Survey Equipment - GNSS / GPS

Posted 01.10.17

GPS was the first global positioning system - a series of satellites orbiting the earth which emit at frequencies which can be tracked by "GPS" receivers on the ground.

Since then, a number of other satellite constellations have been launched and most survey grade receivers can access satellites from multiple systems. The correct terminology for the technology is Global Navigation Satellite System (GNSS), though it will take some time before the USA's first to market advantage and the term GPS wears off.

A GPS/GNSS receiver is a vital tool to the land surveyor performing a new survey – many topographic mapping tasks in the UK should relate to the Ordnance Survey (OS) National Grid. If survey stations already exist, they can be used directly with a total station - though checking the coordinates with GNSS is still advisable.

The OS National Grid is the national coordinate reference system for Great Britain. Heights for mainland are determined through Ordnance Datum Newlyn (ODN). This was set at the mean sea level at Newlyn, Cornwall at the start of the 20th century.

The OS maintain 190 Fundamental Bench Marks.  Each benchmark has a value, "above ordnance datum"(AOD). Historically, these were used to create a much denser network of lower order bench marks, each with an AOD value.

Land surveyors who undertook topographic surveys had a much harder time of it before GPS. To position the survey within the OS National Grid, previously mapped features would have to be observed using traditional methods. To ensure levels were relative to the Ordnance Datum, a local low order bench mark would have to be observed. Despite the density of the benchmark network, the closest one could still take up a good chunk of the day to reach.

According to OS, approximately 500,000 of these relics remain - but they are no longer maintained. Instead GPS/GNSS receivers are used by the surveyor to coordinate survey stations, typically situated on the outer perimeter of a site survey.

As with many surveying technologies, the rate of evolution has been high. Many land surveyors now utilise receivers which can calculate coordinates in real time. This enables the surveyor to take measurements and assess the accuracy on site, repeating measurements as necessary.

It can also offer great efficiency for measuring levels and details within the site survey itself. Unlike a total station, a clear line of site between survey stations is not required - though a clear sky view of multiple satellites is.

GNSS receivers can also be mounted to the blades/shovels of earthwork machinery. Constant measurements enable the operator to cut and fill the site accurately, according to a digital 3d model displayed in the cab. Some manufacturers of survey drones are now equipping drones with GNSS receivers capable of operating in real time - with claims that ground control points are no longer required…

Sub cm accuracy can be achieved with GNSS receivers in the right conditions and operating mode. On the whole though, the tolerances are higher than that achieved with total stations. As more satellites are launched and antennae technology improves, this difference is likely to become smaller.

Further reading

RICS Client Guides Virtually Level - Transition from traditional benchmarks to heighting using GNSS Map Projection Scale Factor - Avoid the potential dangers of scale-factor


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