# Raster Specification The following chapters include information about OS VectorMap District raster, file compression, symbology, georeferencing and formats. ## **Specification** * **Data Source:** Derived data * **Number of tiles in Great Britain:** 2863 (edgematched) * **Tile size:** 10km x 10km Because digital maps frequently cover very large areas they are split down into squares known as tiles, each of which covers part of an overall area. * **Availability:** National, country and tile supply available * **Resolution:** 254 dots per inch * **Data structure:** Raster * **Data Specification:** Selection of map features combined * **Colour Palette:** 256 fixed colours * **Transfer format:** TIFF 8-bit LZW\* compressed * **Storage Volume per tile:** Compressed: approx. 6.7mb * **Greyscale:** A high-quality alternative to colour raster data is to use a GIS to convert data to a greyscale format. \*If LZW compressed formats are used then registration may be required. OS VectorMap District is supplied in the following raster formats: TIFF 8-bit LZW compressed (6.7mb) --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.os.uk/os-downloads/products/maps-and-imagery-portfolio/os-vectormap-district/os-vectormap-district-technical-specification/raster-specification.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.