Geographic & Cartographic Glossary
Clear, mathematically rigorous definitions of 36 foundational concepts in geodesy, coordinate reference systems, map projections, GIS data standards, and spatial analysis.
Geodesy & Datums
Geodesic Distance
Geodesic distance is the shortest physical path between two points along the curved surface of an oblate ellipsoid (such as Earth). On Earth modeled by the WGS84 ellipsoid, geodesics account for polar flattening and provide sub-millimeter precision for aviation, satellite navigation, and surveying.
Great-Circle Distance (Orthodrome)
Great-circle distance is the shortest path between two points on the surface of a perfect sphere, formed by the intersection of the spherical surface with a plane passing through the center of the sphere.
Rhumb Line (Loxodrome)
A rhumb line (or loxodrome) is a line on Earth surface that crosses all meridians of longitude at the same constant angle or compass bearing.
World Geodetic System 1984 (WGS84)
WGS84 (EPSG:4326) is the international geodetic reference system and datum maintained by the US National Geospatial-Intelligence Agency (NGA) and used globally by GPS satellites, smartphones, and web GIS.
North American Datum 1983 (NAD83)
NAD83 (EPSG:4269) is the horizontal control datum for the United States, Canada, Mexico, and Central America, based on the GRS80 reference ellipsoid.
European Terrestrial Reference System 1989 (ETRS89)
ETRS89 (EPSG:4258) is the standard geodetic datum for Europe, fixed to the stable part of the Eurasian continental plate.
Geoid & Mean Sea Level
The geoid is the equipotential surface of the Earth gravity field that best fits global mean sea level, serving as the fundamental reference surface for physical elevations.
Orthometric Height vs Ellipsoidal Height
Orthometric height (H) is the elevation of a point above the physical geoid (mean sea level), while ellipsoidal height (h) is the geometric distance above the mathematical WGS84 ellipsoid measured directly by GPS receivers.
Reference Ellipsoid (Oblate Spheroid)
A reference ellipsoid is a mathematically defined oblate surface formed by rotating an ellipse around its minor (polar) axis, approximating the geometric shape of the Earth.
Spatial Reference System (CRS / SRS)
A Coordinate Reference System (CRS or SRS) is a standardized framework comprising a coordinate system, datum, and projection parameters that unambiguously binds numeric coordinates to physical locations on Earth.
Azimuth & Bearing
Azimuth is the horizontal angular direction of a line from a reference meridian, measured clockwise from true North from 000 to 360 degrees.
Magnetic Declination (Magnetic Variation)
Magnetic declination is the angle between magnetic North (the direction the north end of a compass needle points) and true geographic North.
Projections & Grids
Web Mercator (EPSG:3857 / Pseudo-Mercator)
Web Mercator (EPSG:3857, formerly EPSG:900913) is the projected coordinate system used by virtually all commercial interactive web map tile providers (Google Maps, OpenStreetMap, MapLibre, Apple Maps).
Universal Transverse Mercator (UTM)
UTM is a global projected grid coordinate system that divides the world into 60 six-degree longitudinal zones, measuring positions in metric Cartesian units (Easting and Northing).
Military Grid Reference System (MGRS)
MGRS is the standard geographic coordinate format used by NATO militaries, search and rescue teams, and tactical disaster response agencies for unambiguous ground location reporting.
Geohash Spatial Index
Geohash is a public domain hierarchical spatial data structure that encodes a 2D latitude/longitude bounding box into a short string of letters and numbers using Base32.
Plus Code (Open Location Code)
A Plus Code (Open Location Code / OLC) is an open-source, alphanumeric street address substitute developed by Google to give any spot on Earth a reliable postal code, especially in unaddressed rural or developing areas.
False Easting & False Northing
False Easting and False Northing are arbitrary linear values added to planar coordinates in map projections to ensure all (x, y) coordinate values remain positive across the entire mapped territory.
Central Meridian
The central meridian is the line of longitude chosen as the geometric center and axis of symmetry for a projected coordinate system zone.
Tissot Indicatrix of Distortion
Tissot Indicatrix is a mathematical diagram developed by Nicolas Auguste Tissot (1859) that overlays infinitesimal circles on a globe to visually quantify angular, linear, and areal distortion across map projections.
Astronomy & Solar
Subsolar Point (Sun Zenith)
The subsolar point is the exact geographic coordinate on Earth surface where the Sun is directly overhead at the zenith (90 altitude angle) at any given moment.
Solar Terminator (Day-Night Line)
The solar terminator is the moving boundary line on Earth that separates the daylight hemisphere from the dark nighttime hemisphere.
Solar Zenith Angle
The solar zenith angle is the angular distance between the Sun center and the local celestial zenith (directly overhead). It is the complement of solar elevation (90 - elevation).
Solar Azimuth Angle
The solar azimuth angle is the compass direction from which sunlight comes, measured in degrees clockwise from true geographic North (0 = North, 90 = East, 180 = South, 270 = West).
GIS & Formats
GeoJSON Standard (RFC 7946)
GeoJSON is an open standard geospatial data format based on JSON, standardized under IETF RFC 7946, designed for encoding simple geographic features and their non-spatial attributes.
ESRI Shapefile (.shp, .shx, .dbf, .prj)
A Shapefile is a vector GIS storage format created by ESRI in the early 1990s, composed of at least three mandatory files: .shp (geometry), .shx (spatial index), and .dbf (dBASE attribute table).
Keyhole Markup Language (KML / KMZ)
KML is an XML-based geospatial notation language created for Keyhole (now Google Earth) and maintained by the Open Geospatial Consortium (OGC) to express geographic annotation, 3D building models, and camera perspectives.
GPS Exchange Format (GPX)
GPX is an open XML schema designed specifically for transferring GPS data between navigation hardware (Garmin, Wahoo), fitness apps (Strava), and cartography software.
FIPS Geographic Identification Code
Federal Information Processing Series (FIPS) codes are standard numeric identifiers maintained by NIST and the US Census Bureau to uniquely identify states, counties, and census tracts across the United States.
Spatial Analysis
Isochrone Map (Drive-Time Contour)
An isochrone is an isoline or boundary polygon on a map connecting all geographic locations that can be reached from a starting origin within a specified duration of travel time (e.g. 15, 30, or 45 minutes).
Isodistance Buffer
An isodistance is a polygon or contour line connecting all points on a road network located at the exact same driving or walking road distance from a starting point.
Antipode (Antipodal Point)
An antipode is the geographic location on Earth surface diametrically opposite to a given position. A straight line connecting two antipodal points passes directly through the Earth center of mass.
Geofence (Virtual Geographic Perimeter)
A geofence is a virtual geometric perimeter or boundary for a real-world geographic area that triggers automated software events (alerts, notifications, check-ins) when a mobile device enters or exits.
Geographic Buffer Zone
A buffer zone is a geometric zone of a specified distance drawn around a geographic point, line, or polygon feature to define proximity, environmental setbacks, or service areas.
Centroid (Geometric Center of Mass)
The centroid is the arithmetic mean position of all points in a geometric shape or set of geographic coordinates.
People Also Ask About Cartographic Terminology
Essential questions explaining datums, spatial mathematics, projections, and GIS vocabulary.
QWhat is the difference between a geoid, ellipsoid, and sphere in geodesy?
A sphere is a simplified uniform mathematical model; an ellipsoid (such as WGS84) accounts for the Earth flattening at the poles; and the geoid represents the true equipotential gravity surface of global mean sea level.
QWhat is an isochrone map and how is it calculated?
An isochrone map connects points that can be reached from a starting origin within a specified travel time threshold (e.g., 15-minute or 30-minute drive-time contours) based on network routing algorithms.
QWhat is the difference between a great circle and a rhumb line (loxodrome)?
A great circle is the shortest line between two points on a curved surface (with compass headings that change continually), whereas a rhumb line maintains a constant compass bearing but is longer in total travel distance.
QWhat is a geodetic datum and why is WGS84 the international standard?
A datum defines the coordinate system orientation, origin point, and scale for spatial data. WGS84 is the global geocentric standard used by GPS, Google Maps, and satellite navigation.
QHow does map scale affect spatial measurement precision?
Large-scale maps (e.g. 1:1,000) show small areas with high detail and millimeter precision, while small-scale maps (e.g. 1:10,000,000) generalize features and exhibit projection distortion over continental distances.
QHow are terms categorized in the GeoMap Suite cartography glossary?
Our glossary organizes 36 essential definitions across 5 domains: Geodesy & Datums, Projections & Grids, Astronomy & Solar, GIS & Formats, and Spatial Analysis, complete with mathematical formulas and companion calculators.