Georeferencing

Georeferencing is the process of registering raster images within a geographic framework to allow tiles of map data to be placed in their correct geographic position, both in relation to the map projection and to each other. This is achieved by assigning map coordinates to the raster image data and resampling the pixels of the image to conform to the map projection grid.

Great Britain is surveyed and mapped using the Transverse Mercator (or Gauss-Krüger) projection. All raster tiles will be mapped to this projection, as it applies to the Ordnance Survey National Grid, when using world or TAB files supplied by Ordnance Survey. Further information on map projections is available in the Guide to Cartography on the More than Maps platform.

World files

A world file is a plain text file containing six parameters used by geographic information systems (GIS) to georeference raster map images. The file specification was originally introduced by Esri.

The base filename of a world file matches the raster's base filename, but with a different extension:

Raster format	Raster file name	World file name
TIFF	image.TIF	image.tfw
JPEG	image.JPG	image.jpw
Bitmap	image.BMP	image.bpw
ECW	image.ECW	image.eww
MeSID	image.SID	image.sdw

Example world file (SX99.tfw for raster image SX99.TIF in 1:25 000 Scale Colour Raster product):

2.5
0
0
-2.5
290001.25
99998.75

The generic meaning of the six parameters in the above world file is as follows:

• Line 1: pixel size in the x-direction in map units/pixel

• Line 2: rotation about the y-axis

• Line 3: rotation about the x-axis

• Line 4: pixel size in the y-direction in map units/pixel (almost always negative)

• Line 5: x-coordinate of the centre of the upper left pixel

• Line 6: y-coordinate of the centre of the upper left pixel

TAB files

The MapInfo TAB format is a geospatial vector data format for GIS software. It is developed and regulated by Precisely as a proprietary format.

TAB files for georeferencing are provided in an ASCII format, allowing an image to be rendered as a map layer

Example TAB file (SX99.TAB for raster image SX99.TIF in 1:25 000 Scale Colour Raster product):

!table
!version 300
!charset WindowsLatin1

Definition Table
    File "SX99.TIF"
    Type "RASTER"
    (290000,90000) (0,4000) Label "Pt 1",
    (300000,90000) (4000,4000) Label "Pt 2",
    (300000,100000) (4000,0) Label "Pt 3",
    (290000,100000) (0,0) Label "Pt 4"
    CoordSys Earth Projection 8, 79, "m", -2, 49, 0.9996012717, 400000, -100000
    Units "m"

The definition table in the above TAB file can be broken down as follows:

• Line 1: file name

• Line 2: type (always "RASTER")

• Lines 3-6: ground control points (real world coordinates to pixel coordinates) in the order bottom-left, bottom-right , top-right , top-left

• Line 7: definition for the Transverse Mercator projection [see table below]

• Line 8: units (always "m")

Component	Description
CoordSys Earth Projection 8	The 8 relates to a MapInfo identifier, which in this case is the Transverse Mercator projection.
79	A MapInfo identifier; in this case, this relates to Ordnance Survey of Great Britain 1936, Airy ellipsoid.
"m"	This relates to the unit of measurement, which in this case is 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	This figure 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.

GeoTIFF

GeoTIFF is an extension of the standard TIFF (Tagged Image File Format) that includes embedded geographic metadata. This metadata allows the image to be georeferenced, meaning it can be accurately placed in a spatial context on the Earth's surface.

Key benefits of GeoTIFF over regular TIFF:

• Georeferencing capabilities – GeoTIFF files store coordinate system information (for example, projection, datum, pixel scale), enabling precise alignment with other spatial datasets.

• Interoperability – Widely supported by GIS software (for example, QGIS, ArcGIS), making it easy to integrate into spatial analysis workflows.

• Self-contained metadata – All necessary spatial metadata is embedded within the file itself – no need for external world files (like .tfw), reducing the risk of data loss or misalignment.

• Standardised format – Based on open specifications, ensuring long-term accessibility and compatibility across platforms.

Georeferencing files are not provided for products offered in GeoTIFF format

Ordnance Survey does not provide georeferencing files for any Contextual or Derived Mapping Portfolio products offered in GeoTIFF format. This is because georeferencing is not required when using files provided in GeoTIFF format as the tiles have already been embedded with georeferencing information.