1:250,000 Scale Geology Data

• Introduction
• Polygon Layers
• Linear Feature Layers
• Important Notes

Introduction

The data comprises primarily polygons (or areas) and lines. Each polygon is attributed with information based on the name (often lithostratigraphical) of the unit and its lithology or composition. These polygons are arranged into a single theme: bedrock geology ('solid'). The lines include faults and the coastline. Coverage of point data, e.g. dips and strikes, is limited and is not yet available.

Each DiGMapGB-250 digital tile is based on the latest BGS 1:250 000 scale UTM (Universal Transverse Mercator projection) Bedrock or 'Solid' geological map. The source 1:250 000 published maps have a chronostratigraphic classification but were reclassified, as far as possible, with a lithostratigraphic nomenclature for the digital data.

Some minor changes may have been made to the published linework to correct errors, improve the fit between maps, add formational boundaries or remove purely chronostratigraphic ones. The sources of information specific to each digital tile are available.

Polygon layers

The digital geological maps typically show the polygon theme as described below.

More details can be found in the User Guide provided by the BGS:

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Linear Feature Layers

All linear features are digitised and geologically attributed. Their availability depends on whether they were recorded when the area was mapped. For the 1:250,000 data they are organised into two categories:

• Coastline
• Faults: Faults are attributed as to whether they form the margin of a polygon (contact) or are separate lines (non-contact)

Important Notes

Do not over-enlarge the data; for example do not use 1:250 000 nominal scale data at 1:100 000 or 1:50 000 working scale. The scale of the original information is indicated by the nominal scale attribute (NOM_SCALE) embedded in the data.

The 1:250 000 scale geological maps were largely compiled from 1:50 000 scale maps by cartographic selection, modification, simplification or exaggeration. The generalised geological lines were fitted to topographic bases compiled from Ordnance Survey 1:250 000 maps and Ministry of Defence, Joint Operations Graphic (JOG) sheets. The UTM series (Universal Transverse Mercator projection) sheets were scanned and warped to OS British National Grid for digitising. The digital data have subsequently been refitted to the OS Strategi® coastline and do not necessarily fit other topographic bases, including more modern OS ones.

The 1:250 000 data may be used as a guide to the geology at a regional level, but should not be relied on for local geology. If more detailed geological information is required then the 1:50 000 or 1:10 000 scale maps or digital data, should be consulted.

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