GPS Coordinate System Check
Last Updated: 15 May 2017
Figure 1: Use this table to easily check your data points and it's margin of error. Download the spreadsheet here.
A common question being asked is “Can I survey with any GPS receiver, and landform with another machine control system?”
Firstly, I generally recommend surveying with the same equipment you landform with, but I can understand people you contract may survey and then others may landform, but in our operation we survey and landform with the same equipment. It might not be the same actual piece of equipment but it’s the same model and brand and also the same firmware in that piece of equipment.
If you do want to survey with one model of GPS receiver and landform with another, there’s a check I recommend to do first. The reason for this is that, there can be some differences between the coordinate systems of different GPS equipment, just a brief explanation on that.
The GPS system raw coordinates are latitude, longitude, and altitude. And the coordinate system is called WGS84; but we convert that to eastings, northings, and elevations for our earthworks calculations/designs. It can sometimes be called x, y, and z; but it’s basically a measurement in the eastings direction, north direction, and elevation.
Now, the issue is that there are different ways to convert latitude, longitude, altitude, to eastings, northings, and elevation. The common one is called UTM, but there are many others ways of doing it. And they’re generally similar ways doing it, but over distance they can be significantly difference to each other.
So, what you need to do is check before you do your landforming with certain survey. You need to check that they both matched up. I’ll just explain it right now. Note: You don’t need to do this check every survey. You just need to do it once to confirm your survey equipment is matching your landforming equipment.
Now, the best way to that is to set up your systems with the base stations wherever you want. [Ideally the base stations should be within 1km of the field for accuracy and landforming and this test.]. And mark out 4 (four) benchmarks let’s call it A, B, C and D; and these benchmarks should be at least a kilometer away from each other or half a mile away to make sure we get a descent check. Then, what we are going to do is go to each of these locations with the survey GPS receiver and measure the coordinates of that point. You want these in eastings, northings, and elevation. And then put your landforming equipment blade on that same locations and measure the same eastings, northings, and elevation (x, y, and z) on your equipment and record that on the piece of paper. And here is the example of a piece of paper you might write it down. (Fig. 1) System1 you have Point1, Eastings, Northings, Elevations; and then for System2 at the same thing Point, Eastings, Northings, Elevations. They might be thousands or even hundreds or thousands of meters away from each other but that’s OK at this point. We are trying to get the relative difference between the 2 (two) systems as we move away from the master benchmark, so write that down at each point. And the you with these coordinates, all you have to do is to calculate difference between these numbers. We’re going to calculate these differences of eastings points 1 put that number there and same with Northings put that number there, and Elevations put that number there.
Now, ideally those difference will be a constant difference. So it could be 101.111, let say, and then, if these are all 101.111 or similar then your Eastings between the two different systems are quite similar and same with the Northings and the Elevations. Our error should be to the accuracy of the GPS receiver which for RTK grade survey equipment, should be this orders in lets say 30mm in x, y or eastings and northings. But we have seen a difference of 15 meters over 1 kilometer. So, the problem with that is, if you do a design of a field that is 1 kilometer long, and if your landform equipment went to the farthest point from the master benchmarking then you could be 15 meters out in the horizontal location.
And then, obviously the Z’s. If there’s a change of elevations even 40 millimeters or 50 millimeters difference over that is a big problem.
So, that’s basically how you check and if they don’t matched up then your only option is to try fixe this conversion up, so they produce the same coordinates.
Sometimes you can’t work out what that conversion is.
Thanks for watching, and if you have got any questions, if you got a query like this, please send it to me we have look at it and give you some thoughts. Thanks for watching.