Application of information

This is a Freedom of Information request please. I would like to clarify how the information in the table you have sent me [FOI1098] can be applied and if Ordnance Survey can give advice and assistance on what it means.

Thank you for your e-mail dated 21^st July 2011 and for your telephone call to further clarify your request. We repond as follows:

In October 2010, Ordnance Survey provided you with a response to your Freedom of Information request reference FOI1098.  In our response we detailed the tolerance or accuracy of County Series and National Grid mapping used at the times detailed in your request, and shown below:

In our response we stated the information provided in the request was an indication of ‘relative accuracy’ of the two variants of the mapping, and that accuracy can be defined in more than one way.

Below, I further explanation the information provided to you in the above table which accompanied our previous response:

County Series

This is the relevant information we hold for our mapping from 1924. The County Series information details tolerance by a maximum discrepancy of links under the Cassini projection (previously explained). A link is 7.92 inches or 0.2 metres.

The maximum discrepancy is:

• For distances of up to 200 metres the tolerance is 1 part in 500 as a ratio, which equates to a maximum discrepancy between the measured distance between two points and the distance scaled from the map of +/- 0.4 metres on the ground;
• For distances of up to 1.5 miles, the maximum discrepancy is 15 links equating to +/- 3 metres on the ground  (15 x 0.2 metres).

National Grid

The National Grid information details the tolerance of 1:2,500 ‘overhaul’ mapping using the Cotswold method, worked on a ratio:

• For a distance of up to 200 metres the ratio is 1 in 100 of the measured distance.  Therefore the tolerance in every measured distance below 200 is calculated as a ratio.  For example the map tolerance for a measured distance of 190 metres equates to +/- 1.9 metres on the ground.
• For measured distances between 200 metres to 1,000 metres a tolerance on the map equivalent to +/- 2 metres on the ground applies; 
• For a distances of over 1,000 metres a tolerance of 1 in 500 applies which equates to a ground distance of +/- 2 metres per 1000 metres measured.

During your clarification telephone conversation with me, you asked me to explain how the National Grid tolerances could be applied to specified distances, specifically those less than a distance of 100 meters.  Mathematically the principle of a tolerance being applied to any distance within the scope of the tolerance is valid.  In theory, 1 part in 100 applies as much to distances of 100 metres (allowable tolerance 1m) as to 1 metre (allowable tolerance 1 cm).

However, for County Series and first edition National Grid mapping the reality is that we have to consider that much of the detail was plotted by hand onto a hard copy master survey drawing of the previously published map.  This meant there was a physical limit to the smallest distance which the surveyor could actually physically draw even with a very sharp pencil point. This was one of the reasons why we have/had minimum widths for building juts, streams depicted by two lines, close walls/fences etc – the surveyor couldn't actually plot anything closer at that scale.

Scaling between two very close points (say 5 metres apart) would mean that a tolerance of 1 part in 100 would not be practically achievable since the tolerance would equate to +/- 5cms which is well below the minimum plot-able distance equivalent to about 40 cms.

Your enquiry has been processed according to the Freedom of Information Act (FOIA) 2000. As all the requested information has been provided, we have determined that in all the circumstances of this case the Public interest consideration (section 17 FOIA) is not applicable in this instance.

If you are unhappy with our response, you may raise an appeal to our Appeals Officer at:

FOI Appeals Officer
Customer Service Centre
Ordnance Survey
Adanac Drive
SOUTHAMPTON
SO16 0AS

Please include the reference number below. The Appeals Officer will ensure that the process has been followed correctly, questioning any decisions taken regarding the original response and recommending disclosure of additional information if appropriate.

Thank you for your enquiry.

Reference number: FOI11202/July 2011