Diablo Canyon Power Plant LiDAR Survey & Photogrammetry — Pacific Gas & Electric
The Tetra Tech Geomatic Technologies Group performed a Light Detection and Ranging (LiDAR) and photogrammetry study of a 12 mile region on the Central California Coastline. The project was done for the PG&E Geosciences group in support of their analysis of seismic activity near the Diablo Canyon Power Plant. A digital terrain model representing as much of the coastline was a high priority deliverable, requiring acquisition to take place at maximum low tide (-1.6) which occurred at 3:40 p.m. on January 29th. The aerial photography included 6 Flight Lines and 44 Color exposures @ 1:12000 and was captured slightly before the LiDAR, balancing the requirements of low tide and optimal sun angles. Meanwhile, the LiDAR was collected using the Leica ALS60 Sensor pulsing at 200k pulses per second at a density of 8 pts/ sq. m. Both flights were flown successfully as scheduled, despite the narrow window presented by storms and sea swells. Watershed Sciences was our LiDAR acquisition partner on this project.
The terrain of the rocky coastline presented an additional challenge for compilation technicians who integrated LiDAR data points with photogrammetric stereo compilation of rock outcroppings and cliffs for the generation of contours on a half-meter interval. Tetra Tech’s methodology is to introduce topographic breaklines into the LiDAR data to ensure that the contours are of acceptable cartographic quality. Our method uses imagery (when available) to inform the compiler’s placement of breaklines but it relies on the LiDAR elevations for the vertices of the breaklines that are drawn. The resulting contours have the smooth appearance of traditional photogrammetric work but take full advantage of the vertical accuracy of the LiDAR.
The LiDAR data was delivered in three forms. After the data classification, QC review and edit, the Bare Earth data set was used to interpolate ArcGIS GRID files for all the delivery tiles. Because these grids were generated on a 1-meter interval they represented a significant thinning of the raw laser point data that we felt did a disservice to the detail recovered in the inter-tidal zone, the focus of the study. Accordingly we also generated a quarter-meter grid concentrating on the cliffs and reef outcrops. Due to the increased resolution these grids were quite large, but being ArcGIS GRID files we thought that they would be easier to view than simple ASCII listings of the ground classified points in the inter-tidal zone. In addition to a DEM supplied in both ArcGrid and AutoCAD formats, deliverables also included planimetric mapping at 1:2000 scale in both ESRI .shp files and AutoCAD .dwg format, topographic mapping at half-meter contour intervals, color orthophotography at 15cm pixel resolution and a technical report.