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Glossary
Antipodes - Two points on opposite sides of a planet.
Arc-second - 1/3600th of a degree (1 second) of latitude or longitude.
ARC/Info - A largely UNIX-oriented GIS developed and distributed by the Environmental Systems Research Institute, Inc.
Aspect - The conceptual placement of a projection system in relation to the Earth's axis (direct, normal, polar, equatorial, oblique, and so on).
Authalic projection - See Equal-area projection.
Azimuth - The angle a line makes with a meridian, taken clockwise from north.
Azimuthal projection - A projection on which the azimuth or direction from a given central point to any other point is shown correctly. When a pole is the central point, all meridians are spaced at their true angles and are straight radii of concentric circles that represent the parallels. Also called a zenithal projection.
Bathymetry - The measurement of water depths of oceans, seas, lakes, and other bodies of water.
Bowditch, Nathaniel - A late 18th/early 19th century mathematician, astronomer, and sailor who "wrote the book" on navigation. John Hamilton Moore's The Practical Navigator was the leading navigational text when Bowditch first went out to sea, and had been for many years. Early in his first voyage, however, Bowditch began noticing errors in Moore's book, which he recorded and later used in preparing an American edition of Moore's work. The revisions were to such an extent that Bowditch was named the principal author, and the title was changed to The New American Practical Navigator, published in 1802. In 1868 the U.S. Navy bought the copyright to the book, which is still commonly referred to as "Bowditch" and considered the "bible" of navigation.
Cartography - The art or practice of making charts or maps.
Central meridian - The meridian passing through the center off a projection, often a straight line about which the projection is symmetrical.
Central projection - A projection in which the Earth is projected geometrically from the center of the Earth onto a plane or other surface. The Gnomonic and Central Cylindrical projections are examples.
Choropleth - A map consisting of areas of equal value separated by abrupt boundaries and colored or shaded according to those values.
Complex curves - Curves that are not elementary forms such as circles, ellipses, hyperbolas, parabolas, and sine curves.
Composite projection - A projection formed by connecting two or more projections along common lines such as parallels of latitude, necessary adjustments being made to achieve fit. The Goode Homolosine projection is an example.
Conformal projection - A projection on which all angles at each point are preserved. Also called an orthomorphic projection.
Conceptually projected - The convenient way to visualize a projection system, although it may not correspond to the actual mathematical projection method.
Conic projection - A projection resulting from the conceptual projection of the Earth onto a tangent or secant cone, which is then cut lengthwise and laid flat. When the axis of the cone coincides with the polar axis of the Earth, all meridians are straight equidistant radii of concentric circular arcs representing the parallels, but the meridians are spaced at less than their true angles. Mathematically, the projection is often only partially geometric.
Constant scale - A linear scale that remains the same along a particular line on a map, although that scale may not be the same as the stated or nominal scale of the map.
Contour - All points that are at the same elevation above or below a specified datum.
Conventional aspect - See Normal aspect.
Correct scale - A linear scale having exactly the same value as the stated or nominal scale of the map, or a scale factor of 1.0. Also called true scale.
Cylindrical projection - A projection resulting from the conceptual projection of the Earth onto a tangent or secant cylinder, which is then cut lengthwise and laid flat. When the axis of the cylinder coincides with the axis of the Earth, the meridians are straight, parallel, and equidistant, while the parallels of latitude are straight, parallel, and perpendicular to the meridians. Mathematically, the projection is often only partially geometric.
Dead reckoning - From "deduced reckoning," the estimation of geographic position based on course, speed and time.
DEM (Digital Elevation Map/Model) - Elevation data in the form of a matrix map, generally on a regular grid. DEM also refers to the five primary types of digital elevation models produced by the U.S. Geological Survey, one of which is the 1-degree (3-arc-second resolution) model that is interfaced through the Mapping Toolbox.
Departure - The arc length distance along a parallel of a point from a given meridian.
Developable surface - A simple geometric form capable of being flattened without stretching. Many map projections can be grouped by a particular developable surface: cylinder, cone, or plane.
Direct aspect - See Normal aspect.
Distortion - A variation of the area or linear scale on a map from that indicated by the stated map scale, or the variation of a shape or angle on a map from the corresponding shape or angle on the Earth.
DMS - Degrees-minutes-seconds angle notation of the form ddd° mm' ss''. There are 60 seconds in a minute, and 60 minutes in a degree. In the Mapping Toolbox, when "dms" angles are represented by a single number, the format is dddmm.ss.
Ellipsoid - When used to represent the Earth, a solid geometric figure formed by rotating an ellipse about its minor (shorter) axis. Also called spheroid.
Equal-area projection - A projection on which the areas of all regions are shown in the same proportion to their true areas. Shapes may be greatly distorted. Also called an equivalent or authalic projection.
Equatorial aspect - An aspect of an azimuthal projection on which the center of projection or origin is some point along the Equator. For cylindrical and pseudocylindrical projections, this aspect is usually called conventional, direct, normal, or regular rather than equatorial.
Equidistant projection - A projection that maintains constant scale along all great circles from one or two points. When the projection is centered on a pole, the parallels are spaced in proportion to their true distances along each meridian.
Equireal projection - See Equal-area projection.
Equivalent projection - See Equal-area projection.
Flat-polar projection - A projection on which, in normal aspect, the pole is shown as a line rather than as a point.
Free of distortion - Having no distortion of shape, area, or linear scale. On a flat map, this condition can exist only at certain points or along certain lines.
General matrix map - In the Mapping Toolbox, a matrix map defined with latitude-longitude coordinate matrices, allowing irregular, non-rectangular orientations.
Geoid - The true shape of the Earth, an irregular and complex shape which is usually modeled with a sphere or an ellipsoid. In the Mapping Toolbox, this term refers to the spherical or ellipsoidal model of the Earth or other planet in use rather than the true shape.
Geoid vector - In the Mapping Toolbox, a vector describing the geoid, or ellipsoid, model. The geoid vector has the form:
Geometric projection - See Perspective projection.
Geographic data structure - In the Mapping Toolbox, A MATLAB data structure containing the data and other information for the proper display of map objects. Valid fields in the structure include type, tag, and altitude.
GIS (Geographic Information System) - A system, usually computer based, for the input, storage, retrieval, analysis, and display of interpreted geographic data.
Globular projection - Generally, a nonazimuthal projection developed before 1700 on which a hemisphere is enclosed in a circle and meridians and parallels are simple curves or straight lines.
Graticule - A network of lines representing a selection of the Earth's parallels and meridians for the purpose of projection. The vertices of the graticule grid are precisely projected, and the map data contained in any grid cell is warped to fit the resulting quadralateral. A finer graticule grid results in a higher projection fidelity at the expense of greater computational requirements.
Great circle - Any circle on the surface of a sphere, especially when the sphere represents the Earth, formed by the intersection of the surface with a plane passing through the center of the sphere. It is the shortest path between any two points along the circle and therefore important for navigation. All meridians and the Equator are great circles on the Earth taken as a sphere.
HMS - Hours-minutes-seconds time notation of the form hh° mm' ss''. In the Mapping Toolbox, when "hms" times are represented by a single number, the format is hhmm.ss.
Homalographic/homolographic projection - See Equal-area projection.
Hydrography - The science of measurement, description, and mapping of the surface waters of the Earth, especially with reference to their use in navigation. The term also refers to those parts of a map collectively that represent surface waters.
Hydrology - The 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 - The scientific study of the Earth's topologic configuration above sea level, especially the measurement and mapping of land elevation.
Indexed map - A matrix map in which entries are an index value into another data source. The worldmtx workspace contains an example of an indexed map. Each entry in the matrix map is an index into a data structure containing the names of the world countries.
Indicatrix - A circle or ellipse having the same shape as that of an infinitesimally small circle (having differential dimensions) on the Earth when it is plotted with finite dimensions on a map projection. Its axes lie in the directions of and are proportional to the maximum and minimum scales at that point. This is useful in illustrating the distortions of a given map projection. Often called a Tissot indicatrix after the originator of the concept. In the Mapping Toolbox, Tissot indicatrices may be displayed using the tissot command, and indicatrices for all supported projections are provided in the "Projections Reference" chapter of the online Mapping Toolbox reference documentation.
Interrupted projection - A projection designed to reduce peripheral distortion by making use of separate sections joined at certain points or along certain lines, usually the Equator in the normal aspect, and split along lines that are usually meridians. There is normally a central meridian for each section. The Mapping Toolbox does not include interrupted projections, but the user can separate data into sections and project these independently to achieve this effect.
Large-scale mapping - Mapping at a scale larger than about 1:75,000, although this limit is somewhat flexible.
Latitude (geographic) - The angle made by a perpendicular to a given point on the surface of a sphere or ellipsoid representing the Earth and the plane of the Equator (positive if the point is north of Equator, negative if it is south). One of the two common geographic coordinates of a point on the Earth.
Latitude of opposite sign - See Parallel of opposite sign.
Legs - Line segments connecting waypoints.
Limiting forms - The form taken by a system of projection when the parameters of the formulas defining that projection are allowed to reach limits that cause it to be identical with another separately defined projection.
Logical map - A binary matrix map consisting entirely of 1s and 0s. An example of a logical matrix map can be created with the topo map by performing a logical test for positive elevations (topo>0). Each entry in the matrix map contains a 1 if it is above sea level, or a 0 if it is at or below sea level.
Longitude - The angle made by the plane of a meridian passing through a given point on the Earth's surface and the plane of the (prime) meridian passing through Greenwich, England, east or west to 180 (positive if the point is east, negative if it is west). One of the two common geographic coordinates of a point on the Earth.
Map A representation of geographic data. In the Mapping Toolbox, a map is any variable or set of variables (electronically) representing or assigning values to a geographic location or region, from a single point to an entire planet.
Map axes - In the Mapping Toolbox, a normal MATLAB axes altered for mapping display purposes. Several map axes properties are defined and stored in the UserData slot of the MATLAB axes. These properties control the appearance of the map display, much like the properties of the normal MATLAB axes control the appearance of the displayed plot. A map axes must first be defined in order to display maps using the Mapping Toolbox.
Map frame - In the Mapping Toolbox, a projected "box" or quadrangle enclosing the geographic display.
Map grid - A displayed network of lines representing parallels and meridians. The grid is used for visual reference and should not be confused with the graticule.
Map legend - In the Mapping Toolbox, a vector defining the geographic placement and unit cell size of a regular matrix map. A map legend has the form:
MapInfo - A largely PC-oriented GIS developed and distributed by the MapInfo Corporation.
Matrix map - A map consisting of a matrix (or matrices) of values corresponding to specific geographic points. In the Mapping Toolbox, matrix maps can be defined as regular or general, depending on the structure and orientation of the geographic points. See Regular matrix map and General matrix map.
Meridian - A reference line on the Earth's surface formed by the intersection of the surface with a plane passing through both poles and some third point on the surface. This line is identified by its longitude. On the Earth as a sphere, this line is half a great circle; on the Earth as an ellipsoid, it is half an ellipse.
Minimum-error projection - A projection having the least possible total error of any projection in the designated classification, according to a given mathematical criterion. Usually, this criterion calls for the minimum sum of squares of deviations of linear scale from true scale throughout the map ("least squares").
NGVD 29 (National Geodetic Vertical Datum of 1929) - A reference surface established by the U.S. Coast Guard and Geodetic Survey of 1929, used as the datum for which relief features and elevation data are referenced in the conterminous United States; formerly called "mean sea level 1929."
Nominal scale - The stated scale at which a map projection is constructed.
Normal aspect - A form of a projection that provides the simplest graticule and calculations. It is the polar aspect for azimuthal projections, the aspect having a straight Equator for cylindrical and pseudocylindrical projections, and the aspect showing straight meridians for conic projections. Also called conventional, direct, or regular aspect.
Oblique aspect - An aspect of a projection on which the axis of the Earth is rotated so it is neither aligned with nor perpendicular to the conceptual axis of the map projection.
Orthoapsidal projection - A projection on which the surface of the Earth taken as a sphere is transformed onto a solid other than the sphere and then projected orthographically and obliquely onto a plane for the map.
Orthographic projection - A specific azimuthal projection or a type of projection in which the Earth is projected geometrically onto a surface by means of parallel projection lines.
Orthomorphic projection - See Conformal projection.
Parallel - A small circle on the surface of the Earth formed by the intersection of the surface of the reference sphere or ellipsoid with a plane parallel to the plane of the Equator. This line is identified by its latitude. The Equator (a great circle) is usually also treated as a parallel.
Parallel of opposite sign - A parallel that is equally distant from but on the opposite side of the Equator. For example, for lat 30(°N (or +30°), the parallel of opposite sign is lat 30° S (or -30°). Also called latitude of opposite sign.
Parameters - The values of constants as applied to a map projection for a specific map; examples are the values of the scale, the latitudes of the standard parallels, and the central meridian. The required parameters vary with the projection.
Perspective projection - A projection produced by projecting straight lines radiating from a selected point (or from infinity) through points on the surface of a sphere or ellipsoid and then onto a tangent or secant plane. Other perspective maps are projected onto a tangent or secant cylinder or cone by using straight lines passing through a single axis of the sphere or ellipsoid. Also called geometric projection.
Planar projection - A 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. Mathematically, the projection is often only partially geometric.
Planimetric map - A map representing only the horizontal positions of features (without their elevations).
Polar aspect - An aspect of a projection, especially an azimuthal one, on which the Earth is viewed from the polar axis. For cylindrical or pseudocylindrical projections, this aspect is called transverse.
Pole - An extremity of a planet's axis of rotation. The North Pole is a singular point at 90°N for which longitude is ambiguous. The South Pole has the same characteristics and is located at 90°S.
Polyconic projection - A specific projection or member of a class of projections that are constructed like conic projections but with different cones for each parallel. In the normal aspect, all the parallels of latitude are nonconcentric circular arcs, except for a straight Equator, and the centers of these circles lie along a central axis.
Projection - A systematic representation of a curved 3-D surface such as the Earth onto a flat 2-D plane. Each map projection has specific properties that make it useful for specific purposes.
Pseudoconic projection - A projection that, in the normal aspect, has concentric circular arcs for parallels and on which the meridians are equally spaced along the parallels, like those on a conic projection, but on which meridians are curved.
Pseudocylindrical projection - A projection that, in the normal aspect, has straight parallel lines for parallels and on which the meridians are (usually) equally spaced along parallels, as they are on a cylindrical projection, but on which the meridians are curved.
Quadrangle - A region bounded by parallels north and south, and meridians east and west.
Reckoning - The determination of geographic position by calculation.
Regional map - A small-scale map of an area covering at least 5 or 10 degrees of latitude and longitude but less than a hemisphere.
Regular aspect - See Normal aspect.
Regular matrix map - In the Mapping Toolbox, an equiangular (equal-angle) matrix map defined with a map legend vector, limited to a rectangular orientation.
Retroazimuthal projection - A projection on which the direction or azimuth from every point on the map to a given central point is shown correctly with respect to a vertical line parallel to the central meridian. The reverse of an azimuthal projection.
Rhumb line - A complex curve (a spherical helix) on a planet's surface that crosses every meridian at the same oblique angle; a navigator can proceed between any two points along a rhumb line by maintaining a constant heading. A rhumb line is a straight line on the Mercator projection. Also called a loxodrome.
Scale - The ratio of the distance on a map or globe to the corresponding distance on the Earth; usually stated in the form 1:5,000,000 for example.
Scale factor - The ratio of the scale at a particular location and direction on a map to the stated scale of the map. At a standard parallel, or other standard line, the scale factor is 1.0.
Secant cone, cylinder, or plane - A secant cone or cylinder intersects the sphere or ellipsoid 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 ellipsoid along a line that is a parallel of latitude if the plane is at right angles to the axis.
Shaded Relief - Shading added to a map or image that makes it appear to have three-dimensional aspects. This type of enhancement is commonly done to satellite images and thematic maps utilizing digital topographic data to provide the appearance of terrain relief.
Similar (projection) - Subjective and qualitative term indicating a moderate or strong resemblance.
Singular points - Certain points on most but not all conformal projections at which conformality fails, such as the poles on the normal aspect of the Mercator projection.
Skew-oblique aspect - An aspect of a projection on which the axis of the Earth is rotated, so it is neither aligned with nor perpendicular to the conceptual axis of the map projection, and tilted, so the poles are at an angle to the conceptual axis of the map projection.
Small circle - A circle on the surface of a sphere formed by the intersection with a plane. Parallels of latitude are small circles on the Earth taken as a sphere. In the Mapping Toolbox, great circles, including the Equator and all meridians, are treated as special, limiting cases of small circles.
Small-scale mapping - Mapping at a scale smaller than about 1:1,000,000, although the limiting scale sometimes has been made as large as 1:250,000.
Standard parallel - In the normal aspect of a projection, a parallel of latitude along which the scale is as stated for that map. There are one or two standard parallels on most cylindrical and conic map projections and one on many polar stereographic projections.
Stereographic projection - A specific azimuthal projection or type of projection in which the Earth is projected geometrically onto a surface from a fixed (or moving) point on the opposite face of the Earth.
Tangent cone or cylinder - A cone or cylinder that just touches the sphere or ellipsoid along a single line. This line is a parallel of latitude if the axes of the geometric figures coincide.
Thematic map - A map designed to portray primarily a particular subject, such as population, railroads, or croplands.
Tissot indicatrix - See Indicatrix.
Topographic map - A map that usually represents the vertical positions or elevations of features as well as their horizontal positions. The topo workspace contains a simple example.
Transformed latitudes, longitudes, or poles - Graticule of meridians and parallels on a projection after the Earth has been turned with respect to the projection so that the Earth's axis no longer coincides with the conceptual axis of the projection. Used for oblique and transverse aspects of many projections.
Transverse aspect - An aspect of a map projection on which the axis of the Earth is rotated so that it is at right angles to the conceptual axis of the map projection. For azimuthal projections, this aspect is usually called equatorial rather than transverse.
True scale - See Correct scale.
Valued map - A matrix map in which entries represent some value or measurement. The topo workspace contains an example of a valued map. Each entry in the matrix map is an average elevation in meters for the geographic position represented by that cell.
Vector map - A map consisting of ordered latitude-longitude points, possibly connected. In the Mapping Toolbox, such map data is often represented by two vectors, representing latitude and longitude. Segments can be separated by the insertion of NaN's in both vectors.
Waypoint - Points through which a track passes, usually corresponding to course or speed changes.
WGS 72 (World Geodetic System 1972 - An Earth-centered datum, used as a definition of DMA DEMs, presently stored in the USGS data base. The WGS 72 datum was the result of an extensive effort extending over approximately three years to collect selected satellite, surface gravity, and astrogeodetic data available throughout 1972. These data were combined using a unified WGS solution (a large-scale least squares adjustment).
WGS 84 (World Geodetic System 1984) - The WGS 84 was developed as a replacement for the WGS 72 by the military mapping community as a result of new and more accurate instrumentation and a more comprehensive control network of ground stations. The newly developed satellite radar altimeter was used to deduce geoid heights from oceanic regions between 70° north and south latitude. Geoid heights were also deduced from ground-based Doppler and ground-based laser satellite-tracking data, as well as surface gravity data. The ellipsoid associated with WGS 84 is GRS 80.
Zenithal projection - See Azimuthal projection.
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