Elevation Data Collected in 2010 from Sabine National Wildlife Refuge, Louisiana

Position and elevation data were recorded with an Ashtech DGPS receiver and antenna. GPS base stations were erected at temporary USGS benchmarks located within 13 km of the farthest data-collection site. An Ashtech Z-Xtreme GPS receiver recorded the 12-channel full-carrier-phase positioning signals (L1/L2) from satellites via a Thales choke-ring antenna at the base station. A similar GPS instrument combination was used for the rover GPS. The base receiver and the rover receiver recorded their positions concurrently at 1-second (s) recording intervals throughout the survey. Because the SNWR open-water bodies are very shallow (generally less than 50 cm) and commonly have fluid muddy or peaty bottom sediments, an airboat was required to access the study areas, and a combination of stop-and-go (semi-kinematic) and fully kinematic differential GPS (DGPS) surveying techniques were used. At pre-selected sites along the transects, the airboat was brought to a complete stop, spudded onsite, and the rover antenna was lowered to the sediment surface. At these "stop" sites, data recorded in static mode for 2 minutes. Between stop sites, the antenna was pinned in the up position while data continued to record in kinematic mode and the real-time position data from the rover GPS receiver was streamed to a laptop computer running HYPACK 2010 hydrographic survey software. At approximately equal distances between stop sites, the distance from the antenna foot to the sediment surface was measured with a graduated rod and the time and approximate (raw) position of each measurement were marked simultaneously in the HYPACK data file. After post processing, the vertical position of each kinematic epoch with a corresponding HYPACK timestamp was corrected to account for the measured distance between the antenna foot and the sediment surface. As a quality-control check, rod measurements were also made at each of the stop sites before the rover antenna was lowered to the sediment surface, and the corrected kinematic elevation was compared with the static elevation from the same site.

The coordinate values of the GPS base stations are the time-weighted average of values obtained from OPUS. The base station coordinates were imported into GrafNav, version 8.30 (Waypoint Product Group) and the data from the rover GPS were post-processed to the concurrent base-station session data; baseline distances were less than 13 km. The DGPS data were acquired and processed in the North American Datum of 1983 (NAD83) (CORS96) geodetic datum. Processed ellipsoid elevations were transformed in GrafNav to North American Vertical Datum of 1988 (NAVD88) orthometric elevations using the National Geodetic Survey geoid model of 2009 (GEOID09).

The processed stop-and-go and kinematic point data were imported into Esri ArcGIS version 9.3, the kinematic elevations were corrected to account for the measured distance between the antenna foot and the sediment surface, and the differences between the processed static and kinematic elevations at all stop sites were compared. The average kinematic-minus-static difference for all stop sites (N=271) was 0.5 plus-or-minus 8.3 cm (2-sigma), which is comparable to the maximum error (plus-or-minus 10 cm) at line crossings for single-beam acoustical bathymetric surveys in the delta plain (Morton and others, 2010). The stop-and-go and kinematic datasets were then merged; the final point data are the stop-and-go static positions where applicable or the corrected kinematic elevations.

Added keywords section with USGS persistent identifier as theme keyword.