Tetra Tech performed a Light Detection and Ranging (LiDAR) and photogrammetry study of a 12-mile region along the central California coastline. The project was completed for the Pacific Gas & Electric (PG&E) Geosciences group in support of their analysis of seismic activity near the Diablo Canyon Power Plant.

PG&E needed a digital terrain model representing as much of the coastline as possible to support analysis of seismic activity near the Diablo Canyon Power Plant. This high-priority deliverable required data acquisition to take place at maximum low tide. Two flights were successfully completed as scheduled despite the narrow window presented by storms and sea swells.

The aerial photography included six flight lines and 44 color exposures at 1:12000 and was captured slightly before the LiDAR, balancing the requirements of low tide and optimal sun angles. The LiDAR data was collected using the Leica ALS60 Sensor pulsing at 200k pulses per second at a density of 8 points per square meter.

Tetra Tech, at our request for the need to get as much LiDAR data offshore, completed acquisition in winter when only three days for early morning flights would achieve the needed results. In part, luck that the weather and surf allowed and, to a greater part, the readiness to have equipment on site allowed Tetra Tech to successfully complete this difficult job. The combination of LiDAR and air photos provided excellent images for geologic mapping along the coast and allowed confident projection of the onshore strata to the offshore interpretation of MBES data across the surf zone.

Dr. William D. Page, PG&E Project Geologist

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 introduced topographic breaklines into the LiDAR data to ensure that the contours were of acceptable cartographic quality. Our method maximized the utility of the imagery to inform the compiler’s placement of breaklines while relying on the LiDAR elevations for the vertices of the breaklines that were drawn. The resulting contours had the smooth appearance of traditional photogrammetric work but took full advantage of the vertical accuracy of the LiDAR.

Deliverables included digital elevation models supplied in ArcGrid and AutoCAD formats, planimetric mapping at 1:2000 scale in ESRI and AutoCAD format, topographic mapping at half-meter contour intervals, and color orthophotography at 15cm-pixel resolution.

Tetra Tech’s long-term relationship with PG&E and history of successfully executing projects similar in scope and size allowed our team to meet the client’s needs and unique project challenges. Our team has completed 10 other PG&E projects throughout northern California that involved more than 300 miles of transmission lines. Those mapping efforts have supported a wide variety of PG&E projects, including engineering design for reconductor work, seismic and subsidence analysis, and environmental monitoring of multiple rivers.