BGS 50K - Further Information
The following are a few questions which regularly crop up through the use of British Geological Survey Geological Data:Is the geological mapping updated?
Yes, geological mapping for the British Isles is being updated constantly which is often a surprise to many people who are aware that the geology itself only changes over very long periods of time with Superficial (Drift) Geology often remaining largely unchanged for tens of thousands of years and Bedrock (Solid) Geology generally requiring millions of years.
The reason for the updates is therefore often not because the geology itself has undergone change but rather because those mapping the geology have changed their interpretation for several reasons including:
- overall growth in the knowledge of the geological community as cutting edge theories become recognised key principles upon which further advances can be made
- detailed studies in areas only mapped to a basic level previously
- new exposures for study through construction and excavation
- advances in technology including cheaper methods of obtaining geophysical and borehole data which enable more accurate mapping where the bedrock geology has never been exposed for study
Because the geological data is updated by British Geological Survey whilst the surface mapping is updated by Ordnance Survey you may find that some of the geology shown relates to a previous surface.
Good instances are where a new road has been built and artificial geology may not yet be shown for the area below the road has been built up or equally where the same road has required a cutting into the geology and exposed a different rock type.
Why are there changes in rock types at 'sheet' boundaries?
The 1:50,000 Geological data for the UK is based on the combining of information held for the traditional geological 'sheet' maps covering Great Britain. The data supplied by CENTREMAPSlive is based on this combined coverage which means that it is able to cross these 'sheets' and thus provide extracts of mapping anywhere, even across traditional 'sheet boundaries'.
This is discussed further in the information provided from the British Geological Survey. However, the main reason for differences is based around a couple of key issues:
The nature of geological mapping
It is important to understand that Geological mapping does not only rely on the ability to accurately record the position of features which is the critical issue for many other types of mapping such the topographic (surface) mapping produced for example by Ordnance Survey. Instead, the geologist must first make a decision as to what it is that he is looking at before he can map it.
There are plenty of books on geology and geological mapping with far more information than we are able to present here. It is easy to imagine though that the geologist must make difficult decisions to identify where there is a significant enough change in rock type to warrant the change being mapped and whether the change is so localised (perhaps a different rock type revealed at the surface through a series of complex faults) that it should not be mapped.
These issues are then made yet more difficult by the majority of what it is being mapped often being located several if not tens of metres below the surface superficial (drift) geology. Apart from exposures, brief glimpses into this underground domain are provided through Geophysical investigation which relies on expert interpretation and through boreholes which provide a narrow vertical record of the geological layers at the point of the hole.
Different Geologists mapping:
The above difficulties in the process of geological mapping become apparent most clearly through the means in which the Geology of the British Isles was traditionally (and quite appropriately at the time) methodically compiled on a 'sheet' by 'sheet' basis.
This means that to a certain degree a geological feature recognised by one group of geologists producing one map sheet may not have been given the same importance as the Geologists mapping a neighbouring sheet. This is not simply due to the difference in interpretation between one geologist and another but is also due to the fact that geological strata are not simply evenly laid down across the Earth.
For example, a deposit laid down across an area now covered by a number of map sheets may have been laid down more thickly in one location or eroded more heavily in another location. In one sheet, the feature may never be clearly exposed and thus is not shown at all whilst in another sheet the feature can be paricularly well idenitifed in one part of the sheet and so there has been a tendency to continue to show the feature even though it may become less significant across the sheet.
The result of these difficulties is that one specific rock type may suddenly end at a 'sheet' boundary. However, the definition is only missing where there is not a significant change in key aspects of the rock type and so these differences are not as critical as they may at first seem. For example, the identification of a specific type of sandstone may end at one sheet edge but ultimately the area is still sandstone and so whilst there may be a noticable change in colour there is not as significant a change in the implications for instance in terms of the grounds stability.
Are the positions of British Geological Survey geological features guaranteed to be to the same accuracy as the Ordnace Survey surface features?
Whilst it may appear from a geological map that a feature brushes the edge of a building it is not appropriate to consider that by digging at that point the feature will be found.
It is important to bear in mind that as discussed above, simply putting a line on a map is based on a considerable amount of deduction. As such it is certainly true to interpret that map as indicating that the boundary is very local to that point but it would require more detailed mapping to be specific to the point of being able to say 'X marks the spot'.
It is for this reason that the dataset is traditionally only printed at 1:50,000 scale such that a width of a line defining a feature (change of geology or fault) of for example 0.5 mm on the map represents 250 metres on the ground - making a big difference depending on which side of the line you take to be the boundary!
For this reason CENTREMAPSlive recommends the mapping not be printed at any more than the double enlargement option offered as it can lead to mistaken confidence in the accuracy of the mapping.
It should also be noted (as discussed above) that the geological mapping is generally updated less regularly than the Ordnance Survey mapping and as such there may be inconsistencies where the surface has changed enough to affect the superficial or bedrock geology.