# OS Open Zoomstack Technical Specification

OS Open Zoomstack is a comprehensive vector basemap showing coverage of Great Britain at a national level, right down to street-level detail. Described as a “game changer” by users, it's one map, in one file that can be used for GIS, web, mobile or for offline use.

Zoomstack is supplied in GeoPackage and Vector Tiles formats.

This document supports the [Open Zoomstack Getting Started Guide](/os-downloads/products/maps-and-imagery-portfolio/os-open-zoomstack/os-open-zoomstack-getting-started-guide.md) and provides you with detailed technical information relating to the product.

This document is intended for users with technical knowledge in GIS. 

## Supply Formats

OS Open Zoomstack is available in the following formats:

* GeoPackage (zipped) – Approximately 3.8GB zipped and 11.8GB unzipped
* Vector Tiles (MBTiles) – Approximately 2.6GB Coverage

This data covers content for Great Britain (England, Wales and Scotland).

## Comparison between vector tiles and GeoPackage <a href="#bookmark1" id="bookmark1"></a>

#### *Why are they slightly different?*

We have designed the data schemas slightly differently due to the way they are used and applied within a range of software. These differences are:

* Projection – This is the most noticeable difference. The GeoPackage is in British National Grid (EPSG: 27700) whilst Vector Tiles is in Web Mercator (EPSG: 3857).
* Number of layers - They contain a different number of layers (21 for GeoPackage, 18 for Vector Tiles) as we endeavoured to make the data as performant as possible, across a range of technologies. The nature of a Vector Tiles pyramid means that many different geometries can exist in the same layer and only be rendered at given zoom levels – a concept not apparent in many GIS software. Therefore, we made decisions to split layers out based on their spatial resolutions (e.g. roads are split into 3 layers; national, regional and local).
* Depicting the extent of Great Britain – The GeoPackage contains a Land layer to depict the extent of Great Britain whereas the Vector Tiles contain a Sea layer which is the inverse but gives the same visual appearance.


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