# 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 ](https://docs.os.uk/more-than-maps/geographic-data-visualisation/guide-to-cartography/projections)on the [More than Maps](https://docs.os.uk/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](https://www.esri.com/en-us/home). 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 | | MrSID | *image*.SID | *image*.sdw | Example world file (**SX99.tfw** for raster image **SX99.TIF** in **1:25 000 Scale Colour Raster p**roduct): ``` 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 ](https://www.precisely.com/)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): ```q !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"`)
ComponentDescription
CoordSys Earth Projection 8The 8 relates to a MapInfo identifier, which in this case is the Transverse Mercator projection.
79A 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.
-2This is the origin of the projection in respect of longitude.
49This is the origin of the projection in respect of latitude.
0.9996012717This 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, -100000These 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. {% hint style="warning" %} ### Georeferencing files are not provided for products offered in GeoTIFF format Ordnance Survey does not provide georeferencing files for any Maps and Imagery 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. {% endhint %}