A geological map depicts the spatial distribution of rock and surficial deposits at or near the Earth's surface, classified by lithology and age, together with structural features such as faults, folds, contacts, and bedding orientation. It is the primary tool for communicating the three-dimensional geology of an area on a two-dimensional surface.
Why it matters
Geological maps underpin mineral and groundwater exploration, hazard assessment, engineering geology, and land-use planning. By encoding units, contacts, and structures, they let a reader infer the subsurface, predict where resources or risks lie, and constrain cross-sections. Moving these maps from static PDFs into queryable GIS layers is what makes them usable in modern spatial analysis.
Map elements
- Map units (polygons): lithostratigraphic or surficial units, each with a colour and label keyed to the legend.
- Contacts (lines): boundaries between units, classified as observed, approximate, or concealed.
- Faults and folds (lines): with symbology for type, dip, and movement.
- Structural points: strike and dip of bedding, foliation, and joints.
- Legend, scale, and cross-sections to interpret the map.
Concrete example
Standards help interoperability. The USGS FGDC Digital Cartographic Standard for Geologic Map Symbolization defines colours and symbols, while the GeMS (Geologic Map Schema) specification structures map units, contacts, and description-of-map-units tables for GIS databases. Encoding a map in GeMS-style feature classes makes it analysable in QGIS, ArcGIS, or PostGIS.
Common pitfall
Treating a scanned geological map as data. A raster image cannot be queried, intersected, or styled by attribute; it must be georeferenced and digitised into vector units with attributes before it supports analysis. Also respect map scale: a 1:250,000 sheet is reconnaissance-level and should not be used for site-specific decisions.