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Glossary

The glossary offers definitions for many of the terms frequently used in general geography. If definitions are missing, please use the contact form the let me know.

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Abscissa
The horizontal or x-coordinate of a point in a two-dimensional system of Cartesian coordinates.

Altitude
The height of a thing above a reference level, especially above sea level or above the earth's surface.

Altitude difference
The difference in altitude between two points.

Arctic circle
The imaginary circle round the earth, parallel to the equator, at latitude 66°33' North. It marks the northernmost point at which the sun appears above the level of the horizon on the winter solstice.

Astrolabe
A medieval instrument, now replaced by the sextant, that was once used to determine the altitude of the sun or other celestial bodies.

Atlas
A book or bound collection of maps, sometimes with supplementary illustrations and graphic analyses.

Azimuth
The horizontal angular distance from a reference direction, usually the northern point of the horizon, to the point where a vertical circle through a celestial body intersects the horizon, usually measured clockwise. Sometimes the southern point is used as the reference direction, and the measurement is made clockwise through 360°.

Bathymetry
The measurement of the depth of bodies of water.

Cartography
The art or technique of making maps or charts.

Compass
A device used to determine geographic direction, usually consisting of a magnetic needle or needles horizontally mounted or suspended and free to pivot until aligned with the earth's magnetic field.

Coordinates
Any of a set of two or more numbers used to determine the position of a point, line, curve, or plane in a space of a given dimension with respect to a system of lines or other fixed references.

Contour line
A line on a map joining points of equal elevation above a given level, usually mean sea level.

Course
See
Info -> Orthodromy - Course.

Declination
- The angular distance, esp in degrees, of a star, planet, etc., from the celestial equator measured north (positive) or south (negative) along the great circle passing through the celestial poles and the body.
- Magnetic declination: The horizontal angle between the true geographic North Pole and the magnetic north pole, as figured from a specific point on the Earth.

Degree
A measure of angle equal to one three-hundred-and-sixtieth of the angle traced by one complete revolution of a line about one of its ends. Expressed in either decimal degrees or degree-minute-seconds.

Distance
The length of the shortest line segment joining two points. The great circle distance is the shortest distance between any two points on the surface of a sphere measured along a path on the surface of the sphere.

Distorsion
In cartography, a distortion is the misrepresentation of the area or shape of a feature. The Mercator projection, for example, distorts Greenland because of its high latitude, in the sense that its shape and size are not the same as those on a globe.

Equator
The equator is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that divides the Earth into a Northern Hemisphere and a Southern Hemisphere. It has a latitude of 0°.

Geocoding
Geocoding is the process of assigning geographic identifiers (e.g., codes or geographic coordinates expressed as latitude-longitude) to map features and other data records, such as street addresses.

Geodesy
Geodesy, also called geodetics, a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space.

Geography
The study of the earth and its features and of the distribution of life on the earth, including human life and the effects of human activity.

Geoid
The hypothetical surface of the earth that coincides everywhere with mean sea level.

Geomagnetism
The magnetism of the earth.

Geostationary
Of, relating to, or being a satellite that travels above Earth's equator from west to east at an altitude of approximately 35,900 kilometers (22,300 miles) and at a speed matching that of Earth's rotation, thus remaining stationary in relation to Earth.

GIS
Geographic Information System. Computerized system that relates and displays data collected from a geographic entity in the form of a map.

GPS
Abbreviation for 'Global Positioning System'
A navigational system involving satellites and computers that can determine the latitude and longitude of a receiver on Earth by computing the time difference for signals from different satellites to reach the receiver.

Grade
The grad is a unit of plane angle, equivalent to 1/400 of a full circle, dividing a right angle in 100.

Gradient
The degree to which something inclines; a slope. A mountain road with a gradient of ten percent rises one foot for every ten feet of horizontal length.

Graticule
A network of lines representing the Earth's parallels of latitude and meridians of longitude.

Institut Géographique National (IGN)
French national geographical institute.

Interpolation
Estimation of a value between two known data points.

Isogonic line
Any of the lines on a chart or map connecting points of equal magnetic variation.

Latitude
Angular distance of any point on the surface of the earth north or south of the equator. The equator is latitude 0°, and the North Pole and South Pole are latitudes 90°N and 90°S, respectively. The length of one degree of latitude averages about 69 mi (110 km); it increases slightly from the equator to the poles as a result of the earth's polar flattening.

Level
Any of the successive layers of material that have been deposited with the passage of time to build up and raise the height of the land surface.

Location
Position or point in physical space that something occupies on Earths' surface.

Longitude
angular distance on the earth's surface measured along any latitude line such as the equator east or west of the prime meridian (Greenwich).

Longitude zone
Space between two longitude lines.

Loxodromy
The art, science, or practice of sailing obliquely across lines of longitude at a constant bearing to them. See
Orthodromy - Course

Map
A representation, usually on a plane surface, of a region of the earth or heavens.

Meridian
An imaginary great circle on the earth's surface passing through the North and South geographic poles. All points on the same meridian have the same longitude.

Minute
A unit of angular measure equal to one sixtieth of a degree and containing 60 seconds.

Nautical mile
A unit of length used in sea and air navigation, based on the length of one minute of arc of a great circle, especially an international and U.S. unit equal to 1,852 meters (about 6,076 feet). Also called sea mile.

Navigation
The theory and practice of navigating, especially the charting of a course for a ship or aircraft.

Ordinate
The vertical or y-coordinate of a point in a two-dimensional system of Cartesian coordinates.

Orientation
Position or positioning with relation to the points of the compass or other specific directions.

Origin
The point whose coordinates are all zero.

Orthodromy
See
Orthodromy - Course

Parallel
Any of the imaginary lines representing degrees of latitude that encircle the earth parallel to the plane of the equator.

Projection
Any method used in cartography to represent the two-dimensional curved surface of the earth or other body on a plane. Graphical projection contains three broad categories each with its own protocol: orthographic, oblique and perspective.

Proof/Pull
Copy of the map used for printing and publishing purposes.

Quadrangle
A plane figure consisting of four points connected by four lines.

Radian
A unit of angular measure equal to the angle subtended at the center of a circle by an arc equal in length to the radius of the circle, approximately 57°1744.6".

Relief/Hill shading
Shading employed to create a three-dimensional impression of relief, assuming either vertical or oblique illumination of the ground.

Scale
The ratio of the distance between two points on a map to the real distance between the two corresponding points portrayed. The scale may be expressed in three ways: numerically, as a ratio or a fraction, e.g., 1:100,000 or 1-100,000; verbally, e.g., "one inch to one mile" (not "one inch equals one mile"); and graphically, by marking distances on a sample line. The last method has the advantage that the scale remains true even if the map is enlarged or reduced mechanically. The first method is particularly useful since any unit of measurement may be used; e.g., if one uses metric units, a scale of 1:100,000 would mean that one centimeter on the map represents one kilometer on the earth's surface (since 100,000 centimeters equals one kilometer). The more the size of features on the map approaches the features' actual size on the earth's surface, the larger the scale of the map is said to be. A large-scale map usually shows more detail than does a small-scale map, but covers a smaller area than does a small-scale map of the same size.

Second
Angular unit equal to one sixtieth of a minute. 3600 seconds equal one degree.

Surface
Delimited area. The Earth's surface is represented by geographic projection in order to obtain a plane surface.

Theodolite
An optical instrument used in surveying which consists of a sighting telescope mounted so that it is free to rotate around horizontal and vertical axes, and graduated scales so that the angles of rotation may be measured.

Time zone
Any of the 24 longitudinal divisions of the earth's surface in which a standard time is kept, the primary division being that bisected by the Greenwich meridian. Each zone is 15° of longitude in width, with local variations, and observes a clock time one hour earlier than the zone immediately to the east.

Topography
Earth's relief portrayed by means of contour lines, hachures, shading, or coloring to represent elevations, depressions, and depths of water.

Zenith
In astronomy, the point in the sky directly overhead.



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