> 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/release-notes/april-2026.md).

# April 2026

The April 2026 release note for the OS MasterMap Topography Layer – Building Height Attribute product

Building Height Attribute (BHA) is an enhancement to, and forms part of, OS MasterMap Topography Layer. It provides a set of height attributes with a buildings theme within OS MasterMap Topography Layer.

## Building Height Attribute tiles

* **Total tiles**: 9 972
* **New tiles**: 1 (NT4080.csv)
* **Deleted tiles**: 0

## Extent

The April 2026 BHA release contains building heights for **45 864 850** unique buildings, tanks, chimneys and glasshouses across Great Britain.

The total number of features is **46 048 490**, including duplicate TOIDs (these are recorded when features overlap at tile edges).

## Limitations

Some building TOIDs (Topographic Identifiers) in OS MasterMap Topography Layer will have no corresponding height values in the BHA data in areas of coverage. This can occur because of different update cycles or in instances when a building was constructed after the capture of the source BHA data. This is also the case when a building has failed quality validation checks and there is no existing data to replace it.

Building height values are generated from a fully automated process and, as such, the accuracy of the values is subject to tolerances governed by the process. Ordnance Survey specifically disclaims any warranty that OS MasterMap Topography Layer – BHA is of satisfactory quality or fitness for a particular purpose. We further disclaim any warranty that OS MasterMap Topography Layer – BHA is error free or accurate.

## Next release

The next release of OS MasterMap Topography Layer – BHA is scheduled for **October 2026**.

## Improvements

The April 2026 release includes the below improvements:

* An updated **BHA\_ProcessDate** for each OS\_TOPO\_TOID.
* Enhanced **height validation rules** have been introduced, resulting in two small changes that may be noticeable in the product:
  * Building features with a geometry area less than 8m² will not have a height value.
  * The AbsHMax will always be greater than the AbsH2, whereas previously it could be the case that the roof edge was higher than the roof maximum.
* Enhancements to the calculation and population of the **Confidence Level attribute**. This provides new, clearer definitions for each confidence level to facilitate more informed data-driven decisions. These definitions and source data will be consistent with the height data already being published in the [OS NGD Buildings Theme](https://docs.os.uk/osngd/data-structure/buildings).


---

# 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 by asking a question.

Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters:

```
GET https://docs.os.uk/os-downloads/products/buildings-and-infrastructure-portfolio/os-mastermap-topography-layer-building-height-attribute/release-notes/april-2026.md?ask=<question>&goal=<user_goal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case.

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.