How to get the best GPS accuracy from GPS Survey Equipment.
Perhaps the title should be “How to prevent poor accuracy when using GPS Surveying Equipment”.
There is no doubt that GPS surveying equipment has greatly increased the working range for engineers and surveyors. No longer are they tied to lines of sight and distance measurement from a Total Station. But the engineer or surveyor should be aware that using GPS Surveying Equipment can be fraught with issues that will give poor results without necessarily warning the user. In this article I will go through 4 ways that should prevent you from getting poor results from your GPS Surveying Equipment and provide a defence should you be questioned about your work. And this in turn should mean that you get the best accuracy form the GPS Survey Equipment that you are using.
1.) Always have a localisation or calibration for your site.
The signals received by the GPS survey equipment will only give positions in Latitude, Longitude and ellipsoidal height. For Surveyors and Engineers this information is useless as predominantly we will be working in Eastings, Northings and Elevation (A.O.D.) and on a flat plane, not the curved surface that GPS works with. Surveyors and Engineers will need to convert these Latitude, Longitude and Ellipsoidal Heights into Eastings, Northings and Elevations that will correlate with those being given or observed with Total Stations, Levels and other optical surveying equipment. This is done by creating a localisation or a calibration for the site. All the points of the localisation or calibration should be on the perimeter of the site.
Topcon refers to having a localisation, Trimble refers to having a Calibration. Personally, I don’t like using the term Calibration for GPS Equipment. Calibration is the term given to ensuring that the equipment being used is functioning correctly and to the manufacturer’s specification. There is no formal way of calibrating a GPS since this would involve checking the space segment (satellites), control segment (ground station run by the American Military) as well as the ground segment (you). The only thing you can do for GPS Equipment is have a ‘Certificate of Conformity’ that shows the kit was working mechanically on the day of a test.
2.) Check on a known point each day prior to carrying out any surveying or setting out work.
As there is no formal way of calibrating GPS surveying equipment, it is important that we can prove that the positions being given by the GPS surveying equipment are accurate and correct to known positions on the ground. The only way this can be done is by carrying out observations on at least one known point (within the localisation) that prove the GPS surveying equipment is giving the right readings. The way I do this is by setting out the known position each morning, inspecting the observed position compared to the known position and recording this in the data logger. I usually expect to find the observed and known points to be within 10mm of each other. By recording the difference every day in the data logger, I can prove that any work done is accurate.
3.) Be Aware that Network Rover GPS Equipment can give inconsistent results.
When using GPS Surveying Equipment, I prefer to use a RTK system, that is a base station serving corrections to the GPS Rover Units. I find that this system gives more consistent results than the Network Rover Systems. Don’t get me wrong though, the Network Rover Systems are very good and are perfectly usable in everyday surveying and setting out tasks. I first noticed the inconsistency with Network GPS Rovers back in 2013 when I was checking control stations and found 100mm error in elevation between reading on the same point.
To show what I mean by getting inconsistent results I did a test with a Network GPS Rover over the course of a week. Observing the same point over the course of several days gave results varying in 30mm in the Eastings, 19mm in the Northings and 64mm in Elevation. The article on the real world accuracy of GPS Network Rovers is well worth a read as it shows the potential differences that can be observed.
The point to note is that GPS Network Rovers can give results that are further apart than the instantaneous accuracy the data logger may be indicating.
4.) Be Aware that buildings, trees, structures and overhead powerlines will affect the GPS Signals.
GPS Systems need a clear view of the sky (satellites) to achieve the accuracy we require to do any surveying or setting out. The GPS Receiver needs to receive signals from at least 5 Satellites with a good spread throughout the sky. Any buildings, trees, structures and metal signs will block or reflect these signals form the satellites. This will affect the quality of the position that the GPS rover can achieve. Should this quality degrade sufficiently then the surveyor or engineer should be aware that the GPS Solution Quality drops from a Fixed Solution to a Float Solution.
The GPS Satellites send out the signals to the GPS Receivers. These signals are radio waves in the microwave part of the electromagnetic spectrum. As these signals travel through space, then though the atmosphere down to the ground where we are, the signals are subject to distortions. These distortions introduce errors that affect the perceived position of the GPS Receiver. To calculate these errors, we need to have a fixed or known point that can collect these signals and correct them. This is what the GPS Base Station does. The GPS Base Station doesn’t move on the ground, so any movement in position from the GPS Signals must be due to the distortions of the GPS Signals. These distortions can then be relayed to the GPS Rover Units to correct their position and hence give the desired accuracy.
Now that we know we need to correct the position of the GPS Receivers due to errors in the signals, we can see that the closer the GPS Rover is to the GPS Base Station the more accurate the position could be. Being further away from the GPS Base Station the greater the potential error for the GPS Rover unit could be. And, as the GPS rover unit can move freely, the potential for errors from obstructions or other Electromagnetic (Powerlines etc) sources can be significant on the signals received by the Rover unit compared to the Base unit.
GPS Surveying Equipment can bring a huge amount of freedom for the Surveyor or Engineer. It can increase the productivity of the surveyor or engineer as sites can be covered with easily compared to a Total Station that might need several setup positions to see the whole site. But using GPS Surveying Equipment is not without its pitfalls. Hopefully the information provided here will help steer you clear of the major ones and ensure that you get the best gps accuracy.
