Orthomosaic imagery for Whiskeytown Lake and surrounding area, northern California, 2019-11-12

Aerial imagery was collected by the 111th Group, Inc. on 2019-11-12 between 20:34 and 22:16 Universal Coordinated Time (UTC) (12:34 to 14:16 Pacific Standard Time (PST)), using a Hasselblad A6D-100c mounted in a nadir orientation. The imagery was acquired from an approximate altitude of 890 meters (2,920 feet) above ground level, to produce a nominal ground sample distance (pixel size) of 5 centimeters (2 inches). A total of 1,734 images were acquired in the survey area, with an approximate 70 percent along-line image overlap (forelap) and 50 percent side-to-side overlap (sidelap). An onboard dual-frequency GPS receiver was used to record the precise shutter time and position of each image.

The GPS data from the aircraft onboard dual-frequency GPS was post processed using Precise Point Positioning (PPP) in the Novatel GrafNav GNSS post-processing software package. Precise positions for camera shutter event marks were interpolated using the Grafnav Camera Event Mark tool and exported in NAD83(2011) spherical and projected coordinates.

Ground control was established by measuring the positions of approximately 68 photo-identifiable marks or objects using survey-grade post-processed-kinematic (PPK) GPS equipment. All measurements were referenced to a static base station operating on a temporary benchmark within the survey area. The position of the base station was established using the National Geodetic Survey Online Positioning User Service (OPUS).

Imagery from the raw Hasselblad 3FR format was converted to Hasselblad FFF format using the Hassleblad Phocus software package. The imagery exposure value (EV) was increased by +0.25 stops, and the chromatic aberration and vignetting corrections were applied under the Lens Correction tab. The distortion correction was not applied. The resulting adjusted images were exported using the JPG Max profile (JPG with Quality set to Maximum).

Structure from Motion (SfM) processing techniques were used to create the Digital Surface Model and Orthomosaic in the Agisoft Photoscan/Metashape software package using the following work flow: 1. The aerial imagery was loaded into Agisoft, and the camera positions were updated using the PPP post-processed onboard GPS data. 2. Initial image alignment was performed with the following parameters - Accuracy: 'high'; Pair selection: 'reference', 'generic'; Key point limit: 0 (unlimited); Tie point limit: 0 (unlimited). 3. Optimization and lens calibration parameters f, cx, cy, k1, k2, k3, p1, and p2 were included. 4. Ground control points (GCPs) previously measured with PPK GPS were manually identified and placed in the imagery. 5. Sparse point cloud error reduction was performed using an iterative gradual selection and camera optimization process with the following parameters: Reconstruction Uncertainty: 10; Projection Accuracy: 3; Reconstruction Error: 0.3. 6. A dense point cloud was created using the 'high' accuracy setting, with 'aggressive' depth filtering. 7. An initial Digital Surface Model (DSM) with a native resolution of 0.111 meters per pixel was created using points from the dense point cloud. An exterior boundary was digitized and used as a clipping mask to exclude obvious edge artifacts and large areas of interpolation. The DSM was exported to a GeoTIFF format with a 0.250-meter pixel resolution. 8. An RGB orthomosaic with a native resolution of 0.059 meters per pixel was created using the DSM as the orthorectification surface. The orthomosaic was exported to a GeoTIFF format at two resolutions: 0.060-meter pixels, and 0.250-meter pixels. 9. The RGB orthomosaic was converted to a cloud optimized GeoTIFF format for compatibility with cloud storage services using the GDAL software package. The orthomosaic was scaled to byte values and compressed using JPEG compression, with a quality of 90. The higher-resolution orthomosaic was divided into an east and west tile to reduce the file size, while the lower-resolution version was kept as one file.