> For the complete documentation index, see [llms.txt](https://docs.os.uk/os-downloads/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.os.uk/os-downloads/products/buildings-and-infrastructure-portfolio/os-mastermap-topography-layer-building-height-attribute/os-mastermap-topography-layer-building-height-attribute-overview/product-data.md).

# Product data

## Data currency and completeness <a href="#bookmark17" id="bookmark17"></a>

Building Height Attribute data is derived from the latest source Digital Terrain Model (DTM) and Digital Surface Model (DSM).

## Header information <a href="#bookmark18" id="bookmark18"></a>

For the purposes of simplifying the merging process, the individual 5x5km² tiles are supplied without header information. Instructions on the merging process can be found in the [Getting Started Guide](/os-downloads/products/buildings-and-infrastructure-portfolio/os-mastermap-topography-layer-building-height-attribute/os-mastermap-topography-layer-building-height-attribute-getting-started-guide.md).

The format and structure of the individual columns within the data will be consistent (see table below).

### Building Height Attribute header information

<table data-header-hidden data-full-width="true"><thead><tr><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th><th></th></tr></thead><tbody><tr><td><strong>OS_Topo_ TOID</strong></td><td><strong>OS_Topo_TOID_ Version</strong></td><td><strong>BHA_Proces sDate</strong></td><td><strong>Tile Ref</strong></td><td><strong>AbsH Min</strong></td><td><strong>Abs H2</strong></td><td><strong>AbsH Max</strong></td><td><strong>Rel H2</strong></td><td><strong>RelH Max</strong></td><td><p><strong>BHA_</strong></p><p><strong>Conf</strong></p></td></tr></tbody></table>

The **Building Height Attribute header file** can be downloaded from the ['OS MasterMap Topography Layer – Building Height Attribute Downloads' page](/os-downloads/products/buildings-and-infrastructure-portfolio/os-mastermap-topography-layer-building-height-attribute/os-mastermap-topography-layer-building-height-attribute-downloads.md).

## Data precision <a href="#bookmark19" id="bookmark19"></a>

All heights have been calculated in metres to one decimal place (decimetre).

## Height datum <a href="#bookmark20" id="bookmark20"></a>

The Building Height Attribute dataset uses Ordnance Datum Newlyn (ODN) as the national height datum for all height values across mainland Britain. Other British height datums (for example, Lerwick for the Shetland Islands) are used where applicable. All height datums are incorporated within the National Geoid Model OSGM15.

The following two guides can help you gain further insight into coordinate systems in Great Britain:

* [A Guide to Coordinate Systems in Great Britain](https://docs.os.uk/more-than-maps/deep-dive/a-guide-to-coordinate-systems-in-great-britain)
* [Beginner's guide to grid references](https://getoutside.ordnancesurvey.co.uk/guides/beginners-guide-to-grid-references/)

<figure><img src="/files/2p5qzDogc0oE2q3cy9xk" alt="Example image depicting OS MasterMap Topography Layer – Building Height Attribute displayed over OS Terrain 5 and OS MasterMap Imagery Layer." width="563"><figcaption><p><em>Image depicting OS MasterMap Topography Layer – Building Height Attribute displayed over OS Terrain 5 and OS MasterMap Imagery Layer.</em></p></figcaption></figure>


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## 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, and the optional `goal` query parameter:

```
GET https://docs.os.uk/os-downloads/products/buildings-and-infrastructure-portfolio/os-mastermap-topography-layer-building-height-attribute/os-mastermap-topography-layer-building-height-attribute-overview/product-data.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

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.