2003-2005 South Carolina Oyster Mapping Orthoimagery

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Image Acquisition: Overview--The study area consists of 1,527 square miles along the coast of South Carolina and is organized into Digital Ortho Quarter Quads (DOQQ). In 2003, imaging was done at the DOQQ level, that is, each DOQQ was a designated Region of Interest (ROI). In 2004, imaging was done at the DOQQ-Q level. In both years, post processing of the images was done at the DOQQ-Q level. A fleet of ten of GeoVantage's GeoScannerSM multispectral aerial imaging sensors acquired 1,280 by 960 single frames in a direct digital manner. Each GeoScanner has an integral IMU and collects GPS and ground basestation data. GeoVantage's GeoPost application processed the DOQQ's single frames and created image mosaics orthoprojected to 7.5 minute Digital Elevation Models. In order to maximize the ability to detect intertidal oyster reefs the GeoScanner imagery was collected under the following conditions: Tidal stage - Imaging was timed to coincide with normal 0.0 or negative low tides (Mean Lower Low Water) + 1.5 hours. Season - The flight windows were also based on the period of high Spartina biomass (between May 15, 2003(2004) and October 15, 2003(2004)). Sun elevation - Missions were flown only when the sun angle was at least 45 degrees above the horizon to minimize shadowing on fringing reef. Occasionally acceptable imagery was acquired under high overcast conditions that provided even illumination. Winds - Imagery was only acquired when winds were calm or onshore less than 15 mph as determined at the local airport where flights were staged for that day. Offshore winds of any speed less than that which would interfere with aircraft operations were also acceptable. Clouds/Haze- No clouds or cloud shadows obscured the intertidal portion of the study area. Clouds and shadows did not cover more than 10% of the terrestrial portion of any mission. Rain - No imagery was collected while rain was falling in the mission area. Illumination -The imagery was acquired under even illumination conditions. This can be either through clear skies or under a high overcast. Of the two, clear conditions are preferable. Water Levels- No imagery was acquired if more than 12 inches of rain had fallen in the local area during the previous 72 hours

Image Rectification: The original mosaics were generated to the USGS DEM using either GeoVantage Base Station differential corrected data or multiple CORS stations to control airborne GPS and IMU data. In order to maximize spatial accuracy all data collected in 2003 and 2004 went through the following process: Subset Area Selection--Subset of 100 image frames were post-processed using best available CORS data (only stations within 150 miles of project area). Each set of 10 x 10 frames was selected to cover area with most identifiable features that could be visually compared. If possible this was the center of the DOQQ mosaic. Image Point Comparison-A point shapefile was generated from 10 identifiable features in the original mosaic. A corresponding shapefile was generated from the same 10 features in the 100-frame subset mosaic. Tabular Evaluation-A spreadsheet was populated with the original and the CORS-processed subset coordinates for each of the 10 points. This table showed both coordinate pairs and deltas for both x and y. This allowed a quantitative review of any offsets between the two mosaics to determine if a systematic error existed. Re-Nav/Reprocess Decision Process-If a systematic offset was observed in either x or y-axes, the original DOQQ mosaic and any mission mosaics was nudged by this distance. There was no threshold distance below which a spatial nudge would not occur. If the offsets were determined to be non-systematic, as judged by a CE 90 error greater than 3 meters after the offset was removed, then the entire original frames, mission mosaics, and DOQQ mosaic was reprocessed using the closest CORS stations. Reporting on this process-The table of 10 coordinate pairs and the nudge distances was included in the ReadMe file for the DOQQ and included on the delivery drive. This provided documentation of the process allowing the user to better understand the lineage of the data. After the original untrimmed mosaic was nudged, it was then trimmed for delivery and put through a visual quality review. This review was to identify and correct any anomalies. The "nudging" process was not applied to the individual image frames. The direction and distance of the nudge applied to the mosaic was documented, so that users desiring to work with individual image frames could apply it if needed.