# Updating data holdings

The underlying database for OS MasterMap Topography Layer is live and undergoes continuous revision. When a user orders COU, a ‘change since’ date is specified, and all features that have changed since midnight on the date specified are supplied. This will normally be the date the data was last extracted from the Ordnance Survey main database but could be a previous date. The last extraction date can be found on the [Product Refresh Dates page](https://docs.os.uk/os-downloads/resources/product-resources/product-refresh-dates#os-mastermap-topography-layer) along with the update history and release notes for the product.

To be able to resolve changes to the data holding, the system used to translate or load the data must check the TOID and version of every feature in the update against the current data holding to determine whether it should be loaded, and if so, what existing feature(s) it replaces. This makes it possible to request and load COU with a date preceding the last data supply date without damaging the data holding. This process can be used to correct a data holding if inconsistencies have occurred due to partially loaded or non-sequential COU, by ordering a single COU with a change since date that precedes the problem updates.

## Managed Great Britain Sets <a href="#id-4.6.1-managed-great-britain-sets" id="id-4.6.1-managed-great-britain-sets"></a>

For users taking a Full Great Britain (GB) Supply of OS MasterMap Topography Layer, there is a Managed Great Britain Sets (MGBS) service. The MGBS service is available for GML or vector tiles formats only (full GB coverage of OS MasterMap Topography Layer in GeoPackage format as a single AOI will be too large for the majority of GIS to handle).

OS Data Hub users will be automatically notified (via email) when their Full Supply or COU MGBS is available from their 'Data Package' area on the OS Data Hub, in accordance with the dates stated on the [Product Refresh Dates page](https://docs.os.uk/os-downloads/resources/product-resources/product-refresh-dates#os-mastermap-topography-layer).

OS Orders users will automatically receive their updates (Full Supply or COU) on DVD or hard drive, either quarterly or every six weeks (whichever option was selected at initial order).

MGBS users and OS Licensed Partners that take GB coverage can benefit from:

* Data arriving faster and in a more predictable and timely manner
* Seeing the same version of features as other organisations
* Easier data management
* A Feature Validation Dataset (FVDS) with all COU orders or with Full Supplies on request (GML format only)


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