A definition for registering raster images within a geographic framework is the process of assigning map coordinates to the raster image data and resampling 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 MapInfo format (MIF) record header, the following information will be found under COORDSYS:
CoordSys Earth Projection 8, 79, "m", -2, 49, 0.9996012717, 400000, -100000 Bounds(4.17232513428e011, 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 follows (not all fields have to be used):
| MIF record header 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.
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.
The image file directory for TIFF will contain a selection of the following entries:
File Byte Order: OS defaults to the use of MM = Motorola Byte Order; the order in which the data is interpreted.
Tag 256 (ImageWidth): The number of columns in the image, i.e. the number of pixels per row, for example, value = 4000.
Tag 257 (ImageLength): The number of rows of pixels in the image, for example, value = 4000.
Tag 258 (BitsPerSample): The number of bits per component, for example, value = 8.
Tag 259 (Compression): Compression scheme used on the image data, for example, value = 5 (LZW).
Tag 262 (PhotometricInterpretation): The colour space of the image data, for example, value = 3 (Palette).
Tag 270 (ImageDescription): A string that describes the subject of the image, for example, value = 1:50 000 TILE SU40.
Tag 273 (StripOffsets): For each strip, the byte offset of that strip, for example, first four values = 5640 19678 35692 51409.
Tag 277 (SamplesPerPixel): The number of components per pixel.
Tag 278 (RowsPerStrip): The number of rows in each strip, for example, value = 8.
Tag 279 (StripByteCounts): For each strip, the number of bytes in that strip after compression, for example, first four values = 14038 16014 15717 15442.
Tag 282 (XResolution): The number of pixels per Resolution Unit in the Image Width, for example, value = 254.
Tag 283 (YResolution): The number of pixels per Resolution Unit in the Image Length, for example, value = 100/1.
Tag 284 (PlanarConfiguration): Look-up table, for example, value = first four values = 22873 31354 39321 59110.
Tag 296 (ResolutionUnit): How the components of each pixel are stored.
Tag 306 (DateTime): Date and time of image creation, for example, value = 2007:06:30 12:38:41.
Tag 317 (Predictor): A mathematical operator that is applied to the image data before an encoding scheme is applied.
Tag 320 (ColourMap): Look-up table, for example, value = first four values = 22873 31354 39321 59110.
Tag 339 (SampleFormat): Specifies how to interpret each data sample in a pixel.
Tag 33432 (Copyright): Copyright notice, for example, value = ORDNANCE SURVEY CROWN COPYRIGHT 2019.
The preceding values are relevant to 1:50 000 scale TIFF data with LZW compression.
This technical specification provides detailed technical information about the 1:50 000 Scale Colour Raster. It is targeted at technical users and software developers.
The following sections contain information about 1:50 000 Scale Colour Raster, including file compression, legends, georeferencing and formats.
816 (edge matched)
England: 421
Scotland: 338
Wales: 86
20 km by 20 km
National coverage
254 and 660 dots per inch
Raster
TIFF palette 8-bit (256 colours) LZW* uncompressed
TIFF palette 8-bit (256 colours) with LZW* compression
GeoTIFF with LZW* compression
Windows® BMP 8-bit colour (256 colours) uncompressed
* If LZW compressed formats are used, then registration may be required. Information is available on the Unisys® website.
0.3–18 Mb
Quarterly: March, June, September and December (change-only updates)
Maintained versions (in English and Welsh) of the 1:50 000 Scale Colour Raster legend, which defines all of the map symbols used in the product, can be found on the 1:50 000 Scale Colour Raster Product Support page of the OS website:
A full list of the common mapping abbreviations used in 1:50 000 Scale Colour Raster is available on the OS website.
The purpose of this page is to provide a glossary of the terms used in the definition of products, services, licensing and other terms and conditions for 1:50 000 Scale Colour Raster.
A standard binary coding system used to represent characters within a computer.
The smallest possible unit of data, resulting from a choice between 0 and 1.
A boundary forms the division between two similar real-world objects, for example, property boundary or administrative boundary, and is defined by one or more lines.
A unit of computer storage of binary data, usually comprising 8 bits, equivalent to a character.
A distinctive mark; an inscribed letter; one of a set of writing symbols.
The binary representation of a single element of a character set; for example, EBCDIC, ASCII.
A coordinate pair is an easting and a northing.
A computational process of converting an image or map from one coordinate system to another.
Read-only memory (CD-ROM). A data storage medium. A 12-cm disc similar to the audio CD. It is an alloy disc pitted with tiny holes and then coated in plastic. A laser head reads the pattern of the holes and translates them into binary data.
The sole legal right to print or publish a work. Crown Copyright subsists in all Ordnance Survey publications for a 50-year period, from the end of the year in which they were first published, by virtue of the Copyright Designs and Patents Act 1988.
An organisation or individual that makes use of Ordnance Survey’s data supply facilities. This includes both direct sales customers of Ordnance Survey as well as customers of Licensed Partners. It does not include anyone, or any organisation, that has access to Ordnance Survey material without charge.
A representation of facts, concepts or instructions in a formalised manner suitable for communication, interpretation or processing.
An organised, integrated collection of geographic data, which may or may not be spatial data. It is stored so that specific applications can access the data by different logical paths. A database is accessed and managed by a database management system (software for managing database information).
A specification that defines the order in which data is stored or a description of the way data is held in a file or record.
Attributes of a dataset that define its suitability for a particular purpose, such as completeness, positional accuracy, currency and so on.
The defined logical arrangement of data as used by a system for data management; a representation of a data model in computer form.
This is the means by which computer files are transferred from one computer to another. Transfer media may be subdivided into communications media and physical media.
Data as supplied in a particular format to customers, whether internal or external to Ordnance Survey.
A measure of the number of units of data held on a stated length of storage surface. For example, some magnetic tapes may be recorded at a density of 1,600 bits per inch (bpi). Often referred to as packing density.
The method of supply of data to a customer (such as offline and online).
A name describing a real-world object or feature (for example, School) as shown on the 1:50 000 Scale Colour Raster map.
The name as shown on the 1:50 000 Scale Colour Raster map.
Data that is expressed as numbers (digits) in computer-readable form.
The supply of revised digital data to a customer at a predetermined interval of time.
A direct transaction between Ordnance Survey and a customer.
A text feature which forms a name that distinguishes it from other text features of the same type, for example, Millbrook School.
The resolution, or fineness, of a raster image.
A data storage medium.
See rectangular coordinates.
An image named after the Joint Photographic Experts Group, which uses a lossy compression format. It is designed for compressing full colour or greyscale images of natural, real-world scenes and works well on photographs. It is the de facto standard for photographs on the web.
The process of converting information to a computer-readable form, for example, digitising maps.
A geographic entity such as a building or stream, either taken from a map or surveyed directly from the real world. Can be a point/symbol, text or line.
The specified arrangement of data, for example, the layout of a printed document, the arrangement of the parts of a computer instruction, the arrangement of data in a record.
The cartographic process of simplifying the depiction of features to fit the output scale. For example, road widening is necessary at smaller scales to enhance their visibility.
Assigning a geographic location to data, for example, adding coordinates to an address.
Coordinates, usually expressed as latitudes and longitudes, that define a point's position on the Earth's surface.
A definition for registering raster images within a geographic framework is the process of assigning map coordinates to the raster image data and resampling the pixels of the image to conform to the map projection grid.
GeoTIFF (Geographic Tagged Image File Format) is a public domain and OGC metadata standard. It is a TIFF image file with georeferencing information embedded in the file metadata. It can be utilised by a geographic information system (GIS). All GeoTIFF raster tiles are LZW compressed.
1 073 741 824 bytes, a measure of data storage capacity (see megabyte).
1,024 bytes, a measure of data storage capacity.
Any organisation that has entered into a formal licence agreement with Ordnance Survey to market map information or to incorporate map data with their application or service.
Map feature in the form of a line (for example, river and boundary) that may or may not represent a real-world (existent) feature.
The local origin of rectangular coordinates is the south-west corner of the 1 km by 1 km National Grid square they identify.
The ratio between the extent of a feature on the map and its extent on the ground, normally expressed as a representative fraction, for example, 1:1,250, 1:50 000 and so on.
1 048 576 bytes, a measure of data storage capacity (see gigabyte).
MapInfo format.
The metric grid on the Transverse Mercator projection used by Ordnance Survey on all post-Second World War mapping to provide an unambiguous spatial reference in Great Britain for any place or entity, whatever the map scale.
See rectangular coordinates.
The zero point in a system of rectangular coordinates.
In the 1:50 000 Scale Colour Raster product, a pixel is a single point represented by a square.
A pair of coordinates.
Attribute data expressed as an array of pixels, with spatial position implicit in the ordering of the pixels.
The real-world feature represented by a feature, for example, a building, a fence, a wood.
Also known as XY coordinates and as eastings and northings. These are two-dimensional coordinates that measure the position of any point relative to an arbitrary origin on a plane surface (for example, a map projection, a digitising table or a VDU screen).
Used to produce light or dark shading (for example, building/water fill); this is dependent on the spacing of the dots – the denser the dots, the darker the effect.
A set of items that can be arranged into a sequence according to a rule.
The file format in which the data is supplied to the customer.
MapInfo format.
Tags are unique numbers that are used for identifying specific information in TIFF files, for example, image width, image length, bits per sample, photo interpretation and resolution.
Character, or character string, or field or record used to signal the end of a record, or section, or volume or database.
A unit of map used to divide large areas into regular and more manageable sizes.
TIFF is a tagged image file format-based file format for storing and interchanging raster images with the most recent version (6.0) published in 1992.
The format used to transfer data between computer systems. In general usage, this can refer not only to the organisation of data but also to the associated information, such as attribute codes, which are required in order to successfully complete the transfer.
The physical medium on which digital data is transferred from one computer system to another. For example, compact disc.
An operating system that supports multitasking and is used by many workstations and minicomputers.
The process of adding to and revising existing digital map data to take account of change.
A physical unit of the transfer medium that is a single disk, or a single DVD.