English

(1) The vertical distance of a POINT or a LEVEL, on the surface of the EARTH, measured from a specified vertical DATUM usually MEAN SEA LEVEL. The term elevation is sometimes confused with ALTITUDE. For paper NAUTICAL CHARTS, this term is often synonymous with HEIGHT. (2) For navigational lights, such as lighthouses, the vertical distance of the focal point of the light measured from a specified vertical DATUM. (3) An area higher than its surroundings, as a hill.

Also called height of eye. The visibility distances are relevant with the elevation of the observer's sight. When measuring a long distance by optics a line of sight should be maintained over the convex curvature of the earth and all obstacles between the measuring points.

A plane curve constituting the locus of all points the sum of whose distances from two fixed points, called foci, is constant. Also used to characterize errors in positions and measurements.

A simple mathematical surface which best approximates the shape of the EARTH. It is the surface generated by an ellipse revolving about one of its AXES. See also SPHEROID: OBLATE, SPHEROID: PROLATE and REFERENCE SPHEROID.

The ANGULAR DISTANCE of a body of the SOLAR SYSTEM from the SUN. The angle at the EARTH between lines to the SUN and another CELESTIAL BODY of the SOLAR SYSTEM. The term is usually used in connection with INFERIOR PLANETS. The greatest elongation of such a body is its maximum ANGULAR DISTANCE from the SUN, as observed from the EARTH. The position of a STAR when its AZIMUTH east or west of the MERIDIAN is greatest.

In CARTOGRAPHY, enamelled working surface (normally white), on aluminium, zinc or other dimensionally stable medium. Used to prepare colour separation originals.

The act, process or art of cutting or etching designs or letters on metal plates, wooden blocks, etc. for printing. Any printed impression made from an engraved surface.