Cut Bench Mark - Clayton Street, London, UK
Posted by: Groundspeak Premium Member Master Mariner
N 51° 29.061 W 000° 06.814
30U E 700404 N 5707639
Quick Description: This mark is cut into brick at the corner of Clayton Street and Kennington Oval opposite The Oval cricket ground.
Location: London, United Kingdom
Date Posted: 7/12/2013 5:45:20 AM
Waymark Code: WMHHGC
Published By: Groundspeak Regular Member harleydavidsonandy
Views: 0

Long Description:

Data obtained from the British Ordnance Survey web site...

Square:  TQ
Easting:  3108
Northing:  7783
Mark Type:  Cut Mark
Height: 4.44m
An explanation: The National Grid is the map reference system used on all British Ordnance Survey maps to identify the position of any feature. Other nations have their own systems.

How it works:
The National Grid breaks Great Britain down into progressively smaller squares identified first by letters and then numbers.

The largest unit of the grid is 500km squares each designated by a prefix letter alphabetically from A-Z omitting I - the first letter to be quoted in today's National Grid Reference. Great Britain is covered only by four of these squares: H, N, S and T.

The 500km squares are then further broken down in to twenty-five 100km squares which are identified by a letter, again A - Z omitting I ( the second letter quoted in a reference).

These squares are divided into smaller squares by grid lines representing 10 km spacing each numbered 0 - 9, from the south-west corner in an easterly and northerly direction. You can thus identify a 10km grid square by quoting two grid letters and the eastings and northings; for example, TQ 6 3.

On OS Landranger Maps, you will find the two grid letters on the legend or the corner of the map. The 10km grid is then further broken down into 1km grid squares.

By estimating the eastings and northings to one tenth of the grid interval, you can quote a full six figure grid reference that is accurate to 100m on the ground. For example, the Tower of London's grid reference is TQ 336805.

Geographical origin of the National Grid:
The National Grid applies to all Ordnance Survey maps at all scales. Northern Ireland and the Republic of Ireland use their own National Grid system. It covers the whole of Great Britain and extends 700 km to the east and 1300 km to the north of the false origin. It is based on the Transverse Mercator Projection.

The True origin latitude and longitude coordinates of the national grid are 49 Degrees North: 2 degrees West. The False origin latitude and longitude or coordinates are 49 degrees 45 minutes and 58 seconds North: 7 degrees 33 minutes 23 seconds West.

The False origin which lies slightly southwest of the Isles of Scilly was devised to ensure that all National Grid coordinates were positive (that is, to the east and north of origin 00) 400km are added to all eastings coordinates and 100km subtracted from all northings coordinates. If coordinates were calculated from the true origin, the positions lying west of the central meridian would be negative and the northings, although positive would exceed 1000km for some points in northern Scotland.

Datum height differences:
The difference between the Liverpool and Newlyn Datums varies across the country. This is due to levelling of the day only being fit to form a framework to control lower order levelling and contouring.

The original Liverpool levelling was started in 1840 using a bench mark on St John's Church. In 1844 the datum was changed to the tidal pole in Victoria Dock and tidal observations taken place over a nine day period.

Due to the imperfections with the levelling, it was decided to undertake a second geodetic levelling (1912 to 1921). It was at this time that mean sea level was fixed at Newlyn in Cornwall. Fixed points throughout the country were established fundamental bench marks (FBMs).

In 1950 it was decided to undertake a third geodetic levelling, still based on the Newlyn tidal observations. This started in 1951 and was completed in 1956.

The conversion factors between the two datums vary sometimes between kilometre squares. The rule of thumb is that the conversion factor is negative in the south of the country and positive in the north. The conversion figures are only given as a guide and cannot be given exactly, but can be given with sufficient accuracy for most practical purposes to one decimal place of a foot.

In remoter islands such as Shetland and Hebrides and so on, datums are based on a local determination of mean sea level. Being based on short observations, the figures should be regarded as only approximate.

The mark is generally in good condition. Some re-pointing has taken place across the arrow but there is minimal degradation elsewhere. The mark is on the Clayton Street side of the corner. 

Visit Instructions:
1. A closeup photo of the monument is required.
2. A 'distant' photo including the monument in the view is recommended.
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