By Gerry Mitchell, P.Geo, President PhotoSat
In 2008, I would get a hollow feeling in the pit of my stomach whenever customers phoned or emailed to tell us that the PhotoSat surveys did not match their ground surveys. Back then, I was sure there was a problem with the satellite photos, or that we had made some terrible mistake in our processing.
However, by 2013 our customers were using PhotoSat satellite surveys to check and adjust their ground surveys.
Ground surveys right, PhotoSat surveys wrong?
Back in 2008 it was clear to everyone, including me, that ground survey data was right and satellite survey data wrong. After all, ground survey data was collected by someone who had stood on the ground. It was the “ground truth”.
In comparison, the PhotoSat surveys were produced from satellite photos taken from 750 kilometers above the earth. Of course the ground surveys were right and the satellite photo surveys were wrong. Or so we thought. As we did more and more satellite surveying projects, we began finding obvious ground survey errors. Some of these projects had thousands of ground survey points.
Some ground surveys wrong, PhotoSat right
We began finding projects with two sets of ground survey data, one set matching the PhotoSat surveying perfectly and the other set mismatching. Investigating these cases with customers was eye-opening and often entertaining.
On one project in southern Mexico, all of the ground survey points on or near roads matched the PhotoSat surveying perfectly. The points in remote areas, particularly on the tops of hills, had differences of two to five meters in elevation.
So what happened? The contract surveyor had used his high quality, bulky, GPS surveying equipment for the survey points that he could drive to. But he had sent his young assistant with a hiker’s GPS to all of the survey points on the hilltops. The surveyor was confident that the client would never check those remote points.
By 2010, we had become much more confident in the accuracy and reliability of our PhotoSat surveying. When there was a mismatch between PhotoSat surveying and ground GPS surveying we began suggesting that the PhotoSat surveying was “usually right”. As you can well imagine, many of our customers, and all of their surveyors, were sure that I was delusional.
We had to find another strategy. We began saying, “Thanks for telling us about our mistake. Please send us a copy of your survey data and we will see if we can identify and fix our problem”.
Not all GPS surveys created equal
We also learned that not all ground GPS surveys are created equal. All good quality GPS systems record the GPS signals for later processing, so we began asking for copies of these GPS recordings along with the GPS ground survey coordinates.
The effect of this simple request on the quality of the survey data was remarkable. Some surveyors would immediately go and resurvey the ground points as soon as we asked for the GPS recordings.
By processing the GPS recordings we could see how long the surveyors had been at each point. In many of the significant mismatches, we discovered that the GPS recording times were much too short for ten centimeter accuracy. At first we had pushback from many surveyors when we suggested that their coordinates were probably inaccurate.
Then in June, 2011, the International Association of Oil and Gas producers published a thick report of guidelines for GPS surveying, freely available as a PDF file. For us this was a godsend.
Whenever there was debate about GPS accuracy, we would email a copy of the report saying, “These are the guidelines that we are relying on. Please tell us where they are wrong”. All the discussions about recording times for GPS accuracy stopped.
Most ground surveys are good quality
Of course, it’s worth noting that most of the ground surveys that we receive are very good. Only occasionally are there serious problems that we cannot easily resolve. And of course PhotoSat also makes occasional processing errors and mistakes.
We always investigate whenever the PhotoSat surveying does not match the ground surveying. When we find it to be our mistake, we fix the PhotoSat survey data and resend it to the customer at no cost.
Correcting multiple mismatching ground surveys
By 2013, many of our repeat customers no longer assumed that the PhotoSat surveys were wrong when they did not match their ground surveys. In just five years there had been a 180 degree shift. In 2008, ground surveying always proved that PhotoSat surveying was wrong. By 2013 the PhotoSat surveying was being used to quality check and fix ground surveying.
This is great for projects with several different ground surveys. These are often surveyed by different contractors. For example, we have one case of an oil and gas project with five different ground GPS surveys performed by five different contractors. We proved that none of the ground surveys matched any of the other four ground surveys.
The key ground survey was for an oil well that discovered several hundred million barrels of oil. We matched the PhotoSat survey to the discovery well. All of the other four surveys mismatched the PhotoSat survey, each by different horizontal and vertical distances.
We used the PhotoSat survey to measure the offsets of each of the ground surveys from the oil well. Then we adjusted the other four surveys to match the oil well. This gave the project a consistent set of ground surveys all matched to the oil well.
This case history is described in more detail here.
Please see the experience section of the PhotoSat website for additional case histories and accuracy studies.