Grid Reference Accuracy/ Precision

The production of milestone map files for Google Earth has certainly focused attention on grid references. Many people have expressed concern that their favourite milestones are shown way off position, for example in trees or household gardens, and have been putting in a lot of effort to provide improved grid references.


Many of the current milestone grid references in the National database were produced many years ago when the only method was direct measurement on Ordnance Survey maps. These early surveys adopted 3 figure references for the easting and northing components as this was the maximum precision that could be achieved by direct measurement. Today, there are tools such as SatNavs and other GPS position devices that can provide much more precise references and we are now standardising on 5 figure grid references as a result.


There needs to be a distinction drawn here between accuracy and precision of grid references. Accuracy refers to correctness to the number of figures provided and precision refers to the number of figures provided. The majority of the 3 figure grid references are correct as far as they go in terms of significant figures. However, they are not very precise because 3 figure references only give positioning within a 100 metre square. Conversely,there are many instances of inaccurate grid references as the result of a wrong reading or transcription error that to date have been difficult to pick up until the Google Earth mapping presentation.

Thus, there is therefore really a 2 fold exercise, correcting any inaccuracies in grid references and also bringing all grid references up to 5 figures.

SatNavs and other GPS position devices are not without their problems, however. There are reports that the USA military has deliberately restricted the precision of provided satellite readings. Also, the less expensive devices provide readings only in latitude and longitude coordinates, which you have to convert to OS grid references. We have provided a link to the Nearby site that will do the conversion for you but it is still an extra step. Finally members have reported problems of obtaining settled readings from SatNavs. We would be interested in your experiences of using SatNavs to obtain grid references and the accuracy of readings.

You may not know that you can also use Google Earth or Google Maps to obtain accurate grid references, indulging in what has been termed ‘armchair milestone bagging’. A  further article providing more details will be published next month.


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