Southwest Virginia GPS Control Survey

Project Summary

Timmons Group was hired as a subcontractor to Sanborn, Inc. to provide horizontal and vertical control for the 2007 flight in Southwest Virginia. The control consisted of aerial panels and photo-identifiable points to constrain and orient the stereo models of this project. The area covered by this portion of the project extended from Bedford County west to the state line and south to north from the bordering counties of North Carolina, Tennessee, Kentucky and West Virginia. Observations were started in February, 2007 and concluded in April, 2007.

A total of 310 stations were observed in this survey, consisting of 256 newly established points. 7 CORS (Continually Operating Reference Stations) and 26 NGS monuments were used for horizontal and vertical control. The survey was performed to meet the current Federal Geodetic Control Subcommittee (FGCS) accuracy standard for horizontal first order GPS surveys. Baselines were to meet the required relative horizontal positional accuracy of 10 mm + 10 ppm, at the 95 percent level of confidence. Also, the GPS network was to meet the current Federal Geodetic Control Subcommittee (FGCS) accuracy standard for vertical third order class one GPS surveys. All baselines were to meet the required relative vertical positional accuracy of 30 mm + 100 ppm, at the 95 percent level of confidence.

Five dual frequency geodetic receivers (4 Trimble 4800’s and 1 Trimble 4700) receivers were used in this survey. Fixed height tripods were used to eliminate antennae height measurement errors. Trimble Geomatics Office (TGO) software was used for mission planning, baseline processing and least squares adjustment of the network. A final report was provided to the client containing a written narrative, network plot diagrams, baseline processing reports, results for the minimally and fully constrained least squares adjustment, loop closure reports, session schedules, recovery sheets for each point (including field sketches, horizontal obstruction diagrams, digital photographs, coordinates, elevations and date(s)/time(s) of observation), and final coordinate list.