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05 July 2022

Datum & Grid - Explanation

Surveys are required to be related to other features within the area. This will all only make sense if everything is to the same grid & datum.

Datum & Grid

The survey data recorded is related to the control network established (please refer to the Survey Methodology page). The drawing produced will show the site features relative to each other.
In some cases, all a client needs to know is the relative site situation; for example the difference in level between one part of the site and another etc.. In this case the survey stations can be given an arbitrary grid & datum, i.e. an arbitrary Easting (X) and Northing (Y) co-ordinate grid, and an arbitrary site datum (Z) Level value. This can be assigned by the surveyor, or may be related to some existing site reference point.
However, increasingly now, surveys are required to be related to other features within the area / locality, (i.e. to be related to the same grid & datum); for example, manholes (shown on drainage records used for sewer design), or features on surrounding Ordnance Survey mapping etc.. This will all only make sense if everything is to the same grid & datum, therefore surveys are now generally surveyed to a common standard/reference co-ordinate system. In the UK this is the Ordnance Survey National Grid for Easting & Northing, and Ordnance Datum for level.
The Ordnance Survey National Grid is a standard known grid across the UK, and Ordnance Datum is essentially height above sea level based on a known datum point in Newlyn, Cornwall.
In the past, the level datum was transferred from reference points, referred to as Ordnance Survey (O.S.) Benchmarks (very often chiselled arrow marks on buildings etc.), but these have not been maintained for some years, and have now been abandoned by the O.S.
Also in the past, to relate to O.S. National Grid, survey observations had to be taken to known co-ordinated reference points referred to as O.S. Trig Stations. These were usually in elevated positions, and would need to be observed and incorporated in the survey traverse for the survey being carried out. Direct line-of-sight was required. This was quite a time-consuming exercise.

However, this has now been simplified considerably with the advent of GPS survey equipment. By setting-up GPS units on site and leaving these to gather satellite data, surveys can be related to O.S. datum & grid. This is done by utilising GPS data provided by the Ordnance Survey from their continually operating RINEX reference stations. By combining the surveyor’s GPS recorded data and the O.S. RINEX GPS data recorded for the same period, specialist software can calculate and co-ordinate the surveyor’s control stations to the correct required datum & grid.
More recently, instrument manufacturers have established independent reference networks. Surveyors are able to dial into this GPS network and resolve their position on site.
Total Surveys have the latest GPS equipment. The company has also recently invested in Leica “Smartnet” GPS equipment, which gives immediate real time co-ordinates by utilising the Ordnance Survey Active GPS Network. This gives immediate co-ordinates on site to O.S. grid & datum. Surveys related to Ordnance Datum and National Grid co-ordinates can be superimposed onto O.S. background mapping, to provide additional surrounding information, if required.

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