PhotoSat SPOT-7 stereo satellite mapping accuracy study
Asmara, Eritrea, 1 GCP, 1.4m RMSE

  • 1,458km2 mapped using only one ground reference survey point
  • This SPOT-7 satellite elevation mapping accuracy is accurate to 1.4m RMSE, determined by 7,950 survey checkpoints
  • April 2016 SPOT-7 stereo satellite photos processed by PhotoSat in August, 2016

Summary report below:

Gerry Mitchell, P. Geo, Geophysicist, President PhotoSat

September, 2016

A 3m grid of elevation values, covering an area of 1,458 square kilometres, was produced over the PhotoSat test area in Eritrea. The elevation grid was made using geophysical processing of 1.5m ground resolution stereo satellite photos taken by the Airbus SPOT-7 satellite. The stereo satellite elevation processing was referenced to one ground map point. The elevation mapping accuracy was measured with over 7,000 survey checkpoints. 

SPOT-7 colour image with 2.0m contours from the PhotoSat SPOT-7 map of the Eritrea test area.

Stereo satellite photos:

PhotoSat satellite mapping uses high quality stereo satellite photos. These photos are taken by the satellite as it passes over the mapping area along a north to south satellite orbit. The satellite photographs the same ground area within a minute or two, so the ground conditions are close to identical on each photo. 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:

Map 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 map coordinates from stereo satellite photos. This system is described in a PhotoSat white paperpublished at a 2010 ASPRS conference.

Eritrea satellite photos:

The pair of stereo SPOT-7 satellite photos over the Eritrea test area are shown in Figure 2. The stereo satellite photo look directions, convergence angle, bisector azimuth and bisector angle from vertical are shown in the figure caption. The convergence angle of 30 deg is optimum for mapping elevations in level to moderate terrain.

Eritrea elevation grid:

PhotoSat produced a 3m grid of elevations over the entire project area. We processed the stereo SPOT-7 photos with our geophysical stereo satellite processing system in August, 2016.

Ground reference points:

With this accuracy study we are demonstrating that we can produce highly accurate SPOT-7 elevation mapping with only one ground reference point. Having as little as a single ground reference point for a 1,458km2 satellite mapping project is not uncommon in mining exploration projects.

Accuracy evaluation checkpoints:

The accuracy of the PhotoSat 3m elevation grid was evaluated with 9,828 ground survey checkpoints. These points were originally surveyed for a large mining exploration gravity survey conducted between 2004 and 2008 by MWH Geophysics.

The elevation checkpoints were surveyed to an accuracy of 2cm using Real Time Kinematic GPS map equipment.  The accuracy checkpoints consist of a 250m by 250m regional grid of points.

SPOT-7 1.5m stereo satellite photo. Asmara, Eritrea

Stereo SPOT-7 elevation image created from the 3m PhotoSat

Asmara, Eritrea satellite elevation grid.

Stereo SPOT-7 elevation image showing the location of the ground control point and the 9,828 elevation checkpoints.

Elevation mapping 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 7,950 elevation checkpoints with slopes less than 20% grade have an RMSE of 1.4m.