# Raster data

## Available formats <a href="#id-2.1-available-formats" id="id-2.1-available-formats"></a>

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 overview <a href="#id-3.-geotiff-lzw-overview" id="id-3.-geotiff-lzw-overview"></a>

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.

## Image compression <a href="#id-3.1-image-compression" id="id-3.1-image-compression"></a>

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.

## Raster specification <a href="#id-3.2-raster-specification" id="id-3.2-raster-specification"></a>

### Full Colour and Backdrop <a href="#id-3.2.1-full-colour-and-backdrop" id="id-3.2.1-full-colour-and-backdrop"></a>

* GeoTIFF 8-bit LZW compressed
* 400 dots per inch (DPI)

### Black & White <a href="#id-3.2.2-black-and-white" id="id-3.2.2-black-and-white"></a>

* GeoTIFF 1-bit LZW compressed
* 400 dots per inch (DPI)


---

# 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-local/os-vectormap-local-technical-specification/raster-data.md?ask=<question>
```

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.