G&AI’s Aerial Survey and Mapping Division is the photogrammetric subconsultant to CivilCorp, LLC as part of a multi-disciplined, multi-year, indefinite delivery survey contract for TxDOT’s South Region. Two recent work authorizations required design-grade digital topographic mapping and digital orthophotography for separate areas in the Rio Grande Valley of south Texas.
- Approximate 10-mile corridor at the US 83/US 281 Interchange and required mapping from Second Street to FM 2557 along US 83 and from Polk Avenue to FM 3461 along US 281.
- Approximately 5050 acres west of McAllen for the US 83 Relief Route. The area was both along and north of the existing US 83 right-of-way where a new highway is being planned.
Work for both projects included the acquisition of low-altitude color aerial photography (1”=250’ photo scale, 1500’ above ground), analytical aerial triangulation from client-furnished horizontal and vertical ground control (196 points), digital topographic mapping (1”=50’ scale standard with a 1-foot contour interval) delivered in MicroStation format, and high-resolution color digital orthophotos (1”=50’ scale standard with a 3-inch pixel resolution). The planimetric data was delivered as 2D MicroStation design files, and DTM data was delivered as 3D MicroStation design files. GEOPAK TIN files were also provided. The aerial photography for both projects was acquired with a Leica RC30 precision aerial mapping camera and consisted of approximately 320 overlapping exposures. The imagery was then scanned with a Leica DSW500 precision photogrammetric scanner, and the map files and ortho-rectified imagery were produced using both DAT/EM Summit Evolution and Cardinal Systems photogrammetric software. All map data was prepared in strict accordance with TxDOT mapping standards as well as accepted photogrammetric procedures as published by the American Society for Photogrammetry and Remote Sensing (ASPRS) for Class 1 accuracy standards. These standards require that spot elevations and DTM points have an RMSE no greater than seventeen one-hundredths of a foot.