FOI16706 – Request for information
Thank you for your email of 29 September 2016, requesting the following information in accordance with the Freedom of Information Act (FOIA) 2000:
I am requesting information on the detailed instructions on the carrying out of the PAI (Positional Accuracy Improvement) programme in the early 2000s.
Would you please supply me with a copy of the instructions issued to the photogrammetric surveyors on how to produce the revised OS maps using the stereo aerial photographs available to them for PAI program and the old 1;2500 maps.
I understand fully that the overall objective was to improve the absolute accuracy of the position of features in relation to the Ordnance Survey National Grid.
I am specifically interested, for the rural town's part of the programme, in whether the surveyors were instructed to just adjust the position of "features" that existed on the previous 1:2500 map, to bring them into line with the National Grid. Or, when they saw a "feature" that had obviously changed, what were their instructions with regards a) buildings/structures and b) other features such as fences, hedges etc. which seemed to be boundaries.
- [a] Specifically, where they could see on the stereo photograph that a building had changed shape significantly due to, for example, a large extension or new garage that had no previously been recorded, were they to correct the outline of the building/add the garage or just move the previous 1:2500 outline to a more accurate position relative to the OS National Grid?
- [b] Similarly, where they could see on the aerial photograph a fence/hedge etc. which looked to be a boundary feature and where a boundary feature line also existed in a similar position on the previous 1:2500 map but the boundary feature in the aerial photograph was plainly at a different angle to the 1:2500 feature, were they to adjust the angle of the feature, or just to copy the old feature across correcting just its position to bring it into line with the National Grid?
- [c] Similarly, where they could see on the aerial photograph a fence/hedge etc. which looked to be a boundary feature and where a boundary feature line also existed in a similar position on the previous 1:2500 map but the boundary feature in the aerial photograph was plainly in a different position relative to the associated properties than in the 1:2500 map feature, were they to adjust the position of the feature, or just copy the old feature across correcting it only to bring it in line with the National Grid? There would obviously be a different decision for a 100mm difference than for a 1000mm or a 5000mm difference. I understand completely the position with regards to the General Boundaries Rule.
Unfortunately, Ordnance Survey does not hold a copy of the requested instructions issues to photogrammetric surveyors for the PAI Program.
While confirming that the information was not held, the relevant business areas have provided answers to the questions in the “background” section of your request, which are provided below, under Section 16 of the FOIA 'the duty to provide advice and assistance'.
- [a] Surveyors instructed to move the building and associated features to the meet Geometric Fidelity, Relative Accuracy, and Absolute Accuracy standards, and to update the shape of the feature to reflect what exists on the photography.
- [b] Surveyors instructed to adjust the angle of the feature to meet Geometric Fidelity, Relative Accuracy, and Absolute Accuracy standards
- [c] Surveyors instructed to adjust the feature to meet Geometric Fidelity, Relative Accuracy, and Absolute Accuracy standards.
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