Request for information – FOI13407
Thank you for your e-mail of 15 August 2013, which we acknowledge, requesting information from Ordnance Survey in accordance with the Freedom of Information Act (FOIA) 2000.
The Freedom of Information Act is for the provision of information which is held by an authority in a 'recorded format'. Ordnance Survey does not hold the requested information in a recorded format and therefore no documentation can be provided in relation to your request.
However, under Section 16 of the FOIA 'the duty to provide advice and assistance' we have provided some detail and knowledge of the issue – as assistance in this matter.
1. I would like to know how features that may vary in size over time e.g. a hedge, are captured and shown on Ordnance Survey mapping and how you decide on the position of the line?
Ordnance Survey surveys the alignment of the centre of roots of hedges.
In recognition of the topographic nature of Ordnance Survey mapping and given the largest scales at which Ordnance Survey maps (1:1250 in urban areas; 1:2500 in developed rural areas including small towns and villages – such as Beckford, Worcestershire) minor irregularities in the centre alignment of the roots of hedges are ignored and our surveyors use experience and judgement to define the most appropriate "general alignment" of the feature.
At the rural scale of 1:2500, traditionally measurements have been taken to 0.4m with the smallest dimension mapped normally being 1.0m. For clarity on the map, where there are adjacent features closer than 2.0m – such as a hedge and adjacent "parallel" fence, the surveyor will determine which of the two features is the most significant and portray that. For example, in the case of a hedge with an adjacent 'protective' wire fence designed to prevent livestock from breaking through the hedgerow, normally the hedge will be shown; whereas in – e.g.: ancient parkland where a substantial iron railing fence has a hedge planted next to it for more ornamental reasons, and where (say) the fence may be a much more continuous feature over distance, then it is likely that the fence would be shown.
Where a hedge runs in close proximity to the edge of a hard metalled surface, such as a tarmac or concrete road carriageway or a gravel track or drive, if the separation is less than 2.0m, the edge of metalling (shown as a broken [pecked] line) will be omitted under the close proximity of features rules.
In most cases, ancient hedges will have been surveyed onto a previous edition of the Ordnance Survey map (perhaps as far back as the 1880s), using ground survey methods. The surveyors will have physically identified the alignment of roots and surveyed them on the ground. Such alignments will have been carried forward to present editions, and unless there is very clear evidence that the feature is not shown in its current position, due to:
- very rarely "creep" when a combination of circumstances arises in that the hedge comprises species which send out 'sucker roots' sideways from the hedge line, and over time differential trimming has effectively forced the main growth to focus on previous offshoot roots;
- a previous survey inaccuracy, perhaps identified by more modern survey equipment;
then previously shown alignments are not changed. Ordnance Survey's revision practices require only additions for new ground features, deletions of now removed or demolished features and correction of identified inaccuracies in previous mapping. Changes to existing features will require the surveyor to be confident of the need for amendment and will apply only where there will be a discernible adjustment to the position on the map.
Using these principles, if photogrammetric methods are used (capture of survey information from aerial photography) then even though only the top of the canopy is visible on the aerial image, the photogrammetric surveyor will not amend a position of an existing hedge unless there is compelling evidence for such changes. It follows that, in taking the centre line of the canopy as an approximation for the centre line of roots, and using the principle of mapping the general alignment of the feature, the effects of differential trimming will be ignored unless there is evidence of a very significant misalignment between the previously mapped position and the now apparent alignment.
For more modern and recently planted hedges, the frequency of Ordnance Survey revision cycles means that such new hedges can be mapped within a reasonably short period of their having been planted. This means that ground survey can detect alignments of roots relatively easily, and if photogrammetric methods are used, the hedge is unlikely to have grown to a size where the centre-line of the canopy misrepresents the centre-line of the roots by a distance detectible at the scale of the mapping.
2. In particular, I am interested in the positions (of the lines) of the double hedge(walk) and the (what was very large) hedge that are either side of the drive area for the properties Wing House, Beckford Hall and Chapel House Beckford Hall GL20 7AA. I believe your ref is SO9735.
Inspection of superseded mapping suggested that the division of the property Beckford Hall, and the creation of the land parcel divisions which led to the formation of the walkway and driveway to the properties mentioned, took place between our 1973 post war overhaul of the 1:2500 map (which would have involved a ground-based survey operation) and the publication of a 1:10,000 scale map in 1978, where some of the relevant features appear. These would have been taken from the 1:2500 continuous revision master survey document held in the local field survey office of the time, and again the then new hedges and driveway would most likely have been surveyed by ground survey methods, quite possibly to support the registration of the newly created property subdivisions of the former Beckford Hall.
Such ground surveys would have involved adding the new features to the existing map by taped measurement and reference to the alignment and position of local features capable of being determined both on the ground and on the map. For example if, on the ground, a new feature such as a building corner fell on an alignment drawn between two other building corners or fence junctions, and these features could be identified on the map, then the new feature must lie along that alignment drawn on the map. Establishing multiple such relationships, supplemented by measurements whose distances are then scaled and plotted onto the map, enables the new features to be positioned correctly in relationship to the existing map detail. This technique is known as "graphic survey" and has been used to great effect in Ordnance Survey for at least 180 years.
As the requested information is not held by Ordnance Survey, we have determined that in all the circumstances of this case the Public interest consideration (section 17 FOIA) is not applicable in this instance.
Your enquiry has been processed according to the Freedom of Information Act (FOIA) 2000. If you are unhappy with our response, you may request an internal review with our FOI Internal Review Officer, by contacting them as follows:
FOI Internal Review Officer
Customer Service Centre
Please include the reference number above. You may request an internal review where you believe Ordnance Survey has:
- Failed to respond to your request within the time limits (normally 20 working days)
- Failed to tell you whether or not we hold the information
- Failed to provide the information you have requested
- Failed to explain the reasons for refusing a request
- Failed to correctly apply an exemption or exception
The FOI Internal Review Officer will not have been involved in the original decision. They will conduct an independent internal review and will inform you of the outcome of the review normally within 20 working days, but exceptionally within 40 working days, in line with the Information Commissioner’s guidance.
The FOI Internal Review Officer will either: uphold the original decision, provide an additional explanation of the exemption/s applied or release further information, if it is considered appropriate to do so.
Appeal to Information Commissioner's Office (ICO)
If, following the outcome of the internal review you remain unhappy with our response, you may raise an appeal with the Information Commissioner’s Office at:
The Case Reception Unit
Customer Service Team
The Information Commissioner’s Office
Telephone helpline: 0303 123 1113 or 01625 545745 for advice, Monday to Friday.
Thank you for your enquiry.