FOI17775 – Request for Information
We refer to your email of 13 June 2017, requesting information from Ordnance Survey in accordance with the Freedom of Information Act 2000 (FOIA) as follows:
Could you please provide me with the precise algorithm that is used to compute the coordinates included in your Code-Point Open dataset from the full Code-Point/Address Point/PAF delivery points data. I am looking for sufficient detail so that I can reproduce your calculations myself, to verify them from the source dataset.
For reference, this request is a result of a twitter thread.
According to Code-Point Open User Guide the coordinates in Code-Point Open are calculated as follows:
The point is given the ADDRESS-POINT® coordinates of the nearest delivery point to the calculated mean position of the delivery points in the unit". This to me suggests that you take the arithmetic mean of the coordinates of all the address points with each postcode, and then return the coordinates of the nearest address point (possibly in that postcode) to that mean position.
However, another commentator has suggested that instead you use a geometric centroid of a postcode polygon and return the coordinates of the nearest delivery point to that centroid. But if you do this, it's not clear how you define the polygon -- is it the postcode polygon from Code-Point with Polygons, or is it just the convex hull of the delivery points. If the former, what do you do if there are multiple disconnected polygons?
If it turns out that you are not using the arithmetic mean of the coordinates, then you might also like to consider updating your Code-Point Open documentation to clarify things for other users.
You have requested the precise algorithm used to compute the coordinates included in Code-Point Open, and you have also asked two hypothetical questions in your paragraph commencing “However, another commentator…”.
In relation to your hypothetical questions, we can confirm that your understanding of the process that we use is the better one, i.e. we do not use a geometric centroid. As such, the hypothetical questions do not arise.
To be clear, we identify the building closest to the mean position from all the current addressable properties that share the same postcode, with a high indicator of positional accuracy, and place the code-point 'dot' in said building. We have, as suggested in your request, considered updating our Code-Point Open documentation to clarify this point, and confirm that we are intending to do so by the end of August.
In relation to the precise algorithm, I confirm that Ordnance Survey does hold the information that you have requested. However, we are not providing a copy of the information, as we consider it to be exempt from disclosure under section 43 of the FOIA, as explained below.
Section 43(2) Prejudice to commercial Interests
Under section 43(2) of the FOIA, information is exempt if its disclosure would, or would be likely to,
prejudice the commercial interests of any person.
In this case, we consider that disclosure of the precise algorithm would be likely to prejudice the commercial interests of Ordnance Survey. The precise algorithm comprises a substantial amount of code, which has taken several weeks of work to produce. As you might expect, this code comprises commercially valuable information; note that parts or all of the code are likely to be useful in future commercial applications.
This is a qualified exemption and we are required to consider the public interest.
Public Interest Test
Ordnance Survey recognises the need for transparency. However, this must be balanced against the public interest in allowing the organisation to protect its commercial information and not place itself at a disadvantage in the competitive market in which it operates. Section 43(2) is a prejudice based exemption, and there is a public interest inherent in avoiding the harm specified. Ordnance Survey considers that the prejudice would be likely to occur and would involve a material adverse effect on Ordnance Survey’s revenues. We are not aware of a significant wider public interest in disclosing the algorithm.
Having considered the above, we are satisfied that the public interest in maintaining the exemption outweighs the public interest in disclosure.
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