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Triangulation stations

OSGB36® National Grid is now defined by the ETRS89 (European Terrestrial Reference System 1989) coordinates of the Ordnance Survey National GPS Network plus the definitive OSTN02™ transformation. To obtain true OSGB36 National Grid coordinates for a point it must be first coordinated in ETRS89 from the National GPS Network. The transformation to OSGB36 via OSTN02 is then considered error free. See www.ordnancesurvey.co.uk/gps/ for more detail.

The coordinates supplied for this triangulation station have not been realised via ETRS89 and OSTN02. They are the original archive coordinates and can therefore no longer be considered as true OSGB36 National Grid Coordinates of the station. It is expected that agreement between ETRS89/OSTN02 derived coordinates and the original archive coordinates of triangulation stations (down to third order) will be at the 0.10 metre root mean square (r.m.s) level. Results returned will be limited to a maximum of 100 results.

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Name

This shows the station name, for example, Manor Farm. This may not be the 'widely accepted' or local name for the point but a unique name selected by the surveyor at the time.

This also shows the full single station details, including (if known) the scientific network (SN) coordinates. Only first- and second-order stations have SN coordinates.

There are two SNs, consisting of OSGB1970(SN)® and OSGB1980(SN). These form the results of adjustments done in 1970 and 1980. The adjustments were necessary because the OSGB36® was adjusted in blocks. To make this network more acceptable to that of Europe, the OSGB36 network had to be adjusted as a whole simultaneous unit and not by single blocks.

Sec blk num

This number - in the form of SU30 - shows in which secondary block the trig station falls. The blocks were formed by joining up the primary trig stations to form polygons (trig blocks). Each polygon had a unique number (secondary block number). Within each block each trig station was named and numbered but not computed. These station numbers and names are unique within each block.

Stat num

This number, in the form of /020 or /T20, shows the station number and its order (accuracy). A straight / and number show the station was a secondary station. A /T and a number shows the station was fixed as a tertiary or fourth-order station. A station number preceded by /H show the station was established by the UK Hydrographic Office and fixed to fourth-order standards. A station number preceded by /DM show the station was a 'distomat' observed station, fixed to fourth-order standards. The distomat stations were only fixed by observing distances and not angles, unlike other stations that were fixed by angle and distance methods. There are stations that have combinations of these numbers, for example, HU22/DM/T012. A secondary block number and /Int and a station number indicate that the station is an intersected point. These are 'unoccupiable' points that were fixed by intersection only, for example, chimney or church spire.

Destroyed

This show whether the station is destroyed: 1 = destroyed, 0 = exist. As this archive is not maintained, stations that have been destroyed could be shown as existing.

Mark type

Triangulation stations are marked in a number of ways:

Ground stations

Triangulation pillar - This consists of either a concrete or natural stone pillar, 1.2 m high with a brass plate set into the top to accept a survey instrument.
Surface mark - This is normally a dome-head brass bolt or rivet set into natural rock or a concrete block at ground level.
Buried mark - This is normally a brass bolt, rod or rivet set into a concrete block 60 cm below ground level.

Roof stations

The station is sited on a flat roof consisting of either a dome-head bolt or rivet positioned with enough space to set up a tripod.

Intersected station

These are 'non-occupiable' stations consisting of church spires, chimneys or mast.

Remarks

Any information relating to the point.

Order

This shows the level of precision to which the station was observed: first was the highest order and fourth was the lowest level of triangulation station.

Flush bracket num

These flush brackets (levelling plates) are only to be found on triangulation pillars within the horizontal archive. They can also be part of the vertical archive (bench marks). These are uniquely numbered levelling brackets. This is the point to where, if levelling has taken place, the height above the datum will be referred.

Levelling class

Shows the precision of the levelling.

Levelling datum

Shows to which datum the levelling refers.

Secondary computation block

This shows which computation block the station was computed and adjusted in. This is relevant when using stations near block edges. Observing stations that fall in different computational blocks should not be encouraged.

Computation year

This shows which year the station was computed and/or recomputed.

Maintenance year

This shows the year when the last maintenance took place. The maintenance has now stopped.

Levelling year

Shows the year that the levelling took place.

Height

Shows the height above Ordnance Datum in metres, referred either to the flush bracket or the top of the pillar.

OSGB36 easting and northing

These show the final adjusted coordinates of the station. They are expressed in OSGB36 National Grid.

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