A list of common terms and what they mean.


Aspect: The orientation of a projection. Cylindrical projections are "normal" if wrapped around the equator, "polar" if wrapped around the poles, or "oblique" otherwise. Azimuthal projections are "polar" if centred on a pole, or "equatorial" if centred on the equator, and are often centred at other points as well. Conics are almost always "normal" which means they have their "point" along the Earth's axis.

Azimuthal: A projection where the map is notionally flat and touches the Earth at a point. In a polar aspect the meridians are an evenly spaced full circle about the pole with the parallels circles centred on the pole and possibly evenly spaced or not. In equatorial aspects the meridians are curves from pole to pole, and the parallels are curves that are 'open' toward the nearer pole.

Axis: The line through the Earth about which it rotates.

Bearing: A compass direction. This is distorted by all projections except Normal Mercator.

Chart: A map designed for navigation, particularly by sea or air.

Conformal: A property of projections. A conformal projection preserves angles. If two lines meat at 45 degrees, then they will still meet at 45 degrees in the map. In a rough sense you can think of conformal maps as preserving shapes, though that's not quite right.

Conic: A projection where the map is notionally wrapped around the Earth to form a cone. The graticule of normal conic projections has the meridians as evenly spaced straight lines radiating from a point in a fan shape, and the parallels as circular arcs centred at that same point and may or may not be evenly spaced depending on the particular projection.

Coordinate System: A way of representing positions with numbers. Latitude and longitude is the best known, although there are others including UTM/UPS, and Geocentric.

Cylindrical: A projection where the map is notionally wrapped around the Earth to form a cylinder. The graticule of normal cylindrical projections consists of a rectangular grid with the meridians evenly spaced. The parallels may or may not be evenly spaced and may or may not be evenly spaced depending on the particular projection.

Ellipsoid: A squashed/stretched sphere. If it is only squashed or stretched in one direction, it is called a spheroid, and an oblate (squashed) spheroid is the usual way of representing the geoid.

Equal Area: A property of projections. Shapes in an equal area map will have the same relative areas. If one island is twice as big as another, it will be twice as big in the map.

Equator: The great circle where the plane perpendicular to the axis and containing the centre of mass (The equatorial plane) intersects the surface.

Equidistant: A property of projections. An equidistant map preserves a certain class of distances. Typically it will preserve distances in a certain direction, or through a certain point.

Extent: The area covered by a map.

Feature: A thing to represent on a map. A town, road, mountain, battle, the projected range of an invasive toad species in 20 years.

Geoid: The true shape of the Earth. Formally, it's an "equipotential surface", which means that's it's defined so that if you dropped something down a hole to the Centre of the Earth from any point on the surface, it would have the same speed when it reached the bottom. Other ways of thinking of it are that if the entire geoid were solid, nothing would roll up or down on it, or if you eliminated all the land, and eliminated all the weather and tides, then the oceans would take the level of the geoid. Roughly it's a flattened sphere, but the 'fat' part is a bit off centre, and it's a bit lumpy.

Graticule: The grid indicating latitude and longitude, or sometimes other coordinate systems like UTM.

Great Circle: The equivalent of a "straight line" in spherical geometry. It is a circle defined by the intersection of a plane through the centre of the sphere and the surface of the sphere. Equivalently a plane through the centre of the earth intersects the geoid, or an ellipsoid, in what is generally called a "great circle" even if it's isn't strictly a circle. Strictly these are all "geodesics", but that term is less frequently used.

Latitude: The angle between a line straight 'down' from a given point on the surface, and the equatorial plane where they meet. Ideally, "down" is defined by the geoid, but in practice, an ellipsoid is used.

Legend: A description of a map's symbology, typically in the form of a table showing examples beside descriptions of what they represent.

Longitude: The angle between the plane defined by the axis and the given point, and the plane of the prime meridian.

Projection: A way of taking a position in 3 dimensions (Such as a point on the surface of the Earth) and representing it in 2 dimensions (such as a flat map.). Some information will always be lost or distorted as a result. In cartography, this is most often a matter of projecting the surface of the Earth onto a flat map, but occasionally other projections of general 3d space are used to represent relief, such as isometric and other axonometric projections.

Relief: The shape of the surface of the Earth, including all the bumps and holes.

Reference Map: A map designed to present a broad and general view for use in a wide range of situations. A typical wall map, the maps in atlases, or topographic maps are examples.

Rhumb Line: A curve along which bearing is constant. If you walk forward, following a particular bearing on a compass, you are walking a rhumb line. Unless you are walking straight north/south or along the equator, this is a spiral which gets tighter and tighter as you approach a pole. Also called a Loxodrome.

Scale: The ratio between how big features are represented on the map, and how big they really are. If a 1 km feature is drawn as 1 mm, then the scale is 1:1,000,000. Note that 1:1,000 (0.001) is much is a much larger number than 1:1,000,000 (0.000001) and so is a "larger scale". This means that maps with a large extent (Cover a large area) will usually have a small scale (Draw things small and with little detail), and vis versa.

Symbology: The way that you represent features in your map. Representing towns as red dots is an example of symbology.

Thematic Map: A map designed to present a specific set of data or to serve a specific role. Climate maps, weather maps, and population maps are good examples.