“Everything that the last speaker just told you is wrong”. This shocked me since I was the last speaker. I was just rejoining the audience after my presentation at a satellite data distributors’ conference in San Diego in 2008.
I had given a presentation on PhotoSat’s experience using satellite photos for elevation mapping. I had shown comparisons between PhotoSat stereo IKONOS satellite elevation mapping and hundreds of mining exploration drill collar elevations. Our results suggested an IKONOS mapping accuracy of better than 1.0m in elevation.
The speaker who followed me showed the published specifications of the IKONOS satellite. He declared that this proved that the results I had just shown were impossible. Then he went on to talk about his own stereo IKONOS mapping results. His results showed 5m to 10m in elevation mapping accuracy.
Looking for a way to unambiguously measure our accuracy
The speaker who challenged PhotoSat’s results in the San Diego meeting actually did us a huge favor. Although it did not feel like that as I sat fuming in my chair. His comments provided the motivation for me to find a way to prove we were right. After this meeting we set about looking for a way to unambiguously demonstrate the accuracy of the PhotoSat stereo satellite elevation mapping.
Searching for a detailed, high quality, ground survey data set
We concluded that to prove our accuracy we needed to find a highly accurate ground survey data set covering hundreds of square kilometers. But where to find it?
About two months later, a light came on. I realized that we might find the elevation survey data that we needed from a large, regional, mining exploration gravity survey. The topographic surveys associated with mining exploration gravity surveys are among the most accurate and carefully checked topographic surveys in the world.
An old friend tells me about an existing ground survey data set
I phoned Kevin MacNabb, president of MWH Geo-Surveys International. Kevin did gravity survey contracts for me when I was a geophysicist at BP. I said “Kevin, I am looking for a large regional gravity survey with thousands of accurate topographic survey points. I want to use the topographic survey data to measure the accuracy of PhotoSat’s stereo IKONOS satellite mapping.”
I added “it would be great if the data is in an area of sparse vegetation in a remote region of the world. This way we can prove the accuracy of our stereo satellite topographic mapping and show that we can do this anywhere in the world.”
Kevin replied “how would 45,000 ground survey points covering over a thousand square kilometers just west of Asmara, Eritrea do?” For us this was the perfect data set. Eritrea was a challenging place to work. It had just emerged from a civil war. A perfect place to be mapping from satellites. For a fuller description, click this link.
The Eritea ground survey data is owned by an existing PhotoSat customer
It turned out that Kevin’s customer for the Eritrea gravity survey, Sunridge Gold, was also a PhotoSat customer for stereo IKONOS mapping. We negotiated the right to use the 45,000 ground survey points for accuracy studies. In return, we did some additional stereo IKONOS mapping for the company.
PhotoSat’s first comprehensive accuracy study
We were immediately able to use 10,000 of the Eritrea ground survey points to measure the elevation accuracy of 200 km² of stereo IKONOS elevation grid. PhotoSat had already produced this elevation grid for Sunridge Gold. We measured the accuracy of the PhotoSat elevation grid as 48cm Root Mean Square Error (RMSE). The full 2008 IKONOS Eritrea Accuracy study is available to review.
PhotoSat accuracy measurement and improvement
Since 2008, PhotoSat has been using the 45,000 Eritrea ground survey points as a test bed to measure accuracy improvements in the PhotoSat processing. This gives us a quantitative measure of accuracy improvements. We have shown the results in many conferences and published them. If there are still disbelievers they are certainly not challenging us publicly.
Satellite companies start to provide stereo test data over the Eritrea site.
In 2009, two commercial high resolution satellite companies, GeoEye and DigitaGlobe, provided stereo satellite photos over the PhotoSat Eritrea test area. The DigitalGlobe data was from the 50cm ground resolution WorldView-1 satellite launched in September 18, 2007. The GeoEye data was from the 50cm ground resolution GeoEye-1 satellite launched on September 6, 2008.
PhotoSat published elevation mapping accuracy reports for both new satellite systems. The stereo GeoEye-1 PhotoSat elevation grid had an accuracy of 31cm RMSE, determined by 8,893 ground survey points. The stereo WorldView-1 PhotoSat elevation grid had an accuracy of 35cm RMSE, determined by over 15,000 ground survey points.
WorldView-2 joint DigitalGlobe and PhotoSat news release on Eritrea accuracy study
Soon after the commissioning of the WorldView-2 satellite in early 2010 DigitalGlobe asked PhotoSat to use its new processing system to conduct an accuracy study over the Eritrea test area using stereo WorldView-2 photos. The PhotoSat Eritrea Accuracy study showed WorldView-2 accuracy of better than 30cm RMSE. These results were issued on March 16, 2010. The news release is available here.
With accuracy improvement PhotoSat mapping becomes PhotoSat surveying
Once the PhotoSat elevation grids achieved an accuracy of better than 30cm many of our customers began using them in place of ground surveying. We consequently renamed our products that have accuracy better than 30cm to surveying products.
The Eritrea ground survey data set has been used for hundreds of PhotoSat accuracy tests and studies
Since PhotoSat first acquired the 45,000 ground survey points in Eritrea, we have used the data for hundreds of accuracy test and studies.
2016 PhotoSat Eritrea accuracy studies
In 2016, we used the current version of the PhotoSat Geophysical Processing System to process a full range a stereo satellite photos over the Eritrea test area. Some of these results are published on our website on the links below. The link names include the satellite name, the number of ground control points used in the processing and the RMSE accuracy.
If you would like more information on PhotoSat surveying you can visit the following links.