.8 m and obviously artificial artifacts in the main channel due to inadequate density and distribution of GCPs. Additional problems include the presence of vegetation that obscured the desired ground surface and made traversing the site difficult. Future work will include a second focused survey with a higher density of GCPs in the main channel for topographic change detection analysis.