Transverse Mercator Projection - TM

The Transverse Mercator Projection is the mapping projection used with all Ordnance Survey Mapping
The Transverse Mercator, a conformal orthomorphic version of the Cassini projection is especially suitable for a country which has its greatest extent in a north south direction and therefore was chosen as the best projection for Great Britain.

In the simple Transverse Mercator projection the surface of the ellipsoid chosen to represent the Earth is represented on a cylinder which touches the ellipsoid along a chosen meridian and is then unwrapped. The scale is therefore correct along this central meridian and the amount of distortion, equal in an east west as well as north south direction, varies with the square of the distance from it. Meridians and parallels always intersect at right angles. The main difference compared with the simple Cassini was that the projection stretched the topography equally in all directions rather than only in a north south direction. This gave it the property of conformality or orthomorphism, in which there is a minimal distortion of shape over small areas and the scale, although it varies throughout the projection as a whole, is equal in all directions at any one point. For large scale mapping these attributes are especially valuable. Scale distortion can be measured by a 'Local Scale Factor'

The Transverse Mercator projection used for all Ordnance Survey maps has a central meridian at longitude 2 degrees West and a central meridian scale factor or 0.9996.

Source: Information taken from

'The Ellipsoid and the Transverse Mercator Projection' - Geodetic Information Paper No 1
'A Guide to Coordinate Systems in Great Britain' - Information Paper
'Ordnance Survey Maps - a descriptive manual - JB Harley OS 1975.

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