About Geological Maps
- What is a geological map?
- How are geological maps put together?
- Why do geological maps of the same area look different?
- New ways to view geological maps
- GIS-based Map Production and Data Storage
What is a geological map?
Geological maps show where different rock types, faults and many other geological features occur, against a background of rivers, coastline, roads and other topographic features. A geological map provides descriptions and the context of these features through a geological legend and cross sections that display subsurface relationships.
The rock types are typically subdivided according to their depositional or emplacement age and their composition. Coloured map units thus usually represent rocks with internally similar characteristics. Geological maps of more general scale usually omit thin rock units or those of limited coverage, and should not be used for site-specific purposes, such as designing the foundations of a building.
How are geological maps put together?
1. Acquire new ground-based observations of rock outcrops along coastlines, rivers, cliffs, ridge crests and road cuts.
2. Combine field data and interpret with new and existing paleontology, petrography, geochemistry and geochronology. Hand draw onto compilation sheets.
3. Extrapolate and interpolate observations through structural analysis, geomorphic interpretation of aerial photographs and digital terrain models, and increasingly through interpretation of satellite imagery and airborne geophysics.
4. Capture, assemble and print maps using computer software.
Typically the whole process takes 3-6 years per map.
Why do geological maps of the same area look different?
Geological maps are interpretations based on available observational data and current understanding of geological processes at the time they were made. Inevitably geological maps lose currency with the acquisition of new information and understanding. So while the rocks don’t change (other than extremely slowly), our knowledge and interpretation of them does.
New ways to view geological maps
Since the 1990s, geological maps have been captured and constructed using specialist computer mapping software known as Geographic Information Systems (GIS). Printed geological maps are still produced, but the digital geological map data are also used to make specialist map products and analyses.
Digital data comes in two forms, raster and vector.
Raster data are effectively bitmaps that image the geological map and are able to store only limited information. Raster data formats are used for the Geological Map Catalogue and some raster image products are available from our Publication Sales.
Vector data are more extensively used in GIS as the data can have large amounts of information attached to thematic layers containing polygon, line and point and other special feature types. The vector GIS data used in the digital geological maps highlighted here can be used in any combination, recoloured or symbolised differently, reclassified and analysed with other data using GIS functions, such as proximity and overlap.
GIS-based map production and data storage.
Modern Geographic Information Systems (GIS) form the core of our mapping projects. GIS software captures, stores, displays and publishes our geological maps from a digital map database. The database contains rich information about geological features organised within themes such as geological rock units, faults, folds, structural measurements and horizons. These themes can be mixed and matched with each other and third party data themes such as mineral occurrences, topography, and census information. We have detailed Technical Standards and Specifications about how we store and process our digital geological GIS data.