- 146 km2 surveyed using 153 ground reference points
- This WorldView-3 satellite elevation survey is accurate to 13 cm RMSE, determined by comparison to 6,854 elevation check points from a highly accurate LiDAR survey
- November 2014 WorldView-3 stereo satellite photos processed by PhotoSat in August 2016
Gerry Mitchell, P. Geo, Geophysicist, President PhotoSat
Summary report below:
A 1m grid of elevation values, covering an area of 150 square kilometres, was produced over a PhotoSat test area in California. The elevation grid was made using geophysical processing of 50cm ground resolution stereo satellite photos taken by the DigitalGlobe WorldView-3 satellite. The stereo satellite elevation processing was referenced to 153 ground survey points. The elevation surveying accuracy was determined by direct comparison to 6,854 elevation check points extracted from a highly accurate LiDAR survey.
WorldView-3 color image with 50cm contours from the PhotoSat WorldView-3 survey of the California test area.
The Garlock Fault was surveyed with a LiDAR in April 2008 by OpenTopography. www.opentopography.org.
An elevation image showing the portion of the Opentopography.org LiDAR survey used in this accuracy study. The dimensions of the area are 12.2 km east–west by 12 km north–south. Lower elevations are blue and higher elevations are red. The oval is the Honda California Test Track.
Stereo satellite photos:
PhotoSat satellite surveying uses high quality stereo satellite photos. These photos are taken by the satellite as it passes over the survey area along a north to south satellite orbit.
The satellite photographs the same ground area within a minute or two. The ground conditions are close to identical on the two photos. The difference in appearance of ground features on the photos is due to the different look directions of the satellite camera.
PhotoSat Geophysical Stereo Satellite Processing System:
Survey coordinates of ground features are determined by measuring the apparent shift in location of the features between the two satellite photos. PhotoSat uses a proprietary geophysical processing system to generate survey coordinates from stereo satellite photos. This system is described in a PhotoSat white paper published at a 2010 ASPRS conference.
Stereo WorldView-3 elevation image covering the area of the LiDAR survey. This figure shows the 153 ground reference points used to control the stereo WorldView –3 processing. These points were derived from the LiDAR survey. This elevation grid has an elevation point every meter. Low elevations are blue and high elevations are red.
6,854 elevation checkpoints for areas with slopes of less than 20% grade.
California elevation grid:
PhotoSat produced a 1m grid of elevations over the entire project area. We processed the stereo WorldView photos with our geophysical stereo satellite processing system in August 2016.
Ground reference points:
When we use a single ground reference point to control this WorldView-3 stereo pair the elevation surveying accuracy is 13cm RMSE. With this accuracy study we were testing to see if the accuracy of the elevation survey using this WorldView-3 stereo pair would be improved by using a large number of ground survey points.
Global shift of stereo satellite survey to match ground reference:
The WorldView ortho photo and elevation grid needed a constant shift of only 1.3m E, 0.90m N and -0.94m in elevation to match the ground surveying. The global accuracy of most WorldView stereo satellite photos is better than 3 m.
Accuracy evaluation check points:
The accuracy of the PhotoSat 1m survey grid was evaluated with 6,854 elevation checkpoints derived from the LiDAR elevation grid.
Elevation survey accuracy statistics:
The Guidelines for Digital Elevation Data of the US National Digital Elevation Program (NDEP) recommends that elevation checkpoints should be chosen in areas with slopes less than 20% grade. The 6,854 elevation checkpoints with slopes less than 20% grade have an RMSE of 13cm and LE90 of 21cm.
Histogram of the elevation differences between the WorldView-2 stereo satellite elevations for the 14.4 km by 12 km area and the 6,854 elevation checkpoints with slopes less than 20% grade. The Guidelines for Digital Elevation Data of the US National Digital Elevation Program (NDEP) recommends that elevation checkpoints should be chosen in areas with slopes less than 20% grade. RMSE 13 cm, LE90 21 cm.
No accuracy difference between using one ground reference point and using 153:
There was no significant improvement in the accuracy of this WorldView-3 elevation survey using 153 ground reference points. For this specific WorldView-3 stereo pair both a single ground reference point and 153 ground reference points the elevation surveying accuracy is 13cm RMSE.