WorldView-2, Eritrea, 21 GCP, 14cm RMSE, July 2016

PhotoSat WorldView-2 stereo satellite surveying accuracy study, Asmara, Eritrea RMSE 14cm

  •  100 km2 surveyed using twenty one ground reference survey points.
  •  This WorldView-2 satellite elevation surveying accuracy is accurate to 14cm RMSE, determined by 775 survey checkpoints.
  • January 2010 WorldView-2 stereo satellite photos processed by PhotoSat in June 2016.

Gerry Mitchell, P. Geo, Geophysicist, President PhotoSat

July 2016

Summary report

A 1m grid of elevation values, covering an area of 100 square kilometers, was produced over the PhotoSat test area in Eritrea. The elevation grid was made using geophysical processing of 50cm ground resolution stereo satellite photos taken by the DigitalGlobe WorldView-2 satellite. The stereo satellite elevation processing was referenced to twenty-one ground survey points. The elevation surveying accuracy was measured with 775 survey checkpoints.

Figure 1. WorldView-2 color image with 50cm contours from the stereo WorldView-2 elevation mapping for the Asmara Eritrea test area.

Eritrea satellite photos:

The pair of stereo WorldView-2 satellite photos over the Eritrea test area were taken on January 27, 2010, at approximately 10:30 AM local time.

Eritrea stereo WorldView elevation grid:

PhotoSat produced a 1m grid of elevations over the entire 100km2 project area. We processed the stereo WorldView photos with our geophysical stereo satellite processing system in June 2016. An image of the 1m elevation grid is shown below.

Ground reference points:

With this accuracy study, we are demonstrating that we can produce highly accurate WorldView-2 surveys with a reasonable distribution of ground reference survey points.

On operating mine sites and oil and gas project sites, there are usually hundreds of existing ground survey points.  PhotoSat uses these existing ground survey points to identify and attenuate the distortions in the WorldView satellite photos to achieve elevation surveying accuracies of better than 15cm.

Accuracy evaluation check points:

The accuracy of the PhotoSat 1m survey grid was evaluated with 775 ground survey check points. These points were originally surveyed for a large mining exploration gravity survey conducted between 2004 and 2008 by MWH Geophysics. The distribution of the elevation check points is in the check points figure below.

The elevation check points were surveyed to an accuracy of 2cm using Real Time Kinematic GPS survey equipment.  One of the MWH Geophysics survey teams and their equipment are shown below. The accuracy check points were extracted from a 250m by 250m regional grid of survey points.

Area of the 10km by 10km Eritrea Stereo WorldView-2 1m elevation grid showing the 21 ground survey points used for ground reference and the 775 gravity survey stations used as elevation checkpoints to determine the accuracy of the stereo satellite survey.

MWH Geo-Surveys differential GPS survey crew and equipment. The Magellan RTK base with a ProMarkTM 500 GPS rover are shown in this photo.

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 775 elevation checkpoints with slopes less than 20% grade have a Root Mean Square Error (RMSE) of 14cm.

Histogram of the elevation differences between the WorldView-2 stereo satellite elevations for the 10km by 10km area and the 775 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 14cm, LED90 23cm.