Map & Radius Glossary
Plain-English definitions of 43 terms used in radius mapping, geocoding, routing, and geospatial analysis. Written for marketers, planners, and curious readers — not GIS specialists.
A
- Antipode
- The point on Earth diametrically opposite a given location. The antipode of London is in the Pacific Ocean southeast of New Zealand.
- As the crow flies
- Straight-line distance between two points, ignoring roads, terrain, and obstacles. On Earth this is the great-circle distance, calculated with the Haversine formula.
B
- Bounding box
- A rectangle defined by minimum and maximum latitude/longitude pairs that encloses a geographic region. Often abbreviated bbox and used to constrain map queries.
- Buffer (GIS)
- A zone of a given distance around a feature on a map. A radius circle around a point is a buffer; so is a corridor around a road.
C
- Catchment area
- The geographic region from which a business, school, or service draws its customers or users. Often modeled as a radius or a drive-time isochrone.
- Centroid
- The geometric center of a polygon — the point you would balance the shape on. For an irregular polygon it may fall outside the shape.
- Choropleth map
- A thematic map where regions are shaded by a data value, like population density per county or election results per state.
- Coordinate system
- A framework that assigns coordinates to every point on Earth. WGS 84 is the standard used by GPS and most consumer maps.
D
- Decimal degrees
- A coordinate format using fractional degrees, like 51.5074, -0.1278 (London). The dominant format in modern web maps and APIs.
- DMS coordinates
- Degrees, minutes, seconds — the traditional notation for latitude and longitude, e.g. 51°30'26"N 0°7'40"W. One degree equals 60 minutes; one minute equals 60 seconds.
- Driving distance
- The distance along the road network between two points, computed by a routing engine. Almost always longer than the straight-line distance because roads detour around obstacles.
E
- EPSG code
- A numeric identifier from the EPSG registry of coordinate reference systems. EPSG:4326 is WGS 84 (lat/lng); EPSG:3857 is Web Mercator (the projection used by web maps).
G
- Geocoding
- Converting a human-readable address ("221B Baker Street, London") into latitude/longitude coordinates. The reverse — coordinates to address — is reverse geocoding.
- GeoJSON
- An open JSON format for encoding geographic features — points, lines, and polygons — with properties. Widely supported by GIS tools and JavaScript map libraries.
- Geofence
- A virtual boundary on a map, usually a circle or polygon. Geofences trigger location-based actions: a delivery driver entering a zone, a phone leaving a parent-defined area, an ad firing inside a stadium.
- Great-circle distance
- The shortest distance between two points on a sphere, measured along the surface. On Earth it is what "as the crow flies" approximates and what the Haversine formula calculates.
H
- Haversine formula
- The standard formula for computing great-circle distance between two latitude/longitude points on a sphere. Accurate to within a fraction of a percent for Earth-scale distances.
- Heatmap
- A map visualization that uses color intensity to show density of data points — for example, where Twitter mentions cluster, or where 911 calls originate.
I
- Isochrone
- A boundary enclosing every point reachable from a starting location within a given travel time. A 30-minute drive-time isochrone shows everywhere you can drive to in half an hour. Computed by routing engines like OSRM or Valhalla.
K
- KML (Keyhole Markup Language)
- An XML-based file format for geographic data, originally developed for Google Earth. Now an OGC standard, supported by Google My Maps, ArcGIS, QGIS, and most consumer mapping tools.
L
- Latitude
- Angular distance north or south of the equator, ranging from -90 (South Pole) to +90 (North Pole). Often abbreviated "lat" and listed first in coordinate pairs.
- Leaflet
- An open-source JavaScript library for interactive maps in web browsers. Lightweight, mobile-friendly, and works with OpenStreetMap and most tile providers. Map With Radius is built on Leaflet.
- Longitude
- Angular distance east or west of the prime meridian (Greenwich, London), ranging from -180 to +180. Often abbreviated "lng" or "lon" and listed second in coordinate pairs.
M
- Map projection
- A method of representing Earth's curved surface on a flat plane. Every projection distorts something — area, shape, or distance — so different projections suit different uses.
- Map tile
- A 256×256 image (or vector packet) at a specific zoom level and grid coordinate. Modern web maps stitch many tiles together as you pan and zoom.
- Mercator projection
- A cylindrical map projection from 1569 that preserves shapes and angles but exaggerates area near the poles — the reason Greenland looks larger than Africa on most world maps.
- Mile
- Imperial unit of distance. 1 statute mile = 1.609 kilometers = 5,280 feet. A nautical mile (used in aviation and shipping) is slightly longer at 1.852 km.
N
- Nominatim
- An open-source geocoder built on OpenStreetMap data. Used by Map With Radius for address search; available as a free public service or self-hostable.
O
- OpenStreetMap (OSM)
- A free, editable map of the world built and maintained by a global community of volunteers since 2004. The data is openly licensed and powers many independent mapping tools, including this site.
- OSRM
- Open Source Routing Machine — a fast routing engine optimized for OpenStreetMap road data. Used to compute driving, walking, and cycling routes and isochrones.
P
- POI (Point of Interest)
- Any named location of interest on a map: a restaurant, school, hospital, or landmark. POIs typically have a category, name, and coordinates.
- Polygon
- A closed shape defined by an ordered ring of coordinates, used to represent areas like neighborhoods, parks, or country borders. A multipolygon represents a feature made of multiple disconnected pieces.
- Polyline
- An open shape defined by a sequence of coordinates, used to draw routes, rivers, roads, or any path. Different from a polygon, which is closed.
R
- Radius
- The distance from the center of a circle to its edge. On a map, a radius is typically expressed in miles, kilometers, meters, or feet — and used to define what is "within range" of a point.
- Raster tiles
- Map tiles delivered as pre-rendered PNG or JPEG images. Simple to display but the styling is fixed at the server, so the client cannot restyle the map at runtime.
- Reverse geocoding
- The opposite of geocoding — converting latitude/longitude coordinates into a postal address or named place. "What city am I in?" is a reverse-geocoding question.
S
- Shapefile
- Esri's vector GIS file format, dating to the early 1990s. A shapefile is actually a set of files (.shp, .shx, .dbf, often .prj) that together describe geometry and attributes. Still widely used in professional GIS.
- Slippy map
- A draggable, zoomable web map made of map tiles — the kind you find on Google Maps, OpenStreetMap, and almost every modern site. Named for the smooth panning ("slippy") experience.
V
- Valhalla
- An open-source routing engine that uses OpenStreetMap data, with strong support for multimodal routes (drive, walk, bike, transit). Often used for isochrones and matrix calculations.
- Vector tiles
- Map tiles delivered as binary geometry data and rendered in the browser. Compared to raster tiles, vector tiles are smaller, sharper at any zoom level, and styleable on the fly.
W
- Web Mercator (EPSG:3857)
- The variant of the Mercator projection used by virtually all consumer web maps — Google Maps, OpenStreetMap, Bing, Mapbox. Optimized for square tiles; distortion grows toward the poles.
- WGS 84 (EPSG:4326)
- The World Geodetic System 1984 — the global coordinate reference system used by GPS satellites, aviation, and most consumer mapping. Coordinates are latitude/longitude in decimal degrees.
Z
- Zoom level
- An integer specifying how detailed the map is. Zoom 0 fits the whole world in one tile; each higher zoom doubles linear resolution. Most web maps support zooms 0–22.