> 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/areas-and-zones-portfolio/os-open-built-up-areas/os-open-built-up-areas-technical-specification/methodology.md).

# Methodology

## Methodology <a href="#id-1.4-methodology" id="id-1.4-methodology"></a>

The OS Open Built Up Areas dataset is created by following the methodology detailed below:

{% stepper %}
{% step %}

### **Identify built-up cells**

25m by 25m cells are flagged as built up based on the proportion of the cell that is classified as Building, Road, Residential Garden, Made Surface, or built-up types of Land Use Site.
{% endstep %}

{% step %}

### **Assign names to each cell**

Each cell is assigned a name from the Settlement Named Area dataset.
{% endstep %}

{% step %}

### Create initial built-up area polygons

The following rules are used to create the intitial polygons:

* Adjacent, identically-named 25m x 25m cells are merged together.
* Adjacent built-up areas that are not Cities or Towns are merged together.
* Smaller built-up areas are merged into nearby larger built-up areas.
  {% endstep %}

{% step %}

### Filter to create final geometry

* Built-up areas >=200 000m2 are retained and become Built Up Extents.
* Built Up Areas and Non Built Up Extents are then generated from the Built Up Extents.
  {% endstep %}

{% step %}

### **Refine names**

The name or names of the most significant constituent settlements are applied to Built Up Areas that are not Cities or Towns. The following rules are used to refine settlement names:

* If two or more Built Up Areas have the same name, the name of the lowest-tier local authority in which the Built Up Area is located is appended in brackets.
* If two or more Built Up Areas have the same name within a single local authority, the name of the parish in which the Built Up Areas is located is appended in brackets.
* If two or more Built Up Areas have the same name within a single parish, the name of the nearest Built Up Area is appended in brackets.
  {% endstep %}

{% step %}

### **Assign GSS codes**

GSS codes, supplied by ONS (for England and Wales) and Scottish Government, are applied to the three feature types.
{% endstep %}
{% endstepper %}


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