OS MasterMap plus Getmapping viewing differences
Getmapping PLC's detailed 25cm resolution imagery provides an extra dimension of detail to the Ordnance Survey's OS MasterMap dataset. However, there are a number of factors which may lead to some differences in accuracy and appearance:Positional Accuracy Improvement Scheme:
The PAI scheme is a process which Ordnance Survey has been undergoing for a number of years. In order to combat data errors caused by the original digitsing of data from paper plans, whole areas have been moved and adjusted to more accurately represent their true position with respect to the National Grid.
As much of this work was done after the intial creation of the Getmapping Millennium Map for England and Wales, the user may find that the imagery is offset slightly from the Ordnance Survey mapping by up to a number of metres. Whilst this is visible in the resulting mapping, it does not detract from the additional information contained in the aerial imagery.
It should be noted that in general, these effects are more commonly seen in 'rural' and in particular 'moorland' areas. This is as a result of the errors in the original paper mapping being moved into less critical areas.
More recently, Getmapping now corrects it's mapping using alternative means with the result that new and remapped areas are in-line with the Ordnance Survey mapping.
Leaning Buildings:
Ordnance Survey OS MasterMap's topographic layer provides mapping indicating the location of surface features including a buildings 'footprint'. Aerial imagery captures the highest surface and where a building was not directly below the aircraft, it may be seen to lean away from the camera. The result being that the roof line is not in the same position as the footprint and thus the visible building line being offset to the Ordnance Survey mapping.
This effect becomes more noticable the taller the building and the further the building from the centre of the original photo.
Data currency:
Ordnance Survey OS MasterMap is updated continually, with updates made live to the public in regular intervals. Due to the nature of capture of aerial imagery, the process involves mapping the country in a steadily built up process repeated over a number of years with some areas such as Scotland being captured more slowly due to shorter flying seasons.
The result is that whilst OS MasterMap is corrected where changes have occurred and is able to continually be updated, the aerial imagery may become several years out of date before being updated.
There are however occassions where Ordnance Survey are not made aware or do not have access to changes and are thus unable to update their mapping. By it's very nature the aerial imagery recaptures every feature rather than only those which have changed and thus an area will be completely correct at the time of capture.
The result is that one format may contain information which has either since changed or which was not captured in the other.
Interpretation of features:
Ordnance Surveys' OS MasterMap topographic layer is a representation of surface features. As such, the detail captured in the data may be simpler in appearance when compared directly to the aerial imagery. This is generally only found where there are 'soft' features such as the edges of gravel drives etc which may have been obscured at the time of mapping but not in the aerial imagery.
It is also the case that some features are simply outside of the remit of data capture for the Ordnance Surveys' dataset and this is one of the real benefits of aerial imagery.
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