Ref No: FOI18853 - Request for information
In response to your email of 28 August 2018, in which you made an additional request for information from Ordnance Survey in accordance with the Freedom of Information Act (FOIA) 2000, as set out in the extract below:
OS previously confirmed that its Absolute Accuracy Statement cannot be relied on to calculate accurate measurements as it is based on the National average error taken from the National Grid to a surveyed feature.
Please provide from the records used to establish the National average error the largest error from the National Grid to a surveyed feature known to exist in the County of Dorset.
I confirm that Ordnance Survey does hold the information you have requested. The largest error found in the County of Dorset was 2.012m in a 1:1250 accuracy region and 3.04m in a 1:2500 accuracy region. Under the duty to provide information and assistance in accordance with section 16 of FOIA, we can provide the following information, to explain how the figures have been obtained.
Ordnance Survey have reviewed the National positional accuracy test results from which the National average errors were calculated, and we have filtered the results to the county of Dorset. Within the test results the largest error was returned against a survey standard that no longer exists: 1:2500 overhaul accuracy standard. Those geographic areas originally digitised against this standard were systematically improved as part of our positional accuracy improvement (PAI) programme. This programme finished in 2006. Find more information our PAI programme web page.
Not taking into account the 1:2500 overhaul accuracy standards the largest error found was 3.04m. The test date was March 1990. Given that this test point was measured within a 1:2500 resurvey and reformed accuracy standard area this measurement is still expected to be valid today. The point in error was 1 of 151 points tested. The location of the test (Test ID 736) is identified in the extract at figure 1 of the Portland area (jpg image). The root mean squared error (RMSE) (relevant average for this type of error) for the test was calculated at 0.603m. Statistically we would expect 63% of features to be more accurate than the RMSE.
In reviewing our test data, there are other results that we considered may be more relevant to you and your property. We have identified two test areas (Test ID 611 and 612, see figure 2 of the Bournemouth and Poole area (png image) that are more local and are also within the same capture specification region as your property: 1:1250 accuracy standard. This accuracy region, covering urban conurbations, is of a higher accuracy than our 1:2500 accuracy standards, current or historical. All the data within 500m of your address has been captured to 1:1250 Accuracy Standards.
Both tests were undertaken in November 1991. The largest error found in these tests was 2.012m. Combining the data from both test locations (611 and 612) 845 points were tested in total and result in an RMSE of 0.435m. Using this data we can also provide additional statistical confidence for this specific area:
- 95% of points in this area can be expected to be within 0.753m of their true position.
- 99.9% of points in this area can be expected to be within 1.143m of their true position.
For your reference only the nearest points measured in these tests were approximately 1500m from your address.
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