Accuracy and Tolerance of the Ordnance Survey Map

Accuracy and the OS Map

A whole range of answers can be given to the simple question “How accurate is the map?” The varying attributes of OS maps are influenced by the surveying and production processes, by the projection employed for the national surveys as a whole, as well as by the detailed specifications for individual map series.

OS define accuracy in three ways:

Absolute or Positional Accuracy - compares the location of a position scaled from a map with the true position on the ground i.e. how closely the coordinates of a point on the map agree with the coordinates of the same point on the ground (in the British National Grid reference system).

Relative Accuracy - compares the position of a data point or feature on the map in relation to other local data points or map feature.

Geometric Fidelity (Shape) – the ‘trueness’ of features to the shapes and alignments of the objects they represent i.e. any real-world alignment or shape must be accurately reflected in the map to the required specification

In general, relative accuracy is more important to map users than the positional. The absolute and relative accuracies of OS Large Scale Mapping are measurable and definitive statements of the expected errors of these by survey scale are given. Geometric fidelity (Shape) cannot be closely defined and is a matter for subjective judgement. The guideline is that the detail must be acceptable in terms of geometric fidelity when plotted or displayed at a scale no larger than the storage scale.

The OS carry out tests to determine how accurate the map is and if the result is unacceptable corrective action of either resurveying or changing the processes used will be taken. However on any one map sheet, different techniques may have been used to collect the detail thereon and so the accuracy may not be uniform. The following figures are given for guidance and relate to O.S. “basic scale” information - i.e. the actual scale of survey for any given area. Conversion tables are available to convert between metric and imperial or Scots units of measurement.

Useful Links

Conversion Tables

1. Absolute or Positional Accuracy

Compares the location of a position scaled from a map with the true position on the ground. The adjoining table represents the absolute accuracy applicable to the scale at which the product was surveyed.

It is important to note that that the Positional Accuracy Improvement (PAI) programme completed by OS in 2006 has resulted in two separate levels of Absolute accuracy for 1:2500 scale maps.

Accuracy is expressed as the Root Mean Square Error (RMSE), which is a measure of the distance from the true position within which about 67% of points would be expected to lie. The maximum expected geometric error on a map is about three times the RMSE.

Note: Certain types of feature, such as road centrelines, vegetation, tidelines, landform limits and underground features, are surveyed to a lesser degree of accuracy. Road centrelines, vegetation and landform limits are subjective and are not surveyed while underground features may have been supplied by third parties.

Original Survey Scale	99% of points should be in error by no more than	95% of points should be in error by no more than	Root Mean Square Error (RMSE)
1:1250	0.9m	0.8m	0.5m
1:2500 (outside of built-up areas in defined rural towns)	2.4m	1.9m	1.1m
1:2500 (built-up areas in defined rural towns)	0.7m	0.9m	0.4m
1:10,000	8.8m	7.1m	4.1m

2. Relative Accuracy

Compares the position of a data point or feature on the map in relation to other local data points or map feature. In general, relative accuracy is more important to map users than the positional.

Relative accuracy is normally expressed as a constant plus an amount proportional to the distance measured. The constant is related to survey practise - the accuracy to which measurements are taken and detail plotted).

It is important to note that that the Positional Accuracy Improvement programme completed by OS in 2006 has resulted in two separate levels of Relative accuracy for 1:2500 scale maps.

The relative accuracy criteria are perhaps best understood by using some examples:

1:1250 - If the distances between two well defined points of detail 60.0m apart were measured in the real world, there would be an expectation that 95% would be represented in OS Mapping by a scaled distance of between 59.1m and 60.9m
1:2500 - If the distances between two well defined points of detail 100.0m apart were measured in the real world, there would be an expectation that 95% would be represented in OS Mapping by a scaled distance of between 98.1m and 101.9m
1:2500 (outside of built-up areas in defined rural towns) - If the distances between two well defined points of detail 100.0m apart were measured in the real world, there would be an expectation that 95% would be represented in OS Mapping by a scaled distance of between 98.2m and 101.8m
1:2500 (built up areas in defined rural towns) - If the distances between two well defined points of detail 100.0m apart were measured in the real world, there would be an expectation that 95% would be represented in OS Mapping by a scaled distance of between 99.3m and 100.7m
1:10 000 - If the distances between two well defined points of detail 500.0m apart were measured in the real world, there would be an expectation that 95% would be represented in OS Mapping by a scaled distance of between 492.3m and 507.7m

Original Survey Scale	99% confidence level	95% confidence level	Root Mean Square Error (RMSE)
1:1250	+/- 1.1m (up to 60m)	+/- 0.9m (up to 60m)	+/- 0.5m (up to 60m)
1:2500	+/- 2.5m (up to 100m)	+/- 1.9m (up to 100m)	+/- 1.0m (up to 100m)
1:2500 (outside of built-up areas in defined rural towns)	+/- 2.3m (up to 100m)	+/- 1.8m (up to 100m)	+/- 0.9m (up to 100m)
1:2500 (built-up areas in defined rural towns)	+/- 0.9m (up to 60m)	+/- 0.7m (up to 100m)	+/- 0.4m (up to 60m)
1:10,000	+/- 10.1m (up to 500m)	+/- 7.7m (up to 500m)	+/- 4m (up to 500m)

3. Geometric Fidelity, or the Accuracy of Shape

The principle of geometric fidelity is that any real-world alignment or shape must be accurately reflected in the data to the required specification, for example:

detail that is square on the ground is represented as square in the data, and shapes must be accurate;

alignments that are straight in the real world are represented as straight lines within the data;

lines of sight that pass through ground points should, when plotted at the scale of the original survey, pass through the plan positions of the corresponding points; and

adjacent features are in sympathy with each other as regards alignment and orientation.

Normally, geometric fidelity takes priority over relative and absolute accuracy.

It should also be noted that generalisation of features on the map will have an effect on Geometric Fidelity. The amount of generalisation will be dependent on the criteria for the base scale of the particular map.