Horizontal positions of individual points for the point cloud were calculated by photogrammetric software and ground control points. There are several sources of potential error that affect the horizontal accuracy:
1. The horizontal error estimate provided by Agisoft Photoscan for the Charles Point project as a result of alignment, optimization, and ground control processing procedures is 10.85 cm. This value is an RMS estimate of positional error at discrete locations within the orthomosaic.
2. Ground control points were collected using a Spectra model SP80 GNSS receiver set to record XY locations with the national adjustment of 2011 (NAD83(2011) UTM zone 18N EPSG::6347), which differs from the Photoscan project coordinate system (NAD83/UTM zone 18N (EPSG::26918)). This difference may introduce an average additional 0.9 cm horizontally at the 95% confidence level according to the National Geodetic Survey (https://www.ngs.noaa.gov/web/surveys/NA2011/)
3. Additionally, an assessment of repeated survey sites including transect points and ground control points, using the same equipment by the USGS Aerial Imagery and Mapping (AIM) group estimates an accuracy of the ground control point locations of 1.76 cm (horizontal) and 0.54 cm (vertical).
The combination of the potential horizontal error is on the order of 13.5 cm for the Charles Point products. Although some portions of the map may contain much larger errors, greater than 2 m horizontally, especially along the boundary of the project that are far from a ground control point, and across water bodies. The horizontal coordinate system for the Charles Point Point Cloud, DEM and Orthomosaic is NAD83/UTM zone 18N (EPSG::26918).
It is also important to note: The UAS image locations are derived from a mRo GPS (u-Blox Neo-M8N / 3DR SOLO Upgrade), located on the UAS, which receives signals from GPS and GLONASS satellites in WGS84 (G1150) EPSG::7660. This location information is used by the Photoscan software to help with the initial alignment, however after photo alignment, these location data have no bearing on the project or the derivative product. Therefore the positional accuracy and potential errors of the GPS data attached to the input photographs is not propagated to the Photoscan project and therefore does not contribute to the overall horizontal accuracy of the products (Point Cloud, DEM and Orthomosaic).