GIS & Geology Glossary
Clear, source-grounded definitions of the coordinate, terrain, remote-sensing, and spatial-data terms that come up in real geological and GIS work.
Reference
Definitions for technical teams
Each entry explains what the term means, why it matters in practice, and links to deeper workflow articles.
.prj File
A .prj file is the sidecar that stores a shapefile's coordinate reference system as a WKT string, telling GIS software how to place its coordinates on Earth.
Aspect
Aspect is the compass direction that a terrain slope faces, derived from a DEM and measured in degrees clockwise from north (0-360).
Axis Order
Axis order is the sequence in which coordinate values are stored or read for a CRS, such as latitude-longitude versus longitude-latitude.
Band Ratio
A band ratio divides one spectral band by another to suppress illumination effects and highlight materials such as iron oxides or clay alteration minerals.
Borehole
A borehole is a narrow shaft drilled into the ground to sample, log, or monitor the subsurface, providing the depth-referenced ground truth behind geological models.
Cloud-Optimized GeoTIFF (COG)
A Cloud-Optimized GeoTIFF is a GeoTIFF structured with internal tiling and overviews so HTTP range requests can stream only the pixels needed, no full download.
Contour Line
A contour line connects points of equal elevation on a map, depicting terrain shape, steepness, and landforms through the spacing and pattern of the lines.
Coordinate Reference System (CRS)
A Coordinate Reference System defines how coordinates map to real positions on Earth, combining a datum, coordinate system, and (for projected CRS) a map projection.
Datum
A datum is the reference framework of ellipsoid and control points that fixes how coordinates map to positions on the Earth's surface.
Digital Elevation Model (DEM)
A Digital Elevation Model is a raster grid of elevation values representing terrain height, used to derive slope, aspect, hillshade, and hydrology.
Digital Surface Model (DSM)
A Digital Surface Model (DSM) is a raster elevation model representing the top reflective surface, including buildings, tree canopy, and other features.
Digital Terrain Model (DTM)
A Digital Terrain Model (DTM) is a raster representation of the bare-earth ground surface with vegetation, buildings, and other objects removed.
EPSG Code
An EPSG code is a unique numeric identifier for a coordinate reference system, datum, or projection in the EPSG Geodetic Parameter Dataset.
False Easting and Northing
False easting and northing are constant offsets added to projected coordinates so that all values within a map zone stay positive and avoid negative numbers.
File Geodatabase
A File Geodatabase is Esri's folder-based spatial database that stores many feature classes, rasters, and tables with large capacity and rich data types.
Flow Accumulation
Flow accumulation is a DEM-derived raster counting how many upstream cells drain through each cell, revealing drainage networks and catchment structure.
GeMS (Geologic Map Schema)
GeMS is the USGS Geologic Map Schema, a standardized geodatabase structure for encoding geologic map data consistently for publication and reuse.
Geographic Coordinate System
A geographic coordinate system locates positions on the Earth using angular latitude and longitude referenced to a datum and ellipsoid, measured in degrees.
Geographic Information System (GIS)
A Geographic Information System (GIS) captures, stores, analyzes, and visualizes spatially referenced data, linking location to attributes for mapping and analysis.
Geoid
The geoid is the equipotential surface of Earth's gravity field that best matches global mean sea level, used as the reference for orthometric heights.
GeoJSON
GeoJSON is an open, JSON-based format for encoding vector geographic features and their attributes, standardized as RFC 7946 and widely used in web maps.
Geological Map
A geological map portrays the distribution, type, age, and structure of rock and surficial units at the Earth's surface using standardized colors and symbols.
GeoPackage
GeoPackage is an open, SQLite-based GIS file format storing vector, raster, and tile data with attributes in a single portable .gpkg database file.
GeoParquet
GeoParquet is an open columnar file format that stores vector geometries inside Apache Parquet, enabling fast, scalable spatial analytics and cloud-native workflows.
Georeferencing
Georeferencing is the process of assigning real-world coordinates to an image, scanned map, or dataset so it aligns correctly with other spatial data.
Geospatial Metadata
Geospatial metadata is the structured documentation describing a spatial dataset's CRS, extent, lineage, accuracy, and attributes so it can be used correctly.
GeoTIFF
GeoTIFF is a public-domain raster format that embeds georeferencing—CRS, extent, and pixel size—inside a standard TIFF image file.
Hazard Map
A hazard map shows the spatial likelihood and intensity of a natural hazard such as flooding, landslides, or earthquakes, independent of who or what is exposed.
Hillshade
A hillshade is a grayscale relief image computed from a DEM that simulates sun and shadow to make terrain shape visually intuitive on a map.
KML
KML (Keyhole Markup Language) is an XML-based format for representing geographic points, lines, polygons, and imagery overlays, used by Google Earth.
LiDAR
LiDAR is an active remote-sensing method that measures distance with laser pulses to build dense 3D point clouds of terrain and surface features.
Map Projection
A map projection is the mathematical method for flattening Earth's curved surface onto a plane, inevitably distorting area, shape, distance, or direction.
Multispectral Imagery
Multispectral imagery captures reflected energy in several discrete spectral bands, enabling analysis beyond the visible range for geology and land cover.
NAD83
NAD83 is the North American Datum of 1983, the official horizontal geodetic datum for the United States, Canada, and Mexico used in mapping and GIS.
NDVI
NDVI (Normalized Difference Vegetation Index) measures vegetation greenness from red and near-infrared reflectance, ranging from -1 to +1.
PostGIS
PostGIS is an open-source extension that adds spatial data types, indexing, and hundreds of geometry functions to the PostgreSQL database.
Raster Data
Raster data represents geographic information as a grid of equally sized cells, each holding a value such as elevation, reflectance, or land-cover class.
Reference Ellipsoid
A reference ellipsoid is a smooth mathematical model of the Earth's shape used as the basis for a geodetic datum and for defining latitude and longitude.
Shapefile
A Shapefile is a legacy Esri vector data format that stores geometry and attributes across several files sharing a common base name.
Shortwave Infrared (SWIR)
SWIR is the 1.4–3.0 µm region of the electromagnetic spectrum used in remote sensing to identify minerals, moisture, and alteration through diagnostic absorption features.
Slope
Slope is the steepness of a terrain surface, derived from a DEM as the maximum rate of elevation change, expressed in degrees or as a percent rise.
Spatial Index
A spatial index is a data structure (often an R-tree) that organizes geometries by location so spatial queries run fast without scanning every feature.
SRID
An SRID (Spatial Reference Identifier) is an integer that uniquely identifies the coordinate reference system attached to a geometry in a spatial database.
Strike and Dip
Strike and dip describe the orientation of a planar geological feature: strike is its horizontal trend line, and dip is the angle and direction it tilts downward.
Synthetic Aperture Radar (SAR)
Synthetic Aperture Radar (SAR) is an active microwave imaging technique that creates high-resolution images of terrain day or night, through clouds.
Topology (GIS)
Topology in GIS is the set of spatial relationship rules (adjacency, connectivity, containment) that keep vector features geometrically consistent.
UTM (Universal Transverse Mercator)
UTM is a projected coordinate system dividing the globe into 60 north-south zones, each using a transverse Mercator projection to map coordinates in meters.
Vector Data
Vector data represents geographic features as points, lines, and polygons defined by coordinates, each carrying attributes — the basis of most GIS mapping.
Vector Tiles
Vector tiles are a way of delivering map data as small, pre-cut tiled packets of geometry and attributes that the client renders and styles on demand.
Vertical Datum
A vertical datum is the reference surface from which elevations and depths are measured, such as mean sea level or a geoid model.
Watershed
A watershed is the area of land draining to a common outlet, delineated from a DEM using flow direction and flow accumulation in hydrological analysis.
Web Mercator (EPSG:3857)
Web Mercator (EPSG:3857) is the spherical Mercator projection used by web map tiles, ideal for display but unsuitable for accurate area or distance measurement.
WGS84
WGS84 is the global geographic coordinate system used by GPS, defining positions as latitude and longitude on a standard Earth ellipsoid.