**Datum**—A reference for position on the surface of the Earth. In surveying, a datum is a reference system 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 1983 is based on the Geodetic Reference System 1980 (GRS80) spheroid; it is an Earth-centered datum having no initial point or initial direction. This is the horizontal datum used for National Atlas map layers.Developable surface—A developable surface is a simple geometric form capable of being flattened without stretching. Map projections can be grouped by the developable surface they use: cylinder, cone, or plane.Ellipsoid—A mathematical figure that approximates the shape of the Earth in form and size, and which is used as a reference surface for geodetic surveys. Used interchangeably with Spheriod.

Equal-area—A map projection where every part, as well as the whole, has the same area as the corresponding part on the Earth, at the same reduced scale.

**Equator**—The line which encircles the Earth at an equal distance from the North and South Poles.

**Equidistant**—A map projection that shows true distances from the center of the projection or along a special set of lines. For example, an Azimuthal Equidistant map centered at Washington, DC, shows the correct distance between Washington, DC, and any other point on the projection. It shows the correct distance between Washington, DC, and San Diego and between Washington, DC, and Seattle, but it does not show the correct distance between San Diego and Seattle.Graticule—A network of lines representing a selection of the Earth’s parallels and meridians.Great circle—A circle formed on the surface of a sphere by a plane that passes through the center of the sphere. The Equator, each meridian, and each other full circumference of the Earth forms a great circle. The arc of a great circle shows the shortest distance between points on the surface of the Earth.

Grid—Two sets of parallel lines intersecting at right angles, forming a rectangular Cartesian coordinate system superimposed on a map projection. Sometimes the term “grid” is used loosely to mean the projection system itself rather than the rectangular system superimposed on the projection.

Latitude—Angular distance, in degrees, minutes, and seconds measured from the center of the Earth, of a point north or south of the Equator. Latitude may also be measured in decimal degrees.

Longitude—Angular distance, in degrees, minutes, and seconds measured from the center of the Earth, of a point east or west of the Prime Meridian. Longitude may also be measured in decimal degrees.

Meridian—A great circle on the surface of the Earth, passing through the geographical poles and some third point on the Earth’s surface. All points on a given meridian have the same longitude.Parallel—A circle or approximation of a circle on the surface of the Earth, parallel to the Equator and connecting points of equal latitude.Planar—A map projection resulting from the conceptual projection of the Earth onto a tangent or secant plane. Usually, a planar projection is the same as an azimuthal projection.

Secant and tangent projections on a globe and map

Prime Meridian—The meridian of longitude 0 degrees, used as the origin for the measurement of longitude. The meridian of Greenwich, England, is the internationally accepted prime meridian in most cases.Projection parameters—A series of values that define a particular projection, and that tell how the projection is related to the Earth. Projection parameters may indicate the point of tangency, or the lines where a secant surface intersects the Earth. They also define the spheriod used to create the projection, and any other information necessary to identify the projection.Rhumb line—A rhumb line is a line on the surface of the Earth cutting all meridians at the same angle. A rhumb line shows true direction. Parallels and meridians, which also maintain constant true directions, may be considered special cases of the rhumb line. A rhumb line is a straight line on a Mercator projection. A straight rhumb line does not show the shortest distance between points unless the points are on the Equator or on the same meridian. A navigator can proceed between any two points along a rhumb line by maintaining a constant bearing, or compass direction.

Scale—The relationship between a distance on a map, chart, or photograph, and the corresponding distance on the Earth. Scale is usually given as a fraction or ratio: 1:2,000,000, or 1/2,000,000.

Secant—Cutting the sphere or spheroid along a line or lines. A secant cone or cylinder intersects the sphere or spheroid along two separate lines; these lines are parallels of latitude if the axes of the geometric figures coincide. A secant plane intersects the sphere or spheroid along a line that is a parallel of latitude if the plane is at right angles to the axis.

Spherical – Approximating the shape of a sphere.

Spheroid—A mathematical figure that approximates the shape of the Earth in form and size, and which is used as a reference surface for geodetic surveys. Used interchangeably with Ellipsoid.

Tangent—Touching at a single point or along a single line. A tangent cone or cylinder touches the sphere or spheroid along a single line. This line is a parallel of latitude if the axes of the geometric figures coincide.

Zenithal—A map projection in which the direction from a given central point to any other point is shown correctly. Also called an azimuthal projection.

Source:http://www.nationalatlas.gov/articles/mapping/a_projections.html

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