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 |
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:
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) |
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:
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.