A
Airy projection – The minimum-error Azimuthal projection of the region enclosed by a great or small circle of chosen radius from a given center. Nonperspective; neither conformal nor equal area.
Albers equal-area projection – A type of map which is true along one or two chosen standard parallels, usually but not necessarily on the same side of the Equator. Scale is constant along any given parallel. (The scale factor at any given point along the meridian is the reciprocal of that along the parallel, to preserve area.) It is free of angular and scale distortion only along the one or two standard parallels; distortion is constant along any given parallel.
Antipodes – A pair of points that are on opposite sides of a planet (like the North Pole and the South Pole).
Arctic Circle – An imaginary circle at latitude 66°30' N, around the North Pole.
Axis of rotation – An imaginary line around which the earth rotates.
Azimuth – The angle that a line makes with a meridian (a line of longitude), going clockwise from north.
Azimuthal projection – A type of map in which a flat sheet is placed in contact with a globe, and points are projected from the globe to the sheet. Directions from the projection's center to all points are correct. Also called a zenithal projection.
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B
Braun projection – A type of map, which is defined by a simple geometric construction. It was created by C. Braun in 1867, and it encloses the globe in a cone aligned with the north-south axis, 1.5 times as tall as the globe and tangent at the 30°N parallel. The projection center is the South Pole, and the resulting map fits a perfect semicircle.
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C
Cartographer – A person who makes maps.
Cartography – The study and construction of maps.
Central meridian – A meridian that passes through the center of a projection; often a straight line that is an axis of symmetry of the projection.
Compass rose – A design on a map that shows direction; points which way is north, south, east, west, and some intermediate directions on the map.
Conformal projection – Preserves angular relationships at points during the transformation process; cannot be equal area; also called orthomorphic projection.
Conic projection – Type of map in which a cone is wrapped around a sphere (the globe), and the details of the globe are projected onto the conic surface; then, the cone is unwrapped into a flat surface.
Cylindrical projection – A type of map in which a cylinder is wrapped around a sphere (the globe), and the details of the globe are projected onto the cylindrical surface; then, the cylinder is unwrapped into a flat surface, yielding a rectangular-shaped map. Cylindrical maps have a lot of distortion in the polar regions (that is, the size of the polar regions is greatly exaggerated on these maps).
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D
Datum – Reference system in surveying for computing or correlating the results of surveys. There are two principal types of datums: vertical and horizontal. A vertical datum is a level surface to which heights are referred. In the United States, the generally adopted vertical datum for leveling operations is the National Geodetic Vertical Datum of 1929. The horizontal datum is used as a reference for position. The North American Datum of 1927 is defined by the latitude and longitude of an initial point (Meade's Ranch in Kansas), the direction of a line between this point and a specified second point, and two dimensions that define the spheroid. The new North American Datum of 1983 is based on a newly defined spheroid (GRS80); it is an Earth-centered datum having no initial point or initial direction.
Degree – A unit of measurement; a degree is also written °. There are 360 degrees in a circle. Each degree is divided into 60 minutes, written as the symbol '. For example, 10 and a half degrees is written 10° 30'. Further, each minute is divided into 60 seconds, written as the symbol ".
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E
Equator – Imaginary line formed by passing a plane through the center of the earth midway between the poles; Zero degrees latitude, divides the earth into the Northern and Southern hemispheres.
Equidistant projection – Projection that maintains constant scale along all Great Circles from one or two points. When the projection is centered on a pole, the parallels are spaced in proportion to their true distances along each meridian. An equidistant map projection is possible only in a limited sense. That is, distances can be shown at the nominal map scale along a line from only one or two points to any other point on the map. The focal points usually are at the map center or some central location. The term is also often used to describe maps on which the scale is shown correctly along all meridians.
Equirectangular projection – A map which is a simple modification of Plate Carree, having east-west compression; conceptually projected onto a cylinder secant to the globe along the chosen standard parallels. Scale is true along two standard parallels equidistant from the Equator and along all meridians.
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G
Geodesy – The science that measures the size and shape of the earth.
Geographic grid – Spherical coordinate system used for finding locations on the earth's surface.
Geography – The study of the Earth's surface.
Geographical coordinate system – System that uses latitude and longitude to describe points on the spherical surface of the globe.
Geoid – Term used to describe the shape of the earth.
Geometric Projection – See Perspective projection.
Globe – Spherical model of the earth's surface that includes a map of the earth; also known as a terrestrial globe.
Goode projection – An interrupted, pseudocylindrical, equal-area, composite map projection that can present the entire world on one map. Global land masses are presented with their areas in proper proportion, with minimal interruption, and minimal overall distortion.
GPS – Global positioning system. GPS devices pinpoint your exact longitude and latitude by using information from orbiting satellites.
Graticule – Parallels and meridians on a map projection.
Great circle – Any hypothetical circle that passes through the center of the earth and divides the planet into two hemispheres. The great circle route, following the arc of a great circle on the earth's surface is the shortest route between any two places on earth. The equator is the only line of latitude that is a great circle. All meridians are great circles.
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H
Hemisphere – Half of the earth's surface. There are two sets of hemispheres, Northern and Southern (divided by the equator) and the Eastern and Western (divided by the Prime Meridian and 180°).
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I
Indicatrix – Convenient way of showing distortion on a map: a circle drawn on the surface of the globe will appear squashed or stretched on a distorted map. The size and shape of the Indicatrix will vary from one part of the map to another, thereby displaying the distorting effects of projections. Also known as Tissot's Indicatrix.
International Date Line – Imaginary line near 180° longitude that exists to separate the two simultaneous days that exist on the planet at the same time; an imaginary north-south line (at the 180th meridian), in the Pacific Ocean, at which the date changes. The east side of the IDL is a calendar day earlier than the west side. The actual IDL used is not a straight line, but zigzags around certain populated areas.
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J
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K
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L
Lambert Conformal projection – A type of map in which the representation of areas is preserved so that all regions on the projection will be represented in correct relative size. Equal area maps cannot be conformal, so most earth angles are deformed and shapes are strongly distorted.
Latitude – Position north or south of the earth's equator.
Longitude – Position east or west of the prime meridian.
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M
Map – Graphic representation of cultural or physical environments on the earth's surface. There are many different types of maps that have different uses. Since a map is 2-dimensional representation of a 3-dimensional world, compromises must be made. Different maps differ in the relative accuracy of the depiction of the area, the shapes of objects, actual distances, and compass direction. Maps that accurately reflect area are often called equal-area maps (an example is the Albers equal-area conic map). Maps that maintain the shape of objects are called conformal. Maps that correctly show the distance between areas are often called equi-distant maps (note that the shortest distance between two points on a map is generally not a straight line. but a curve). Navigational maps need accurate compass directions maintained on the map (like the Mercator map).
Map projection – A transformation process; a mathematical formula which assists in representing the curved surface of the earth onto the flat surface of a map; Since a map is a 2-dimensional representation of a curved surface (a globe), the map cannot be perfectly accurate. So cartographers have to project a 3-D surface onto a 2-D map. There are many different types of projections that have different uses. Some projections preserve compass directions but distort areas (like Mercator projections), while others preserve area but distort distances and compass directions (like Robinson projections).
Map projection deformation – Results from the transformation of a spherical surface to the plane surface, may include tearing, shearing or compression.
Map Scale – The relationship between distance on a map and distance on the earth's surface.
Mercator projection – A type of rectangular map (a cylindrical projection) in which the true compass directions are kept intact (lines of latitude and longitude intersect at right angles), but areas are distorted (for example, polar areas look much larger than they really are). Mercator projections are useful for nautical navigation. Geradus Mercator devised this cylindrical projection for use in navigation in 1569.
Meridian – A line of longitude; meridians are numbered from 0° - 180° E or W of the prime meridian.
Mollweide projection – A type of sinusoidal projection map in which the entire surface of the Earth is shown within an ellipse. Lines of latitude are parallel to the equator, but lines of longitude are curved in such a way that area distortion is minimal. The distortion is greatest at the edges of the ellipse. Carl B. Mollweide created this type of projection in 1805.
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N
North Magnetic Pole – The point on the Northern Hemisphere of the Earth toward which a compass' needle always points; at the North Magnetic Pole, a compass' needle will stand vertically. It is now located in northern Canada (and its location changes over time).
North Pole – The point on the Northern Hemisphere of the Earth that is farthest north; It is 90° north of the equator.
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O
Orthographic projection – A type of map, which is essentially a drawing of (one side of) a globe. There is a lot of distortion of area in this type of map, but one gets the idea that the globe is being represented.
Orthophanic projection – Literally: 'right appearing;' a widely used type of map in which the Earth is shown in a flattened ellipse. In this pseudocylindrical projection, lines of latitude are parallel to the equator, but lines of longitude are elliptical arcs. Area is represented accurately, but the distances and compass directions are distorted. Arthur H. Robinson first made this type of projection in 1963; also known as the Robinson projection.
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P
Parallel of latitude – A line of latitude; parallels are numbered 0° (at the equator) - 90 ° (at the north or south pole).
Perspective projection – Projection produced by projecting straight lines radiating from a selected point (or from infinity) through points on a surface of a sphere or ellipsoid and then onto a tangent or secant plane. Other perspective maps are projected onto a tangent or secant cylinder or cone by using straight lines passing through a single axis of the sphere or ellipsoid. Also called Geometric Projection.
Planar projection – A type of map in which the details of the globe are projected onto a plane yielding a rectangular-shaped map. Planar maps have a lot of distortion towards the edges.
Plane – A flat surface.
Plate Carree projection – A type of map in which longitude and latitude points are directly plotted on a regular X,Y graph. The longitude lines on the graph are spaced the same distance apart as the latitude lines, forming squares. Sometimes called the Geographic projection.
Prime meridian – Meridian adopted by most countries as the point of origin (0°) for east and west longitude; passes through the British Royal Observatory in Greenwich, England.
Projection – A representation of one thing onto another, such as a curved 3-dimensional surface (like the Earth) onto a flat 2-dimensional map. There are 3 major types of projections: cylindrical, conic, and planar.
Pseudocylindrical projection – A type of projection that, in the normal aspect, has straight parallel lines for parallels and on which the meridians are usually equally spaced along parallels (as they are on a cylindrical projection) but on which the meridians are curved.
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Q
Quadrangle – A four-sided area bounded by two lines (parallels) of latitude and two lines of longitude (meridians) on a map.
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R
Robinson projection – See Orthophanic projection.
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S
Small circle – Any circle on a spherical surface that is not a great circle; parallels other than the equator are small circles.
Sinusoidal projection – A type of map projection in which lines of latitude are parallel to the equator, and lines of longitude are curved around the prime meridian.
South Magnetic Pole – The point on the Southern Hemisphere of the Earth toward which a compass' needle always points; at the South Magnetic Pole, a compass' needle will stand vertically. Its location changes over time, and it is now located just off the coast of the continent of Antarctica.
South Pole – The point on the Southern Hemisphere of the Earth that is farthest south. It is 90° south of the equator.
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T
Tropic of Cancer – An imaginary line of latitude at 23°30' N. It describes the northernmost points that the sun's rays are vertical to the earth as summer begins in the northern hemisphere.
Tropic of Capricorn – An imaginary line of latitude at 23°30' S. It describes the southernmost points that the sun's rays are vertical to the earth as summer begins in the southern hemisphere.
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U
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W
Winkel Tripel projection – A type of pseudocylindrical projection map in which both the lines of latitude and longitude are curved. The National Geographic Society adopted the Winkel Tripel projection in the late 1990s (replacing the Robinson projection).
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