Reliance of accuracy

We are seeking to use a 1972 Ordnance Survey map sheet at 1:2500 to determine the width of a public right of way at a location in xxxxx.

The boundary to the north of the public right of way is fixed.  However the boundary to the south of the public right of way was formed by the exterior wall of a property that was constructed in 1965 and was still standing in 1972. 

The 1965 property has now been demolished and a new property constructed and it is impossible to tell from the remaining physical features on the ground where the boundary wall of the 1965 property was located. 

The Council are making an assumption that the full extent of the public right of way existed between the two boundary walls when the 1965 property was constructed and that the full width should still to be available to users of the path.

We therefore want to use measurements from the 1972 Ordnance Survey map to determine the extent of the highway.  Can you advise how accurate a 1:2500 scale plan would be and how much reliance we could place on the survey data provided by the Ordnance Survey?  Any information that you can provide on relative and positional accuracy at this scale would be helpful.

Thank you for your e-mail dated 4th November 2011. I confirm that Ordnance Survey does hold information to assist you with your request.

Below, is the relative accuracy surveying tolerances for the National Grid 1:2500 ‘Overhaul’ mapping, applicable to our 1972 mapping of xxxxxx.  This is taken from an internal publication called ‘The Red Book’ dated 1963, which was the surveyor’s instructions/work guide at that time:

Permissible tolerances (relative accuracy)

• Distances up to 200 metres - 1 metre in 100 metres
• Distances 200 – 1,000 metres – 2 metres
• Distances over 1, 000 metres – 1 metre in 500 metres

Under our duty to provide Advice and Assistance - Section 16 of the Freedom of Information Act – I detail the following to clarify the above information.

The above details the tolerance of 1:2500 ‘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 distances of over 1,000 metres a tolerance of 1 in 500 applies which equates to a ground distance of +/- 2 metres pre 1,000 metres measured.

However, when considering tolerances and relative accuracies over short distances, on the first edition of the National Grid 1:2500 scale 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 is 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 40cms.  This means that for practical purposes a tolerance / relative accuracy of circa 0,5m should be used for distances between adjacent features of up to 50m.

Please note your enquiry has been processed according to the Freedom of Information Act (FOIA) 2000. As all the information held by Ordnance Survey to assist with your request 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: FOI11233/November 2011