magnetic declination
See: declination

map
Graphic representation of the physical features (natural, artificial, or both) of a part or the whole of the Earth's surface, by means of signs and symbols or photographic imagery, at an established scale, on a specified projection, and with the means of orientation indicated.

map, base
Map on which information may be placed for purposes of comparison or geographical correlation. The term "base map" was at one time applied to a class of maps now known as outline maps. It may be applied to topographic maps, also termed "mother maps" that are used in the construction of other types of maps by the addition of particular data.

map, bathymetric
Maps delineating the form of the bottom of a body of water, or a portion thereof, by the use of depth contours (isobaths).

map, cadastral
Map showing the boundaries of subdivisions of land, often with the bearings and lengths thereof and the areas of individual tracts, for purposes of describing and recording ownership. It may also show culture, drainage, and other features relating to land use and value. See:plat

map, choropleth
Thematic map in which areas are colored, shaded, dotted, or hatched to create darker or lighter areas in proportion to the density of distribution of the theme subject.

map digitization
Conversion of map data from graphic to digital form.

map, engineering
Map showing information that is essential for planning an engineering project or development and for estimating its cost. It usually is a large-scale map of a small area or of a route. It may be entirely the product of an engineering survey, or reliable information may be collected from various sources for the purpose, and assembled on a base map.

map, flood control
Map designed for studying and planning control projects in areas subject to flooding.

map, forestry
Map prepared principally to show the size, density, kind, and value of trees in a designated area.

map, geologic
Map showing the structure and composition of geologic features.

map hypsographic
Map showing relief with elevations referred to the national geodetic vertical datum of 1929.

map, hypsometric
Map showing relief by any convention, such as contours, hachures, shading, or tinting.

map, isopleth
Map consisting of lines connecting places of equal value of distribution for a given theme such as rainfall or temperature.

map, land use
Map showing by means of a coding system the various purposes for which parcels of land are being used by man.

map, line
Map composed of lines as distinguished from photographic imagery.

map, orthophotographic
See: orthophotographic map

map, photographic
See: photomap

map, planimetric
Map that presents only the horizontal positions for features represented. distinguished from a topographic map by the omission of relief in measurable form. The features usually shown on a planimetric map include rivers, lakes, and seas; mountains, valleys, and plains; forests, and prairies; cities, farms transportation routes, and public utility facilities; and political and private boundary lines. A planimetric map intended for special use may present only those features essential to the purpose to be served.

map projection
Orderly system of lines on a plane representing a corresponding system of imaginary lines on an adopted terrestrial or celestial datum surface. Also, the mathematical concept for such a system. For maps of the Earth, a projection consists of 1) a graticule of lines representing parallels of latitude and meridians of longitude or 2) a grid.

map series
Family of maps conforming generally to the same specifications and designed to cover an area or a country in systematic pattern.

map, slope (clinometric map)
Map showing the degree of steepness of the Earth's surface by the use of various colors or shading for critical ranges of slope.

map, soil
Map that shows the constitution, structure, and texture of the soil and identifies ongoing erosion.

map, storm evacuation
Map designed to identify coastal areas subject to flooding, to indicate recommended areas of refuge, and to emphasize available evacuation routes.

map, thematic
Map designed to provide information on a single topic, such as geology, rainfall, population.

map, topographic
Map that present the horizontal and vertical positions of the features represented; distinguished from a planimetric map by the addition of relief in measurable form.

marsh, coastal
Area of salt-tolerant vegetation in brackish and (or) saline-water habitants subject to tidal inundation.

marsh, freshwater
Tract of low wet ground, usually miry and covered with rank vegetation.

mean high water
Tidal datum that is the arithmetic mean of the high water heights observed over a specific 19-year Metonic cycle (National Tidal Datum Epoch). For stations with shorter series, simultaneous observations are made with a primary control tide station to derive the equivalent of a 19-year value. Use of "mean high tide" is discouraged.

mean high water line
Intersection of the land with the water surface at the elevation of high water. See: shoreline

mean low water
Tidal datum that is the arithmetic mean of the low water heights observed over a specific 19-year Metonic cycle (National Tidal Datum Epoch). For stations with shorter series, simultaneous observations are made with a primary control tide station to derive the equivalent of a 19-year value. Use of "mean low tide" is discouraged.

mean low water line
Intersection of the land with the water surface at the elevation of low water.

mean sea level
Tidal datum that is the arithmetic mean of the hourly water elevations observed over a specific 19-year Metonic cycle (National Tidal Datum Epoch). Shorter series are specified in the name; that is, monthly mean sea level and yearly mean sea level. See: datum

meander line
Metes-and-bounds traverse approximately along the mean high water line of a permanent body of water. By following the sinuosities of the bank or shoreline, the meander line provides data for computing the area of land remaining after the water area has been segregated. A meander line differs from other metes and bounds surveys in that it does not ordinarily determine or fix boundaries.

meanderable
Capable of being depicted by reference to a meander line.

meridian
Great circle on the surface of the Earth passing through the geographical poles and any given point on the Earth's surface. All points on a given meridian have the same longitude.

metes and bounds
Method of describing land by measure of length (metes) of the boundary lines (bounds).

Metonic cycle
Period of 235 lunations or about 19 years. devised by Meton, an Athenian astronomer (5th century B.C.) for the purpose of obtaining a period at the end of which the phases of the moon recur in the same order and on the same days as in the preceding cycle.

metric system
Decimal system of weights and measures based on the meter as a unit length and the kilogram as a unit mass.

monoscopic
Pertaining to the observation of a single photograph or other view.

monument (surveying)
Permanent physical structure marking the location of a survey point. Common types of monuments are inscribed metal tablets set in concrete posts; and metal rods driven in the ground.

mosaic, aerial
Assembly of aerial photographs whose edges usually have been torn or cut selectively and matched to the imagery on adjoining photographs to form a continuous representation of a portion of the Earth's surface.

multiplex
Stereoplotter of the double-projection type characterized by its use of reduced- scale diapositives and stationary lamphouses with condensing lenses.

multispectral scanner (MSS)
Device for sensing radian energy in several channels of the electromagnetic spectrum.