Calibrating GPS Surveying Equipment

Calibration of GPS Surveying Equipment

There is no doubt that GPS surveying equipment is a great tool for surveyors and engineers. They allow us to easily take accurate measurements over very large distances quickly and easily.

But how do we know that these measurements accurate every time?

Usually, all the surveying equipment we use will be calibrated at least yearly. A yearly check that ensures the measurements we take are as accurate as possible. Think of automatic levels, laser levels and total stations. But this is not the case with GPS Surveying Equipment, there is nothing to adjust within the GPS receivers.

That is why we need to check our equipment every time we take a measurement with it.

Can GPS Surveying Equipment be Calibrated?

There is no formal way of calibrating GPS surveying equipment. But 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.

I recently hired a GPS network Rover that came with a calibration certificate this GPS Rover came from one of the major hirers of surveying equipment in the UK. But with no moving parts or optical components the GPS sensors inside the unit can’t be adjusted. So, for me GPS Surveying Equipment cannot be calibrated.

It does seem odd that GPS Surveying Equipment is issued with a calibration certificate, but it does at least confirm that the people supplying the GPS surveying equipment know how to use it!!!

Network GPS Rover

If GPS Surveying Equipment can’t be Calibrated, What do we do?

GPS surveying equipment can’t be calibrated it can only be checked. The sensors of the GPS equipment will only ever detect the GPS signals from the satellites and we have no control over those signals. In fact the signals from the GPS satellites will only pinpoint the sensor to within a few metres.

The only option we have is to check a known point every time that we do some setting out or surveying with GPS surveying equipment.

Why can’t GPS Surveying Equipment be Calibrated?

The ground co-ordinates we use are transformed from the geodetic co-ordinates received by the gps device. There are many different transformations available. We can also create them and are usually referred to as localizations or calibrations.

I prefer the term localization as the term calibration would refer to be independently verified.

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.

Checking the Accuracy of GPS Equipment Daily

How do we increase the accuracy of GPS?

Millimetre accuracy is achieved by using a known reference point and then sending a correction factor to GPS rover units. The GPS rover equipment can then use the correction factors to provide the millimetre accuracy we desire.

The distance of the GPS rover equipment from the GPS base station is crucial in obtaining the greatest accuracy. This is one of the reasons why the real-world accuracy of network GPS rovers does not always live up to expectations.

What Happens When GPS Surveying Equipment is not Calibrated or Localised?

When GPS Surveying Equipment is not Localised or set on the correct grid then the 3D positions we obtain cannot be relied upon.  They have no accuracy and therefore no precision for the work can be obtained.

I was on a job recently where a site engineer (for the client) was given a GPS network rover to do his work with. His job was to position bus stops between two cities so that the services in the ground could be moved before the actual bus stops were constructed.

The length of the job was some 15 miles in total. The original surveys for the whole site were done in eight separate lots. These surveys were all transformed to a scale factor of 1 so they were independent to each other. Each survey had their own localization. This meant that using any GPS surveying equipment in these areas required eight different localizations (or calibrations) to be carried out.

Unfortunately, the site engineer was not conversant with GPS Surveying and used the GPS surveying equipment on OS grid (OSTN15) for all of his work. He also never checked onto a control point. So, while he thought what he was doing was accurate and the equipment was saying he was accurate to within a few mm, in reality he was up to a metre out of position.

To make matters worse for him this issue was old this issue was only discovered when subcontractor was asking him to check control stations in one section of the works. The control stations had been installed some years previously and were difficult to find. This meant that there was no confidence in the primary control network for the works to be measured from.

The client’s site engineer was tasked with identifying the primary control points throughout the section of works. The first control point to be checked couldn’t be found but his GPS surveying equipment was telling him the control station was in the road and not in the splitter island as shown on the drawing. He was at least 600mm (2ft) away from the control point in the splitter island.

The next control station to be checked was at least 500mm from the actual point on the ground. The third control station point was about 400mm difference in position. This is the position that his GPS Surveying Equipment was giving in relation to the found PK Nail in the paved areas.

It suddenly dawned on the engineer that everything that he’d been doing previously was in the wrong position. This is because he hadn’t applied a localization (calibration) to the GPS surveying unit that he was using. He had also failed to check onto any known points throughout the whole time he had been working on the site with the GPS Surveying Equipment.

What is accuracy and precision in surveying?

Accuracy refers to how close the measurement is to the true value. In this case how close is the Easting, Northing and Elevation to the true value.

Precision refers to how close repeated measurements are to each other. In this case how close together are the measured Eastings, Northings and Elevations.

Why have the need for different scale factors?

Most construction projects carried out will always be carried out to a scale factor of 1 Anything that is designed to be built in an area will be designed on a flat table, not the curved surface of the earth. In most cases this is correct it is only when we get large projects or long projects (long in length) that we need to start considering the curvature of the earth.

GPS surveying equipment works on the curvature of the earth they used latitude longitude ellipsoidal height to calculate their position from the signals that they received from the GPS satellites. To use GPS surveying equipment on these scale factor 1 construction sites we need to apply localization (calibration) to them and that is a case of transforming the latitude longitude ellipsoidal height to ground coordinates that we know which are eastings northings and elevations.

Calibration or localizations would usually be carried out at the start of the project this will involve visiting each known control station of with a value eat with the and eastings northings and elevation value to find out the equivalent latitude longitude and these are then transformed says the GPS surveying equipment gives us the ground coordinates all of these control points should be outside of our work area if we start working outside of our control layer outside of our localization the errors from the GPS surveying equipment can increase without our knowledge

The only way we can ever check GPS surveying equipment is by visiting a known point every day when we are going to do some work this has to be considered our control cheques our quality control under calibration system.

With no formal way of calibrating GPS surveying equipment, the only thing we can do is check a known position with the GPS Surveying Equipment daily and ensure that the errors are within our allowable tolerances.

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