Tolerance of County Series

Please can you provide me with the tolerance of the Cassini method and the tolerance of the County series 1924 and 1956 maps of xxxxxx?

Please would you tell me the most accurate of the country series?
Also, is it possible to have a copy of the current mapping of the boundary between xxxxxx and xxxxxx?

Thank you for your e-mail dated 27^th September 2010. We are pleased to provide you with the following information with regard to your request.

The 1924 County Series mapping was originally surveyed using the Cassini Projection in 1886-87, and was subsequently revised in 1900 before the revision of 1924.  These map sheets were later “re cast” onto the National Grid which is based upon a re-centred Transverse Mercator Projection.  The mapping on national grid sheet lines were ‘overhauled’ using a process called the “Cotswold” method, which would have been the method used in the 1956 version to which you refer.

As an indication of local (relative accuracy) of the two variants of the mapping, which would be most relevant to interest in the accuracy of the map over a small area, we quote below the survey tolerances used in the County Series and the National Grid “overhaul” mapping.  The numerical data we have available for tolerance or accuracy (as is it later referred) is as follows:

However, it must be understood that accuracy can be defined in more than one way.  Relative accuracy as referred to in the National Grid overhaul above, is the degree to which the relationship, orientation and separation of one feature from another is as shown on the map to replicate the ‘true’ situation on the ground.

It may also be of assistance to note Geometric fidelity, is the degree to which the shape of a feature on the ground can be represented at the map scale in question.  For example: for 1:2500 mapping this usually means that linear features such as streams are shown with two lines, one for each edge of the water if wider than 2 metres, but with a single line representing the centre of the river if less than 2.0 metres wide.

Unfortunately the figures in the above table do not actually compare like with like since the survey methods for the County Series mapping were predominantly based upon Chain Survey, whereas the overhaul of the National Grid plans relied mainly on a method unique to Ordnance Survey, known as “Graphic Survey” where much of the new map detail was fixed using lines of sight connecting multiple points such as hedge junctions or building corners on the ground being replicated on the map.  These alignments were supplemented where necessary by reference measurements and some detail fixed by chain survey techniques, to complete the geometry of the map.

This, together with the effects of the different projections mean that it is very difficult to make a direct comparison.  Therefore, we are unable to comment which of the County Series mapping is the most accurate. However, since the overall target was to achieve a relative accuracy of no more than 1:500 over the wider area of the map, there is some logic that ‘relative’ accuracy is probably broadly similar between the two editions.

In relation to your third point, I have today send to you an extract of current Ordnance Survey mapping, showing the current captured boundary between xxxxxx and xxxxxx at two scales, 1:1000 and 1:500.  These files would be too large to send electronically, and the scanning and reproduction of such images may distort the information contained within.  However it is important to note that the features were surveyed at a scale equivalent to 1:2500 and so the extracts will simply enlarge any small discrepancies inherent in the base mapping.  The enlargement does not make the extracts any more accurate.

The date of capture of this mapping varies by feature. The last change to this area of mapping was made in 2006, to the southern edge of the road xxxxxx, and not to the boundary between xxxxxx and xxxxxx. The date we have recorded for the last change or update to the boundary in question was in 2004.

Ordnance Survey maps do not show legal boundaries or property ownership. If you feel the position of a boundary fence is incorrect, you should contact Land Registry to establish if they can assist. Ordnance Survey is unable to assist with boundary disputes. To clarify, if Ordnance Survey were to survey xxxxxx and xxxxxx today, we would only capture the features as they appear on the ground. Therefore, the fence between xxxxxx and xxxxxx (which you believe is in the wrong position) would be captured as it is, and would not assist.

Please click on the link below, which details further information about boundary disputes, which appears on the Ordnance Survey website:

http://www.ordnancesurvey.co.uk/oswebsite/site/contact/boundaries.html

In addition, I refer you to Land Registry’s Public Guide 19: Title plans and boundaries, please click the link below to access this information:

http://www1.landregistry.gov.uk/assets/library/documents/public_guide_019.pdf

Please note that 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
Romsey Road
SOUTHAMPTON
SO16 4GU 

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: FOI1098/September 2010