# Supply mechanisms

OS MasterMap Topography Layer orders are fulfilled via the OS Data Hub’s download service. However, in exceptional circumstances, OS will supply the product via CD, DVD, an SFTP server or a download service, on a minimum six-week cycle.

Full Great Britain coverage is available by ordering a National Set (GB) in GML or vector tiles formats only. Smaller areas can be ordered by selecting an AOI through the online ordering system in GML or GeoPackage formats only.

More information about supply options can be found in the product's Overview, which is available from the [OS MasterMap Topography Layer Product Support page of the OS website](https://www.ordnancesurvey.co.uk/business-government/tools-support/mastermap-topography-support).

## Chunking for GML files <a href="#id-1.4-chunking-for-gml-files" id="id-1.4-chunking-for-gml-files"></a>

In GML format, OS MasterMap Topography Layer is split into chunks to allow file sizes to be kept at a manageable level. Two types of chunks are available: geographic and non-geographic. A more detailed description of these options can be found in the product's Overview, which is available from the

[OS MasterMap Topography Layer Product Support page of the OS website](https://www.ordnancesurvey.co.uk/business-government/tools-support/mastermap-topography-support).

### **Geographic chunking**

Geographic chunking divides supply areas into manageable sizes in a geographically meaningful way. Chunks are created to a user-specified size, either 2km by 2km, 5km by 5km, or 10km by 10km. Features that fall within two or more chunks are supplied in both chunks, meaning that some features around the chunk edges will be duplicated.

### **Non-geographic chunking**

Non-geographic chunking divides supply areas into files that have a fixed nominal size, regardless of geographic area. There are three compressed file size options available: 10MB, 30MB or 50MB. Each feature will only appear in one chunk file. It is possible for features from various geographic locations to appear in one file, and for adjacent features to be supplied in different files.


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