Request for Information – FOI14560
Thank you for your email of 15 June 2015 and additional attachments on the 17 June, requesting information from Ordnance Survey in accordance with the Freedom of Information Act (FOIA) 2000.
The area around the village of XXXXXXXX, South Hampshire (typical postcode nearest post code XXXX XXX and approx Grid Ref XXXXXX XXXXX) was updated in September 2004, possibly as part of your PAI Programme (two specific properties were updated on the 29th September 2004.)
I attached copies of OS maps for 9 June 2004 and 19 March 2005 plus a map with the two combined maps together which show the differences.
In 2004 the OS using aerial photography/remote collection "adjusted" almost all the positions of the houses and boundaries for the whole area (OS unable to say whether as part of the PAI programme or not).
As you will see from the combined map, the repositioning of the houses and the boundaries are all very different. This would indicate that the mappers looked at each feature on the aerial photograph and "corrected" its position in relationship to the OS national grid. For example, the north-west boundary of xx moved very slightly to the east, whereas its south-east boundary moved by 1.2548 metres to the east, while xx itself only moved to the north.
My request under FOI is for a copy of the instructions issued to the mappers who undertook the 2004 adjustment. Specifically, how were they instructed to determine the (revised?) positions structures (i.e. houses, garages etc.) and how were they to determine the (revised?) position of boundaries (i.e. if a neatly trimmed domestic hedge which appeared to be on/near a previous boundary line and was 2.0 metre wide, where were they instructed to place the (revised) boundary line, similarly with a one metre wide wall, and a standard post and rail fence?).
I appreciate fully the difference between OS boundaries and legal boundaries etc. and the tolerances on features shown on OS Maps. This FOI request is solely to discover how in 2004, using aerial photographs, the mappers were instructed to determine the position of features - structure and boundaries.
Ordnance Survey does not hold a copy of the instructions issued to the mappers who undertook the 2004 adjustment. However, under Section 16 of the FOIA, ‘the duty to provide advice and assistance’ we can provide the following information which may assist you in this matter.
With regard to the Positional Accuracy Improvement (PAI) programme all staff worked to existing data specification documents with the aim of improving stated accuracies. Both the PAI target accuracy and the survey guidance principles applied in 2004/5 still exist in our current documentation. Relevant extracts from the current Capture and Maintenance Guide (version 1.3.2 June 2015) are quoted below, full document available here (PDF).
Photogrammetrists (surveyors who work off aerial photography) who undertook PAI positioned definite detail in reference to aerial imagery and in accordance with Ordnance Survey specification (represented in our Capture and Maintenance Guide) which stipulates that building outlines represent the face of a building.
Our current specification states that “the external face of all building walls is normally captured”, and that walls depicted by a single line represent the centre-line of the boundary. Current specification wording states that “for walls of normal thickness represent central alignment”, where walls less than 2.0m wide are represented by a single line in 1:2500 mapping.
As far as possible the PAI adjustments were controlled by definite detail and indefinite detail, such as hedges were moved in sympathy. If a hedge was adjusted independently, it was positioned in accordance with the specification which stipulates that hedges depicted by a single line represent the root-line of the boundary.
Current specification wording states that the line is “captured to represent the central alignment of the roots of the hedge”. Differences in could be as a result of the control applied to the imagery used to undertake the PAI activity. This control (higher order accuracy points of detail easily identifiable in the imagery) was essential in ensuring the target accuracy could be achieved.
In practice, as large an area as appropriate was adjusted in sympathy to achieve a best-fit which met the required standard for positional accuracy and at the same time preserved valid existing relative accuracy. Small areas, or in exceptional circumstances individual features, were moved independently where isolated positional discrepancies were identified.
This approach is referenced in a PAI companion guide (2004) produced at that time. This document has been attached and is also still available on our dedicated PAI web page.
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