OR/14/013 Model assumptions, geological rules used
|Monaghan A A, Arkley S L B, Whitbread K, McCormac M. 2014. Clyde superficial deposits and bedrock models released to the ASK Network 2014: a guide for users Version 3 . British Geological Survey Internal Report, OR/14/013.|
Where no boreholes were present, basal units were modelled to the BGS rockhead model (see http://www.bgs.ac.uk/products/onshore/superficialThickness.html). Where the depth of rockhead in boreholes differed from the rockhead model, the borehole depth was preferred.
A consistent approach to the modelling of water was attempted across all the modelled blocks. Firstly, a water envelope (extent) was extracted from the OS 1:10 000 scale Open data digital map. Judgement was then used to extract a subset of the larger water bodies from that map data such that the River Clyde, the lower parts of the White Cart and Black Cart waters and lakes/reservoirs exceeding approximately 150 m in width/length are included in the 3D model.
During modelling, the water bodies were generally fitted to the water envelope and not to the DTM as it is assumed that users will wish to attach the model to an OS map base rather than a derived/simplified NextMap DTM. The result is that in a few places where the DTM and OS map data do not match, water appears as if it goes uphill. In places (e.g. Balgray reservoir) other artefacts exist within the modelled water (e.g. where there is a bridge over the reservoir the DTM/water volume is raised up as the bridge is not modelled as such). In some areas there is made ground beneath water at dams, or along tidal parts of the River Clyde. The OS Open data envelope (extent) for water which extended to high water mark was edited within the intertidal zone in the Clydebank and western Central Glasgow models such that it fitted the NEXTmap® DTM in which ocean surface is levelled to OSGB1936 geodetic datum (i.e. zero metres relative to OD, or low water mark).
The depth of water within the tidal limit of the River Clyde was based approximately on a UK Hydrographic Office Admiralty Chart (International Chart Series c.2000). Where no information is available a sub-rectangular river channel cross-section has been assumed.
The depth of reservoirs was unknown and so they were modelled to water depth of about 10 m.
It is assumed that all data types (borehole, mine plan, map) are included with equal weight in the modelled surface calculation, though in reality some data may be more certain than others. Data points were excluded if clearly erroneous, based on the geologist’s judgement.