I have lifted this directly from www.police.uk as we use the same data.
A key challenge for the www.police.uk website and API is striking a balance between being open and transparent about crime, and our obligations to protect the identity and privacy of individual victims.
For this reason, the icons you see on the Crime Maps page always represent the approximate location of a crime – not the exact place that it happened. Anonymising crime location is one of the steps we have taken to help protect the privacy of victims, based on consultation with the Information Commissioner’s Office.
How are crime locations anonymised?
We maintain a master list of over 750,000 ‘anonymous’ map points. Each map point is specifically chosen so that it:
Appears over the centre point of a street, above a public place such as a Park or Airport, or above a commercial premise like a Shopping Centre or Nightclub.
Has a catchment area which contains eight or more postal addresses (or no postal addresses at all).
When crime data is provided by police forces, the exact location of each crime is compared against the master list to find the nearest map point. The co-ordinates of the actual crime are then replaced with the co-ordinates of the map point.
No other filtering or rules are applied, and the original location information is deleted. This process ensures that we can provide crime information that is accurate, transparent and locally relevant, whilst maintaining the anonymity of victims.
How was the master list of map points created?
In summary, the process used to creating the master list was as follows:
The centre point of every road in England and Wales was taken from the Ordnance Survey Locator dataset.
This was augmented with locally relevant ‘points of interest’ from the Landmark Points of Interest dataset.
Subsequently, each map point was analysed to see how many postal addresses were contained in its ‘catchment area’ (according to the Ordnance Survey Address-Point dataset). Any with between 1 and 7 postal addresses were discarded to protect privacy.
The remaining points were provided to police forces for a human assessment. A small number of additions and deletions were made based on their feedback to make the map points more locally relevant.”
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