Ground control point locations for UAS survey of the Liberty Island Conservation Bank Wildlands restoration site, Sacramento-San Joaquin Delta, California, 2018-10-23

GCP positions were measured using survey-grade dual-frequency GPS receivers operating in real-time kinematic (RTK) mode. The GPS receivers were placed on short fixed-height tripods and set to occupy each GCP for a minimum occupation time of one-minute. The RTK corrections were referenced to a static GPS base station operating on a benchmark approximately 3 kilometers south of the survey area. The position of the benchmark was previously established using three static GPS occupations (2017-06-26 to 2017-06-28) with durations between 4 and 8 hours, processed using the National Geodetic Survey (NGS) Online Positioning User Service (OPUS). The processed positions from each static occupation were averaged to derive the final position of the benchmark, resulting in an estimated elevation uncertainty of 1 cm. Manufacturer reported accuracy for the differentially corrected vertical positions for the GPS rover positions is 1.5 cm + 1 ppm. The maximum baseline from the GPS base stations was approximately 3 km, suggesting the minimum vertical accuracy to be about 3 cm. Uncertainty in the vertical positions associated with the weight of the tripod settling onto the GCP tarp during data collection, and possible subsequent vertical rebound of the center of the tarp after removal is unknown.