Compilation and methodology — version 2

United Kingdom and adjacent Irish waters

The bathymetric contours published in DigBath250 for UK and adjacent Irish waters have principally been taken from a series of 1:250 000 scale geological maps published by the British Geological Survey from 1977 to 1992. These geological maps comprise three themes, Bedrock, Quaternary, and Sea Bed Sediments, with each theme published as individual sheets for named map areas. For some map areas the Quaternary and Sea Bed Sediments theme have been amalgamated into a single sheet. Bathymetric contours are normally published on the Sea Bed Sediments map sheet.

Map showing BGS 1: 250 000 map availability

An index of BGS 1:250 000 maps that can be purchased online at our internet shop is available.

Two primary sources of data were used to compile the bathymetry for the BGS 1:250 000 geological maps. These were:

  • Over 200 000 line kilometres of geophysical survey data collected over the UK Continental Shelf and Slope by BGS during regional surveys from 1966 to 1987. Bathymetric soundings were taken along these lines but no correction was made for tidal variation
  • UKHO Admiralty Charts and Survey data. These have been used to compile or improve the bathymetry where BGS data was inadequate or not available. They have also been utilised within DigBath250 for areas outside the coverage of the published BGS 1:250 000 maps.

During the bathymetric compilation for the published BGS geological maps the data was generally gathered and compiled at a scale of 1:100 000. These compilations were then reduced and re-compiled at a final publication scale of 1:250 000. The original bathymetric data used in the compilation was surveyed at a variety of scales, density and standards over a considerable period of time, some even back to the eighteenth century. Although compiling at 1:250 000 scale entails generalisation, smoothing and loss of detail, no attempt was made to produce a compilation standard in terms of line or feature density for the geological maps or for DigBath250. Therefore no assumptions should be made when comparing adjacent areas on the criteria of presence or absence of detail. Differences may simply reflect the density of available data.

The published bathymetric compilations at 1:250 000 scale have been scanned and attributed for DigBath250. However, to meet the DigBath250 specification further compilation work has been undertaken, both to delete contours in some areas and add contours in other areas.

The published BGS 1:250 000 maps incorporate an Ordnance Survey (OS) based coastline which only covers the UK. However, for DigBath250, the World Vector Shoreline© has been used as the coastline because it has European as well as UK coverage. This has created problems in some UK coastal areas because of incompatibilities between the OS coastline and WVS. These problems have been resolved by recompiling bathymetric lines in the affected areas.

The bathymetric contours for the published BGS geological maps have not been compiled to a common vertical datum across the whole map series. Individual map sheets have generally been compiled to Chart Datum or Mean Sea Level. Some have been published at Ordnance Datum but these may be regarded as Mean Sea Level at the DigBath250 scale. Given the volume, provenance, age and accuracy, both horizontal and vertical, of the bathymetric data originally compiled for the BGS 1:250 000 map series it was deemed impractical and uneconomic, technically and financially, to recompile the original data. DigBath250 has been compiled as a regional scale bathymetry and at this small scale the vertical attribution as mean sea level/chart datum is a practical resolution within UK and adjacent Irish waters. If further detail is required metadata can be provided for the bathymetric data within each published 1:250,000 geological map sheet used in the compilation of DigBath250.