How to create a choropleth (fill) map using ONS geographies

It’s easy to create a choropleth (or fill) map using Office for National Statistics (ONS) geographies – Lower layer Super Output Areas (LSOAs), wards, etc. – in the OS Maps for Power BI Visual, without requiring a separate geometry file.

To make the following steps work, your data will need a column that includes ONS Government Statistical Service (GSS) codes.

GSS codes

GSS (Government Statistical Service) codes are nine-character alphanumeric codes which correspond to geographies that ONS supports, for example, an LSOA code looks like this: E01000917.

Examples of LSOA codes are shown in the table below:

LSOA Code	LSOA Name
E01000916	Camden 027B
E01000917	Camden 027C
E01000917	Camden 027C
E01000914	Camden 028B

To display these geographies as polygons on a map, simply drag and drop the column of data that has these codes into the Features Layer: GSS identifiers geocoding field of the visual.

An example of polygons displayed on a background map within the OS Maps for Power BI Visual.

Incorrect codes

Any incorrect codes won’t display a polygon on the map. The notification panel will display what codes did not work. Currently, the OS Maps for Power BI Visual does NOT support historic codes.

To colour the map and create the choropleth, drag the relevant column of data into the Features Layer: Colour field. This field can accept either numeric or text data. It’s important that every row within your data table has a GSS code (representing the polygon) and the relevant data to colour the polygon (numeric or text).

Interpreting numeric data

If using numeric data, then the Features Layer: Colour field allows several mathematical operations to be performed on the data (for example, Sum, Average, Count, etc.) to interpret it; these may need to be utilised to display the desired data correctly on the map.

How to filter map boundaries by an overarching geography

It’s quite common that a report reader will want to view a defined geography (for example, a local authority such as Cardiff) but see data on the map that represents an underlying geography, such as wards. To do this, it’s important that the underlying data is structured in the correct way. Each row should have both the geography to be displayed on the map (for example, ONS ward code – W05001272) and the geography to filter by (the local authority that the ward code belongs to; which in this case is Cardiff). The data might look like the following table:

Ward	Ward Code	Households	Local Authority
Trowbridge	W05001296	7 009	Cardiff
Whitchurch and Tongwynlais	W05001297	6 832	Cardiff
Allt-yr-yn	W05001627	3 989	Newport
Alway	W05001628	3 403	Newport

A filter can then be created to display the data, such as having a table listing the local authorities alongside the map. When a user clicks a local authority, the relevant wards and any associated data making up that local authority will be displayed on the map.

How to style a choropleth map

Once appropriate data has been dragged into the Features Layer: Colour field, then the map’s colours should update. To customise these colours, go into the Format Visual > Styling: Features Layer section.

Using text data to style the map

If using text data to colour the map, then a maximum of 12 colours will be automatically generated. These colours cannot be manually changed.

By changing the Min or Max Feature Colours, the map colours will update based on the minimum and maximum values of the data used. Opacity can also be altered to style the map.

Outlying data points

The Feature % Clip can be used to ignore outlier data points. It ignores the selected % minimum and maximum data points in the data. As an example, if 5% is specified, then the minimum 5% and maximum 5% of data points will be ignored when colouring the map.

Crime count by area displayed as shaded polygons against a background map of an area of London in the OS Maps for Power BI Visual.