Request for information – FOI13394
Thank you for your e-mail of 23 July 2013, requesting information from Ordnance Survey in accordance with the Freedom of Information Act (FOIA) 2000.
'On 14/08/2013, Ordnance Survey completed a site survey of our property in xxxxxx. Some surprising results emerged and these were at variance with our own GPS commissioned survey. We would therefore request under the Freedom of Information Act, the actual site readings taken by your equipment, based on National Grid references.
I attach a sketch (omitted from our response) showing the particular points of interest. If any points were not read during this survey, would you please provide the 'off Mastermap' coordinates, and advise as such. Our GPS result is for instance, of point 1, is: 317845.913 703495.2869
Can you supply your points for both horizontal and corrected for the considerable height differences? I include a schematic drawing of the key points. These are: 1 , 6, 7, 5/C, 4/B, A, 2E, 3/D, AG, 11, F, GF, 10/A'.
Ordnance Survey does hold the information to respond to your information request, and we respond as follows:
Our data investigation department have taken a 'screen shot' of the raw data as entered by the surveyor at the time of ground survey, for the area in question, and provided the GPS readings (i.e. the true national grid positions) for the control points in the Ordnance Survey data. We have endeavored to use control points that correspond with the supplied sketch but please note that the points below may not correspond exactly to the points indicated on the sketch.
We have provided coordinates read from the GB04 data where there are no control points.
Here is the key for the attached bmp. X and Y co-ordinates followed by the Z (height) value. These values are derived directly from GNSS observations.
1. 317849.75, 703485.83 Z value 206.60
2. 317762.69, 703425.87 Z value 189.48
3. 317737.76, 703404.86 Z value 185.69
4. 317685.72, 703360.08 Z value 176.79
5. 317786.56, 703398.54 Z value 192.28
6. 317784.65, 703389.54 Z value 191.85
Here are the coordinates held by positioning the cursor on the screen where there are no control points.
A. 317846, 703496
B. 317716, 703333
C. 317774, 703389
D. 317822, 703370
This survey was controlled by GPS points of detail, such that the set of features in question can be expected to fall well within the expected accuracy standard for Ordnance Survey mapping at this scale.
In this case, for 1:2 500 standard mapping, the expected root mean square error of absolute accuracy is 1.1m, the expected relative accuracy for measured distances up to 100m is ±.1.0m.
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:
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Thank you for your enquiry.