Georeferencing

A definition for registering raster images within a geographic framework is the process of assigning map coordinates to the raster image data and re-sampling the pixels of the image to conform to the map projection grid. This allows tiles of map data to be located in their correct geographic position relative to the map projection and also to themselves.

Great Britain is surveyed and mapped using the Transverse Mercator (or Gauss-Kruger) projection, so all raster tiles will be mapped to this projection as it applies to Ordnance Survey National Grid if using World or TAB files supplied by Ordnance Survey.

Within the MIF record header the following information will be found under COORDSYS:

• CoordSys Earth Projection 8, 79, "m", -2, 49, 0.9996012717, 400000, -100000 Bounds (4.17232513428e-011, 7.7486038208e-011) (700000, 1300000). This information relates to the Transverse Mercator projection, its position relative to the rest of the world and also an individual tile’s position relative to the projection. The record header is constructed as (not all fields have to be used):

• CoordSys Earth Projection 8 The 8 relates to a MapInfo identifier, in this case the Transverse Mercator projection.

• 79 a MapInfo identifier, in this case this relates to Ordnance Survey of Great Britain 1936, Airy ellipsoid.

• “m” relates to the unit of measurement, in this case metres.

• -2 this is the origin of the projection in respect of longitude.

• 49 this is the origin of the projection in respect of latitude.

• 0.9996012717 indicates the distortion of the tile at the central meridian. A value of 1.0 would indicate no distortion at all. However, distortion within this projection is minimal even at the far western or eastern limits.

• 400000, -100000 these figures indicate the false origin of the British National Grid. They represent the south-west corner of the Transverse Mercator projection, which overlays Great Britain, so all coordinates for any tile, no matter what scale, will always be positive.

• Bounds: (4.17232513428e-011, 7.7486038208e-011) these values represent the minimum bounding X and Y coordinates for the tile. (700000, 1300000) these values represent the maximum bounding X and Y coordinates for the tile.

Legends Backdrop and Full Colour

Both Legends can be found on the OS VectorMap District Downloads page.