ETRS89 realised through OS Net
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GNSS is the standard tool for precise surveying and mapping, used for all OS precise surveying work. Some characteristics of GNSS as a surveying tool are:
GNSS is a three-dimensional positioning system: a precise GNSS fix yields latitude, longitude and ellipsoid height.
The highest precision of GNSS positions is at the 2 mm level horizontally relative to a global datum. To achieve this requires networks of permanently installed GNSS receivers. A typical field GNSS survey gives accuracies of a few centimetres relative to a global datum. Vertical position quality is generally about 2.5 times worse than horizontal.
GNSS is a purely geometric positioning tool; that is, GNSS coordinates do not give you any information in relation to level surfaces, only in relation to the geometric elements of coordinate system axes and ellipsoid. For this reason, GNSS does not give orthometric height information.
GNSS does not require intervisibility between ground reference points; neither is the geometric arrangement of the ground network crucial to the results as it is in theodolite triangulation survey.
With care, GNSS can be used very accurately for terrestrial survey over any distance – even between points on opposite sides of the world. For this reason, global datums are used for GNSS positioning. This feature makes GNSS vastly more powerful than traditional survey techniques.
The datum of OS Net is the European Terrestrial Reference System 1989 (ETRS89), which we looked at in . Since this datum is realised by many European precise GNSS reference points, OS Net is actually just a TRF enabling easier access to this datum in Great Britain.
Currently OS Net realises ETRS89 with an ETRF97 frame with a “parent” ITRF of ITRF97 at epoch 2009.756. This OS Net realisation is known as “OS Net v2009”. Prior to this the original OS Net also had an ETRF97 frame with a parent ITRF at ITRF97 but the epoch was 2001.553 so this OS Net realisation is now designated “OS Net v2001”. A small transformation between the two realisations is given in ''.
OS Net comprises of a network of over 100 continuously operating permanent GNSS receivers (COGRs). Because precise GNSS positioning is always carried out in relative mode (that is, observing the vector difference in coordinates between two simultaneously recording GNSS stations), a network of COGRs with precisely known coordinates is a very useful infrastructure. Precise positioning can then be carried out by one person with a single geodetic-quality GNSS receiver, using data downloaded from the COGRs.
All OS Net stations are coordinated in three dimensions by GNSS in the ETRS89 terrestrial reference system. Hence the user obtains ETRS89 coordinates for their unknown points, which are suitable for use with the Ordnance Survey transformations to the mapping coordinate systems OSGB36 and ODN (see section 6 below). ETRS89 coordinates can also very easily be transformed to the international ITRS datum (see section 6.5 below) – this is usually only required for international scientific applications.