OS VectorMap Local Technical Specification
This technical specification provides detailed technical information about OS VectorMap Local. It is targeted at technical users and software developers.
Last updated
This technical specification provides detailed technical information about OS VectorMap Local. It is targeted at technical users and software developers.
Last updated
OS VectorMap Local is a mapping dataset designed for providing contextual mapping output on paper, PCs, hand-held devices or the Internet. The vector format of the product consists of layers to enable you to customise and style the output to suit your needs.
The product is generated automatically from the OS Large-Scale Topographic Database with no manual cartographic input. This database is used to update other OS products, including OS MasterMap Topography Layer.
This Technical Specification contains detailed technical information about the data in both raster and vector formats. For basic information required to understand and use OS VectorMap Local, please refer to the Overview, which is available on the Product Support page on the OS website.
OS VectorMap Local raster data is supplied in GeoTIFF file format (Geographic Tagged Image File Format). The product is available in three raster formats: Full Colour, Backdrop and Black & White.
GeoTIFF LZW (Lempel Ziv Welch) 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.
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. OS VectorMap Local raster tiles are lossless compressed.
Lossless compression
As its name suggests, lossless compression does not lose information within an image. A lossless compression retains the original quality of an image when it is uncompressed. This process doesn’t 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.
GeoTIFF 8-bit LZW compressed
400 dots per inch (DPI)
GeoTIFF 1-bit LZW compressed
400 dots per inch (DPI)
OS VectorMap Local is available in three vector formats:
Geography Markup Language (GML) 3.2.1
GeoPackage
Vector tiles (MBTiles)
These file formats contain a set of features and identifiers which conform to the data structures outlined in the following sub-sections.
In OS VectorMap Local vector data for GML, vector tiles and GeoPackage formats, each feature is given a unique identifier: a featureID. The featureIDs are maintained between iterations. For GML only, this allows feature-level change-only updates (COUs) to be delivered. The COUs will consist of a selection of inserts and deletes related to real-world and generalisation generated change (see the GML overview section of this document for more information). Vector tiles and GeoPackage formats do not have a COU option.
The data structure is described below by means of unified modeling language (UML) class diagrams and accompanying tables containing text. The UML diagrams conform to the approach specified in the ISO 19103 Conceptual Schema.
Colour conventions have been used in the diagrams and tables to distinguish the properties that have been added in this specification. In the UML class diagrams, feature types in the Ordnance Survey product specification are coloured orange. The tables which follow in this Technical Specification use orange for a feature type, blue for a code list and purple for a data type.
The following stereotypes are used on UML elements:
GeoPackage overview
GML overview
Feature types
Code lists
Structured data types
Feature codes and descriptions
Vector tiles overview
Important Buildings classification
| Stereotype | UML element | Description |
|---|---|---|
FeatureType
Class
A spatial object type [ISO 19136].
Codelist
Class
A controlled set of values for a free text data type that may be extended.
DataType
Class
Structured data type.
