海角社区

A map is a two-dimensional representation of a three-dimensional object, location, or place usually drown on paper or flat surface. The most commonly used maps are 海角社区 maps, which show the shapes and sizes of countries, geographical features, and distances between points or locations. Thus, a map represents a real object or 海角社区 on a much smaller scale. It acts as a guide to places that a person has never been before. Maps are also essential tools for planning and urban development. Because of their numerous benefits, much work goes into creating some of the maps used today.

The art and science of creating and using maps is known as cartography. A person who is involved with map-making is known as a cartographer. Cartographers use data collected over time using various techniques, including aerial surveys, remote sensing, and photography. These data are used to design and create either electronic or physical maps. Besides the data, cartographers also use other tools like drafting scales, lettering aids, and scanners to create visually appealing maps.

Cartography is a complex 海角社区 that includes both the use of complex data and tools and terms that may have different meanings from the ordinary. A glossary of cartography and map terminologies is useful in understanding some of these terms and assist in interpreting the maps. Below are some terminologies you are likely to come across while using maps.聽

Important Terminologies

Definitions Provided By: Maps for America - Third Edition

accuracy: Degree of conformity with a standard. Accuracy relates to the quality of a result and is distinguished from precision which relates to the quality of the operation by which the result is obtained.

adjustment: Process designed to remove inconsistencies in measured or computed quantities by applying derived corrections to compensate for random or accidental errors.

adjustment, land- line: Positioning land lines on a map to indicate their true, theoretical, or approximate location relative to the adjacent terrain and culture, by reconciling the information shown on Bureau of Land Management plats and field records with the ground evidence of the location of the lines.

adjustment, standard accuracy: Adjustment of a survey resulting in values for positions and (or) elevations that comply with the National Map Accuracy Standards.

aerotriangulation: The process of developing a network of horizontal and or vertical positions from a group of known positions using direct or indirect measurements from aerial photographs and mathematical computations.

alidade:Instrument, or part of an instrument , for determining direction , either horizontal or vertical . In its simplest form, a peepsight or telescope mounted on a straightedge and used for plotting directions graphically. In such instruments as transits and theodolites, the alidade is the part containing the telescope and its attachments.

altimeter: Instrument for measuring altitudes or elevations with respect to a reference level, usually mean sea level. The most common type is an aneroid barometer. A radar altimeter determines the height of an aircraft above the terrain by measuring the time required for an electromagnetic pulse to travel from aircraft to the ground and back.

azimuth: Horizontal direction reckoned clockwise from the meridian plane.

backshore: Part of a beach that is usually dry and is reached only by the highest tides; by extension, a narrow strip of relatively flat coast bordering the sea.

bathymetry: Science of measuring water depths (usually in the ocean) to determine bottom topography.

beach (seabeach): Zone of unconsolidated material that extends landward from the low water line to the place where there is marked change in material or physiographic form, or to the line of permanent vegetation (usually the effective lint of storm waves).

bench mark: Relatively permanent material object, natural or artificial, bearing a marked point whose elevation above or below an adopted datum is known.

boundary monument: Material object placed on or near a boundary line to preserve and identify the location of the boundary line on the ground

boundary survey: Survey made to establish or to reestablish a boundary line on the ground, or to obtain data for constructing a map or plat showing a boundary line.

cadastral survey: Survey relating to land boundaries, made to create units suitable for title transfer or to define the limitations of title. Derived from "cadastre" meaning a register of land quantities, values, and ownership used levying taxes, the term may properly be applied to surveys of a similar nature outside the public lands, such surveys are more commonly called "land surveys" or "property surveys."

cartography: Science and art of making maps and charts. The term may be taken broadly as comprising all the steps needed to produce a map: planning, aerial photography, field surveys, photogrammetry, editing, color separation, and multicolor printing. Mapmakers, however, tend to limit use of the term to the map-finishing operations, in which the master manuscript is edited and color separation plates are prepared for lithographic printing.

chain: Unit of length equal to 66 feet, used especially in the U.S. public land surveys. The original measuring instrument (Gunter's chain) was literally a chain consisting of 100 iron links, each 7.92 inches long. Steel-ribbon tapes began to supersede chains around 1900, but surveying tapes are often still called "chains" and measuring with a tape is often called "chaining." The chain is a convenient unit in cadastral surveys because 10 square chains equal 1 acre.

chart: Special-purpose map designed for navigation or to present specific data or information. The term "chart" is applied chiefly to maps made primarily for nautical and aeronautical navigation, and to maps of the heavens, although the term is sometimes used to describe other special-purpose maps.

chart, aeronautical: Charts designed to meet requirements of aerial navigating, produced in several series, each on a specified map projection and differing in scale, format, and content, for use as dictated by type of aircraft and whether flight is to be conducted under visual or instrument flight rules.

chart, nautical: Representation of a portion of the navigable waters of the Earth and adjacent coastal areas on a specified map projection and designed specifically to meet requirements for marine navigation. Included on most nautical charts are depths of water, characteristics of the bottom, elevations of selected topographic features, general configurations and characteristics of the coast, the shoreline (usually the mean high water line), dangers, obstructions and aids to navigation limited tidal data, and information about magnetic variation in the charted area.

color separation: Process of preparing a separate drawing, engraving, or negative for each color required in the printing production of a map or chart.

compilation: Preparation of a new or revised map or chart, or portion thereof, from existing maps, aerial photographs, field surveys, and other sources.

continuous tone: Image not broken into dots by photographic screen; contains unbroken gradient tones from black to white, and may be either in negative or positive form. Aerial photographs are examples of continuous-tone prints. Contrasted with halftone (screened) and line copy.contourImaginary line on ground, all points of which are at the same elevation above or below a specific datum.contour intervalDifference in elevation between two adjacent contours.

control mapping: Points of established position or elevation, or both, which are used to fix references in positioning and correlating map features. Fundamental control is provided by stations in the national networks of triangulation and traverse (horizontal control) and leveling (vertical control). Usually it is necessary to extend geodetic surveys, based on fundamental stations, over the area to be mapped, to provide a suitable density and distribution of control points. Supplemental control points are those needed to relate the aerial photographs used for mapping with the system of ground control. These points must be positively photoidentified; that is, the points must be positively correlated with their images on the photographs.

control station: Point on the ground whose position (horizontal or vertical) is known and can be used as a base for additional survey work.

coordinates: Linear and (or) angular quantities that designate the position of a point in relation to a given reference frame.

coordinates, origin of: Points in a system of coordinates which serves as a zero point in computing the system's elements or in prescribing its use.cultureFeatures constructed by man that are under, on, or above the ground which are delineated on a map. These include roads, trails, buildings, canals, sewer systems, and boundary lines. In a broad sense, the term also applies to all names, other identification, and legends on a map.

datum听(辫濒.听datums): In surveying, a reference system for computing or correlating the results of surveys. There are tow 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.

declination: In astronomy, the angular distance of a celestial body above (north, plus) or below (south, minus) the celestial Equator. Magnetic declination is the angular difference between magnetic north and true (geographic) north at the point of observation; it is not constant but varies with time because of the "wandering" of the magnetic north pole.

depth curve: Line on a map or chart connecting points of equal depth below the datum.

diazo process: Rapid method for copying documents in which the image is developed by exposure to ammonia.

dike: Bank of earth or stone used to form a barrier, frequently and confusingly interchanged with levee. A dike restrains water within an area that normally is flooded.聽

electronic distance measuring (EDM) device: Instruments that measure the phase difference between transmitted and reflected or retransmitted electromagnetic waves of known frequency, or that measure the round-trip transit time of a pulsed signal, from which distance is computed.

elevation: Vertical distance of a point above or below a reference surface or datum.

ER-55 plotter: Double-projection plotting instrument utilizing ellipsoidal reflectors for light projection.

erosion: Group of natural processes including weathering, dissolution, abrasion, corrosion, and transportation that remove material from any part of the Earth's surface.

estuary: That portion of a stream influenced by the tide of the body of water into which it flows; an arm of the sea at a river mouth.f

eature separation: Process of preparing a separate drawing, engraving, or negative for selected types of data in the preparation of a map or chart.

flood plain: Belt of low flat ground bordering a stream channel that is flooded when runoff exceeds the capacity of the stream channel.

formlines: Lines, resembling contour lines, drawn to present a conception of the shape of the terrain without regard to a true datum or regular spacing

geodesy: Science concerned with the measurement and mathematical description of the size and shape of the earth and its gravitational fields. Geodesy also includes the large-scale, extended surveys for determining positions and elevations of points, in which the size and shape of the earth must be taken into account.

geoid: Figure of the Earth visualized as a mean sea level surface extended continuously through the continents. It is a theoretically continuous surface that is perpendicular at every point to the direction of gravity (the plumbline).

graticule: Network of parallels and meridians on a map or chart.

graticule, geographic: System of coordinates of latitude and longitude used to define the position of a point on the surface of the Earth with respect to the reference spheroid.

grid: Network of uniformly spaced parallel lines intersecting at right angles. When superimposed on a map, it usually carries the name of the projection used for the map- that is, Lambert grid, transverse Mercator grid, universal transverse Mercator grid.

hachure: Any series of lines used on a map to indicate the general direction and steepness of slopes. The lines are short, heavy, and close together for steep slopes; longer, lighter, and more widely spaced for gentle slopes.

halftone: A picture in which the gradations of light are obtained by the relative darkness and density of tiny dots produced by photographing the subject through a fine screen.

high water: Maximum height reached by a rising tide. The height may be due solely to the periodic tidal forces or it may have superimposed upon it the effects of prevailing meteorological conditions. Use of the "high tide" is discouraged.

high water line: Intersection of the land with the water surface at an elevation of high water.

high water mark: Line or mark left upon tidal flats, beach, or along shore objects indicating the elevation or the intrusion of high water.

hydrographic survey: Survey of water area, with particular reference to submarine relief, and any adjacent land.聽

hydrography: Science that deals with the measurement and description of the physical features of the oceans, seas, lakes, rivers, and their adjoining coastal areas, with particular reference to their use for navigation.

hydrology: Scientific study of the waters of the Earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of ground water.

hypsography: Topography referred to the national geodetic vertical datum of 1929. The science or art of describing heights of land surfaces with reference to this datum.

hypsometry: Science or art of determining terrain relief, by any method.

imagery: Visible representation of objects and (or) phenomena as sensed or detected by cameras, infrared and multispectral scanners, radar, and photometers. Recording may be on photographic emulsion (directly as in a camera or indirectly after being first recorded on magnetic tape as an electrical signal) or on magnetic tape for subsequent conversion and display on a cathode ray tube.

infrared scanner (thermal mapper): Instrument that detects infrared radiation and converts the detected energy to an electrical signal for recording on photographic film or magnetic tape.

isogonic chart: Chart showing isogonic lines properly labeled with their magnetic declination.

isogonic line: Line joining points on the Earth's surface having equal magnetic declination as of a given date.

Kelsh plotterDouble-projection plotting instrument utilizing swinging lamps to transmit light through contact- size diapositives (positive transparencies).

land use classification system: Coding system of categories and subcategories designed for use on a map to designate land or water use.

landmark: Monument of material mark or fixed object used to designate a land boundary on the ground: any prominent object on land that may be used to determine a location or a direction in navigation or surveying.

latitude: Angular distance, in degrees, minutes, and seconds of a point north or south of the Equator.

lead line: Line weighted with lead for making depth soundings in water.

levee: Artificial bank confining a stream channel or limiting adjacent areas subject to flooding; an embankment bordering a submarine canyon or channel, usually occurring along the outer edge of a curve.

level surface: Surface which at every point is perpendicular to the plumbline or the direction in which gravity acts.

leveling: Surveying operation in which heights of objects and points are determined relative to a specified datum.

line copy (line drawing):聽Map copy suitable for reproduction without the use of a screen; a drawing composed of lines as distinguished from continuous- tone copy.

longitude: Angular distance, in degrees, minutes, and seconds, of a point east or west of the Greenwich meridian.

low water: Minimum height reached by a falling tide. The height may be due solely to the periodic tidal forces or it may have superimposed upon it the effects of meteorological conditions.

low water line: Intersection of the land with the water surface at an elevation of low water. Not to be confused with mean low water line.

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.聽

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聽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, 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, thematicMap 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聽

Metonic cycle:聽(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.聽

mean low water: Tidal datum that is the arithmetic mean of the low water heights observed over a specific 19-year聽

Metonic cycle:聽(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.聽

meander line: Metes-and-bounds traverse approximately along the mean high water line聽of a permanent body of water. By following the sinuosities of the bank or聽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聽metes and bounds聽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.

national geodetic vertical datum of 1929: Reference surface established by the U.S. Coast and Geodetic Survey in 1929 as the datum to which relief features and elevation data are referenced in the conterminous United States; formerly called "mean sea level 1929.

"National Map Accuracy Standards: Specifications promulgated by the U.S. Office of Management and Budget to govern accuracy of topographic and other maps produced by Federal Agencies.

navigable waters: Water usable, with or without improvements, as routes for commerce in the customary means of travel on water.

neatline: Line separating the body of a map from the map margin. On a standard聽quadrangle聽map, the neatlines are the meridians and parallels delimiting the quadrangle.

oceanic survey: Survey or examination of condition in the ocean or any part of it, with reference to animal or plant life, chemical elements present, temperature gradients, etc.

offshore: Comparatively flat zone of variable width that extends from the outer margin of the rather steeply sloping shoreface to the edge of the continental shelf.

orientation: Establishing correct relationship in direction with reference to points of the compass; the state of being in correct relationship in direction with reference to the points of the compass.

origin of coordinates: Point in a system of coordinates that serves as a zero point in computing the system's elements or in prescribing its use.

orthophotograph: Photograph having the properties of an orthographic projection. It is derived from a conventional perspective photograph by simple or differential rectification so that image displacements caused by camera tilt and terrain relief are removed.

orthophotographic map: Map produced by assembling orthophotographs at a specified uniform scale in a map format.

orthophotomap: Orthophotographic map with contours and cartographic treatment, presented in a standard format, and related to standard reference systems.

orthophotoquad: Monocolor orthophotgraphic map presented in a standard quadrangle format and related to standard reference systems. It has no contours and little or cartographic treatment.

orthophotoscope: Photomechanical device used in conjunction with a double-projection stereoplotter for producing orthophotograph.

overedge: Any portion of a map lying outside the nominal map border (neatline).

overlay: Printing or drawing on a transparent or translucent medium intended to be placed in register on a map or other graphic and which shows details not appearing or requiring special emphasis on the base material.

overprint: New material printed on a map or chart to show data of importance or special use, in addition to those data originally printed.

parallel of latitude: A circle, or approximation of a circle, on the surface of the Earth, parallel tot he Equator, and connecting points of equal聽

latitude: a circle of the celestial sphere parallel to the ecliptic, and connecting points of equal celestial latitude.

photogrammetry: Science or art of obtaining reliable measurements or information from photographs or other sensing systems.

photomap (photographic map): Map made by adding marginal information, descriptive data, and a reference system to a photograph or assembly of photographs.

plain: Region of uniform general slope, comparatively level, of considerable extent, and not broken by marked elevations and depressions (it may be an extensive valley floor or a plateau summit); an extent of level or nearly level land; a flat, gently sloping, or nearly level region of the sea floor.

planetable: Instrument consisting essentially of a drawing board on a tripod and some type of sighting device (alidade) with attached straightedge, used for plotting the lines of survey directly from observation in the field.

planimetry: Plan details of a map - those having no indication of relief or contour.

plat: Diagram drawn to scale showing all essential data pertaining to the boundaries and subdivisions of a tract of land, as determined by survey or protraction. As used by the Bureau of Land Management, the drawing which represents the particular area included in a survey, such as township, private land claim, or mineral claim, and the lines surveyed, established, or retraced, showing the direction and length of each such line; The relation to the adjoining official surveys; the boundaries, descriptions, and area of each parcel of land subdivided; and, as nearly as may be practicable, a representation of the relief and improvements within the limits of the survey.

prime meridian: Meridian of longitude 0 degrees, used as the origin for measurements of聽longitude. The meridian of Greenwich, England, is the internationally accepted prime meridian on most charts. However, local or national prime meridians are occasionally used.

public land system: Public lands are subdivided by a rectangular system of surveys established and regulated by the Bureau of Land Management. The standard format for subdivision is by聽townshipsmeasuring 6 miles (480聽chains) on a side. Townships are further subdivided into 36 numbered sections of 1 square mile (640 acres) each.

quad-centered photograph: Middle exposure of a phototriplet (three consecutive aerial photographs) take so that the middle photograph is exposed directly above the center of the聽quadrangle聽and the preceding and following photographs are exposed directly above the boundaries of the quadrangle. The flying height is set such that the quad-centered photograph covers the entire quadrangle.

quadrangle: Four-sided area, bounded by聽parallels of latitude听补苍诲听meridians聽of longitude used as an area unit in mapping (dimensions are not necessarily the same in both directions). Also, a geometric figure of significance in geodetic surveying.

radial-line plotting: Determination of the location of points by the successive intersection and resection of direction lines radiating from the radial centers of overlapping aerial photographs.

rectification, differential: The process of scanning and reprojecting a photograph onto a horizontal plane in differential elements to remove displacements caused by tilt and relief. The process may be accomplished by any one of a number of instruments developed specifically for the purpose.

rectification, simple: Projection of an aerial photograph (mathematically, graphically, or photographically) from its plane onto a horizontal plane by translation, rotation, and (or) scale change to remove displacement due to tilt of the camera.

relief: Elevations and depressions of the land or sea bottom.

relief shading: Technique for making hypsography on a map appear three dimensional by the use of graded shadow effects. Generally, the features are shaded as though illuminated from the northwest.

remote sensing: Process of detecting and (or) monitoring chemical or physical properties of an area by measuring its reflected and emitted radiation.

representational fraction: Scale of a map or chart expressed as a fraction or ratio that relates unit distance on the map to distance measured in the same unit on the ground.

reproduction: Summation of all processes involved in printing copies from an original drawing. A printed copy of an original drawing made by the processes of reproduction

scale: Relationship existing between a distance on a map, chart, or photograph and the corresponding distance on the Earth.

sea level (water level): Height of the surface of the sea at any given time.

section: Unit of subdivision of a聽township; normally a quadrangle 1 mile square with boundaries conforming to meridians听补苍诲听parallels聽within established limits, and containing 640 acres as nearly as practicable.

sensor: Technical means, usually electronic, to extend man's natural senses by detecting emitted or reflected energy. The energy may be nuclear, electromagnetic (including the visible and invisible portions of the spectrum), chemical, biological, thermal, or mechanicalshorelineIntersection of the land with the water surface.

spheroid: Mathematical figure closely approaching the geoid in form and size and used as a surface of reference for geodetic surveys. A reference spheroid or ellipsoid is a spheroid determined by revolving an ellipse about its shorter (polar) axis and used as a base for geodetic surveys of a large section of the Earth (such as the Clarke spheroid of 1866 which is used for geodetic surveys in the United States).

spot elevation: Point on a map or chart whose height above a specified datum is noted, usually by a dot or a small sawbuck and elevation value. Elevations are shown, on a selective basis, for road forks and intersections, grade crossings summit of hills, mountain.

stadia: Technique of distance measurement wherein the observer reads the intercept subtended on a graduated rod between two marks on the reticle of the telescope.

state plane coordinate system: Coordinate systems established by the U.S. Coast and Geodetic Survey (now the National Ocean Survey), usually one for each state, for use in defining positions of points in terms of plane rectangular (x,y) coordinates.

stereocompilation: Production of a map or chart manuscript from aerial photographs and geodetic control data by means of photogrammetric instruments.

stereoplotter: Instrument for plotting a map by observation of stereomodels formed by pairs of photographs.

stereoscopicPertaining to the use of binocular vision for observation of a pair of overlapping photographs or other perspective views, giving impression of depth.

subsidence: Decrease in the elevation of land surface due to tectonic, seismic, or artificial forces, without removal of surface material.

survey: Orderly process of determining data relating to any physical or chemical characteristics of the Earth. The associated data obtained in a survey. An organization engaged in making a survey.

tacheometer (tachymeter): Surveying instrument designed for use in the rapid determination of distance, direction, and difference of elevation from a single observation, using a short base which may be an intergraph part of the instrument.

theodolite: Precision surveying instrument for measuring horizontal and vertical angles.

tide: Periodic rise and fall of the water resulting from gravitational interactions between the Sun, Moon, and Earth. The vertical component of the particulate motion of a tidal wave. Although the accompanying horizontal movement of the water is part of the same phenomenon, it is preferable to designate this motion as tidal current.t

topography: Configuration (relief) of the land surface; the graphic delineation or portrayal of that configuration in map form, as by contour lines; in oceanography the term is applied to a surface such as the sea bottom or surface of given characteristics within the water mass.

township: Unit of survey of the public lands of the United States, normally a quadrangle approximately 6 miles on a side with boundaries conforming to聽meridians听补苍诲听parallels聽within established limits, containing 36 sections. Also, in minor governmental subdivision.

transit: Precision surveying instrument; a theodolite in which the telescope can be reversed in direction by rotation about its horizontal axis.

traverse: Sequence of lengths and directions of lines connecting a series of stations, obtained from field measurements, and used in determining positions of the stations.

triangulation: Method of extending horizontal position on the surface of the Earth by measuring the angles of triangles and the included sides of selected triangles.

trilateration: Method of surveying wherein the lengths of the triangle sides are measured, usually by electronic methods, and the angles are computed from the measured lengths. Compare with triangulation.

Universal Transverse Mercator (UTM) grid: Military grid system based on the transverse Mercator projection, applied to maps of the Earth's surface extending from the Equator to 84 Degrees north and 80 degrees south latitudes

upland: Highland; ground elevation above the lowlands along rivers or between hills.

zenith telescope: Instrument for observing starts near the zenith (a point on the celestial sphere directly above the observer's position). ground below the water table where all the pores in rock, sediment, and soil are filled with water