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Most accurate DTM data

Request for Information – FOI13368

Thank you for your e-mail of the 9 April 2013, which we acknowledge, following the outcome of our internal review of FOI13360 by xxxxxx, Internal Review Officer.

We note you are now requesting the following information from Ordnance Survey:

Please tell me whether the gridded DTM data available as part of OS Terrain 50 represents the most accurate DTM data that you hold in the vicinity of the requested grid reference (in terms both of horizontal grid resolution and what you believe to be the vertical accuracy).

It is important here to be clear exactly what data is what, as for example I see that there are differences between the PANORAMA and Terrain 50 products and that this difference is as much as 2 metres at a point only about 300m east of the requested grid reference.

As this information request differs from your initial FOI request, we are dealing with this as a separate information request under our reference: FOI13368.

Ordnance Survey does hold the information you have requested, and will shortly be uploading a detailed User Guide to our website, which will provide the answers to your information request.

In this instance we are able to consider the engagement of Section 22 of the FOIA ‘information intended for future publication’ and to consider exempting this information, as there is a clear intention to publish at a future date.

However, we have concluded there is greater public interest in the release of this information to you at this time, being the release of this guide is imminent and there is no value or purpose in withholding the information from you until the scheduled time or date of publication.

It will assist you when reviewing the following information to note the grid reference you have previously provided as being of interest falls within an ‘urban’ area.

We respond to your request with the following points:

1. OS Terrain™ 50 is not the highest resolution DTM that Ordnance Survey holds. It is however

  • the most detailed available under OS OpenData™ terms (i.e. for free),
  • derived from the same base content as our high resolution data (OS Terrain™ 5). See point 3 below for more detail on OS Terrain 5.

2. Land-form PANORAMA® is shortly to enter a 'notice period' and will be withdrawn in 12 months of that point. Panorama is a very different product from OS Terrain 50 in that it was produced for 'cartographic' purposes (i.e. as a contour set to be used on paper maps), while OS Terrain 50 is derived from our core height content store (which we call iDTM) which is captured as part of the photogrammetric maintenance programme (www.ordnancesurvey.co.uk/business-and-government/help-and-support/products/continuous-cyclic-revision.html).

3. Currently, our more accurate product is Land-Form Profile.

However, this is shortly to be replaced by an updated product called OS Terrain 5. OS Terrain 5 is also derived from the iDTM with a higher grid density (5m posting as opposed to 50m). OS Terrain 5 will be made available under licence and not under OS OpenData terms.

4. The z values of the source data (iDTM) have to meet positional accuracy requirements according to their geographic location. The terrain has been divided into four classifications; urban, major communication routes, rural and mountain and moorland to ensure that modelling reflects customer requirements. The accuracy of the height value above Newlyn Datum must achieve the root mean square error (RMSE) set for each area. Thus for both Terrain 5 and 50, the following accuracies can be used for the height value:

  • Urban: Urban areas are those with topographic mapping at a planimetric absolute accuracy (RMSE) of +/- 0.42 m plus all other settlements more than 1 km2 in area. ±1.5 m
  • Major communication routes: These are major road networks (as defined in OS MasterMap® Integrated Transport Network™ Layer and all railways represented within the Meridian™ 2 product). ±1.5 m
  • Rural: Areas with topographic mapping at a planimetric absolute accuracy (RMSE) of +/- 1.10 m plus any areas not already defined by the other geographic definitions. ±2.5 m
  • Mountain and moorland: Areas with topographic mapping at a planimetric absolute accuracy (RMSE) of +/- 4.09 m. ±5 m

The above geographic areas have been resolved into a 1 km grid dataset (except for the major communication routes) and are used to ensure the consistent depiction of features across Great Britain. Source height TINs produced for each classification have to achieve a target RMSE accuracy as shown.

Internal review
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
Ordnance Survey
Adanac Drive
Southampton
SO16 0AS

E-mail: foi@ordnancesurvey.co.uk

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 you remain unhappy with our response and handling of your Internal Review, you may now raise an appeal with the Information Commissioner’s Office at:

The Case Reception Unit
Customer Service Team
The Information Commissioner’s Office
Wycliffe House
Water Lane
Wilmslow
Cheshire
SK9 5AF

For further assistance you may e-mail the mail@ico.gsi.gov.uk or contact them on the telephone helpline: 0303 123 1113 or 01625 545745 for advice.

Thank you for your enquiry.

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