Pit #3 Hydroelectric Dam Photogrammetry & LiDAR Survey — Pacific Gas & Electric

Tetra Tech performed a LiDAR and photogrammetry study of the Pit Dam (8,426 acres) in northern California (N. CA) for Pacific Gas & Electric Geosciences. Aerial photography included 22 exposures @ 1:12000 to accommodate planimetric mapping at 1”=100’ and high resolution orthophotography at 6” pixel resolution. An additional stereo pair at 1:7200 of the Pit #3 Dam was designed to provide detailed mapping of the Dam and the immediate vicinity. The two exposures were oriented to provide the best perspective on the face of the dam to support as-built mapping. LiDAR data was collected at 8 points per square meter over terrestrial surfaces. All survey areas were surveyed with an opposing flight line side-lap of =50% (=100% overlap) to reduce laser shadowing and increase surface laser painting. Simultaneous with the airborne data collection mission, we conducted multiple static (1 Hz recording frequency) ground surveys over monuments with known coordinates.

Post processing of the raw laser points included tiling into manageable blocks (less than 500 MB) to perform manual relative accuracy calibration and filter for pits/birds. Ground points were then classified for individual flight lines (to be used for relative accuracy testing and calibration). Using ground classified points per each flight line, the relative accuracy was tested and the position and attitude data were imported. Resulting data were classified as ground and non-ground points. Statistical absolute accuracy was assessed via direct comparisons of ground classified points to ground RTK survey data. Data were then converted to orthometric elevations (NAVD88) by applying a Geoid03 correction. Ground models were created as a triangulated surface and exported as ArcInfo ASCII grids at a 1-meter pixel resolution. Deliveries included DTM data, planimetric mapping, orthophotography, contours at 2’ intervals, highest hit classified LiDAR points, as well as a full report containing introduction, methodology, and accuracy. Images courtesy of Wastershed Sciences Inc.


Peter Ashley


(925) 280-7441