EVALUATING THE TIME-EFFICIENCY OF USING STRUCTURE FROM MOTION METHODS TO GENERATE DEMS FOR USE IN STRIKE-SLIP FAULT OFFSET STUDIES

Light detection and Ranging (LiDAR) is valuable for geologic studies due to its ability to detect sub-meter scale geomorphic features remotely. However, the high cost of air-based systems and lengthy time plus amount of equipment required to operate ground-based terrestrial laser scanners (TLS) over large areas (e.g. hundreds of km²) can limit efficiency in time and money when utilized for certain fault offset studies. Structure from Motion (SfM) can generate DEMs using aerial photographs and is becoming increasingly explored due to its cheaper cost. The time-consuming tasks with SfM involve collecting aerial photographs and GPS ground control points (GCPs). To maximize time-efficiency of SfM, we address the following: What resolution is required for aerial photographs and how many GCPs are needed to achieve LiDAR accuracy? How does uncertainty of GPS GCP measurements impact the SfM DEMs and offset measurements made from them? These questions were tested by collecting TLS data and aerial photographs from a camera mounted on a Cessna flying ~1,000 ft above a 378 km² area of the Benton Springs fault, NV. While sacrificing resolution, collecting photographs from a Cessna required minimal time, as opposed to collecting closer without a Cessna. Four DEMs of the same area were generated with SfM utilizing: (1) 10 tripod based GPS GCPs referenced to stable North America (collected for 30 minutes each), (2) 5 tripod GPS GCPs, (3) 10 handheld GPS GCPs (10 minutes each), and (4) 5 handheld GPS GCPs. The DEM resolution achieved, 0.2 m per pixel, was comparable to the TLS DEM of the same area. Compared to handheld GPS, the two SfM DEMs that utilized tripod GPS had minor differences compared to the TLS DEM, suggesting in some cases time could be saved collecting fewer GCPs with high-accuracy GPS. The SfM DEMs that utilized handheld GPS had greater variability in elevation and geographic placement compared to the TLS DEM. Preliminary measurements of dextral offset magnitudes ranging over ~4 to 33 m on the five DEMs were similar, although the handheld GPS SfM DEMs yielded measurements most dissimilar to those made from the TLS DEM. This study suggests that for dextral offsets of <10 m, high-accuracy GPS becomes increasingly important when utilizing SfM and for even smaller offsets (approaching ~1-2 m) photographs should be taken closer to the ground.