The data volumes for each file format are influenced by the level of data compression.
When an image is compressed, duplicated data that has no value is removed or saved in a shorter form, reducing a file’s size. For example, if large areas of water are the same tone, only the value for one pixel needs to be saved, together with the locations of the other pixels with the same colour. When the image is edited or displayed, the compression process is reversed. When raster data is compressed, not only are the data volumes reduced, but the user can download, display, edit and transfer images more quickly.
There are two forms of compression: lossless and lossy:
Lossless compression: As its name suggests, lossless compression does not lose information within an image. Lossless compression retains the original quality of an image when it is uncompressed. This process does not provide much compression, so file sizes remain large. Lossless compression is used mainly where detail is important, such as when planning to make large prints.
Lossy compression: This process degrades images to some degree, meaning that the decompressed image is not quite the same as the original. The more an image is compressed, the more degraded it becomes. In many situations, such as posting images on the Internet or printing small- to medium-sized prints, the image degradation is not so obvious. If a lossy compressed image is over-enlarged, the degradation will become apparent, and therefore, 1:25 000 Scale Colour Raster is not supplied using this form of compression.
TIFF is one of the most used lossless image formats. TIFF is primarily designed for raster data interchange and is supported by numerous image-processing applications. This permits much more efficient access to exceptionally large files that have been compressed.
Ordnance Survey measures the data in its products in one or more of the ways set out in the table below:
Completeness
Presence and absence of features against the specified data content*
Omission
Features representing objects that conform to the specified data content but are not present in the data
Commission
Features representing objects that do not conform to the specified data content but are present in the data
Logical consistency
Degree of adherence to logical rules of data structure, attribution and relationships
Conceptual consistency
How closely the data follows the conceptual rules (or model)
Domain consistency
How closely the data values in the dataset match the range of values in the dataset specification
Format consistency
The physical structure (syntax): how closely the data stored and delivered fits the database schema and agreed supply formats
Topological consistency
The explicit topological references between features (connectivity) – according to specification
Positional accuracy
Accuracy of the position of features
Absolute accuracy
How closely the coordinates of a point in the dataset agree with the coordinates of the same point on the ground (in the British National Grid reference system)
Relative accuracy
Positional consistency of a data point or feature in relation to other local data points or features within the same or another reference dataset
Geometric fidelity
The ‘trueness’ of features to the shapes and alignments of the objects they represent*
Temporal accuracy
Accuracy of temporal
Temporal consistency
How well-ordered events are recorded in the dataset (life cycles)
attributes and temporal relationships of features
Temporal validity (currency)
Validity of data with respect to time: the amount of real-world change that has been incorporated in the dataset that is scheduled for capture under current specifications
Thematic accuracy (attribute accuracy)
Classification of features and their attributes
Classification correctness
How accurately the attributes within the dataset record the information about objects*
*When testing the data according to the dataset specification against the ‘real world’ or reference dataset.
ISO 19115 compliant UK GEMINI discovery level metadata is provided for the data and can be found on the AGI (The Association for Geographic Information) website.
The following list gives a detailed description of the metadata elements provided on the AGI website:
Title: The title of the product.
Abstract: The abstract gives a brief description of the product.
Currency: The currency takes the form of date of last update for the feature.
Lineage: The lineage metadata takes the form of product specification name and date of product specification.
Spatial extent: The spatial extent is supplied in the form of geographic identifiers (for example, England, Scotland and Wales) and in the form of geographic coordinates.
Spatial reference system: The spatial reference system for all products takes the form of a British National Grid system, namely OSGB36.
Data format: Data format takes the form of the name of the format or formats the product is supplied in.
Frequency of updates: Frequency of updates takes the form of a stated period of time.
Distributor’s contact details: Distributor contact details include postal address, phone number, email address and website.
Data originator: Given as the company having primary responsibility for the intellectual content of the data source; in all cases, this will be Ordnance Survey.
Other metadata available includes keywords, start date of data capture, access constraints, use constraints, level of spatial data, supply media and presentation detail
This document contains additional information relating to the three Colour Raster Mapping products. All users will find the Product Information document useful and informative.
The following pages contain general overview information relating to the 1:25 000, 1:50 000 and 1:250 000 Scale Colour Raster products. Detailed product information relating to each individual product can be found in the following pages:
The following table outlines the formats available for the three OS Colour Raster Map products. These include:
TIFF LZW (Lempel Ziv Welch) is a lossless compression (see the data compression page).
TIFF PackBits is a lossless compression scheme that is supported by virtually all applications that can import TIFF graphics.
GeoTIFF LZW is a TIFF file which has geographic (or cartographic) data embedded as tags within it. The geographic data can then be used to position the image in the correct location and with precise geometry on the screen of a geographic information display.
Windows® BMP is a historic file format for the Windows operating system. A compressed BMP format is also available using Run Length Encoding (RLE). RLE means that the file can be read from start to finish in one pass. A BMP file consists of either three or four parts. The first part is a header that includes the position of the image and the number of colours to be displayed. This is followed by an information section that contains the image width (part 2), height (part 3) and the type of compression (part 4).
TIFF LZW
TIFF LZW
TIFF LZW
GeoTIFF LZW
GeoTIFF LZW
TIFF PackBits
Windows® BMP
TIFF is a file-based format for storing and interchanging raster images, with the most recent version (6.0) published in 1992.
There are two types of architecture for a TIFF. Many mainframe computers use what is known as a big endian (Motorola®) architecture. Most modern computers, including personal computers (PCs), use the little-endian (Intel®) system. 1:50 000 Scale Colour Raster and 1:25 000 Scale Colour Raster TIFFs are supplied with Intel architecture.
Converting between these two systems is possible but, as a general rule, modern software should be expected to handle both of these outputs without operator intervention.
Both the 1:25 000 and 1:50 000 Scale Colour Raster maps conform to the TIFF 6.0 standard. Customers are recommended to contact their system suppliers to ensure that it can read the Intel/little-endian TIFF architecture.
The GeoTIFF LZW compressed format is supplied as an online download which is available from the OS Data Hub. Other TIFF formats are also available from the OS Data Hub but as area of interest (AOI) downloads only (i.e. not as full supply).
The TIFF LZW compressed, TIFF LZW 8-bit uncompressed and Windows® BMP compressed formats are available for customers to request on DVD/HDD through OS Orders.
The product is updated quarterly. OS Partners can select a full supply option or a change-only update (COU) option. Public Sector Geospatial Agreement (PSGA) customers are only able to select the COU option. If you select the COU option, then you will only receive tiles that have changed since the previous supply.