## Glossary

This glossary of geographical terms is drawn extensively from “An Album of Map Projections”, U.S. Geological Survey Professional Paper 1453, by John P. Snyder and Philip M. Voxland.

Because the purpose of this glossary is to assist in understanding and using Mapping Toolbox™ features, it includes some terms specific to the toolbox, and gives some other terms shades of meaning beyond their general definitions.

**Antipodes**Two points on opposite sides of a planet.

**Arc-second**1/3600th of a degree (1 second) of latitude or longitude.

**Aspect**The conceptual placement of a projection system in relation to the Earth's axis (direct, normal, polar, equatorial, oblique, and so on).

**Attribute**In vector geodata, a quantitative or qualitative descriptor of a spatial entity. An attribute can describe a real-world quality (such as population or land area), or a graphic quality (such as patch color or line weight). Attributes are frequently coded as numbers or character vectors in character-coded or binary tabular data files, with one or more attribute per map feature.

**Attribute spec**(Attribute specification) A cell array structure that specifies attributes of geodata to be included in a KML file and defines labels and formats for each attribute. Used with

`kmlwrite`

.**Authalic projection****Axes***See***Map axes**.**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.**Buffer zone**The locus of points that lie within a specified distance from a map feature.

**Cartography**The art or practice of making charts or maps.

*See***Map**.**Categorical geodata**Geospatial data in which raster pixel values (or vector data attributes) are categorical indices, usually coded as integers. The meanings of the categories are usually stored in a separate table. Examples are geocodes, land use categories, and indexed color images.

*See***Numerical geodata**.**Central meridian**The meridian passing through the center of 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 portraying regions of homogeneous classified attribute values, changing abruptly at region boundaries, and colored or shaded according to their attribute values. Thematic political maps are usually choropleth maps.

**Complex curves**Curves that are not elementary forms such as circles, ellipses, hyperbolas, parabolas, and sine curves, such as rivers, coastlines, and administrative boundaries.

**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, except at a finite number of singular points (e.g., the poles in a Mercator projection). Also called an orthomorphic projection.

**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 height above or below a reference datum; generally applied to continuous, single-valued surfaces only, such as elevation, temperature, or magnetic field strength.

**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.

**Data grid**A raster data set consisting of an array of values posted or sampled at specific geographic points. Mapping Toolbox data grids can be implicit (regular) or explicit (irregular, or geolocated), depending on the uniformity of the grid.

*See***Regular data grid**,**Geolocated data grid**.**Datum (vertical)**A base reference level for establishing the vertical dimension of elevation for the earth's surface. A datum defines sea level and incorporates an ellipsoid; thus one can reference a coordinate system to a datum or to a specified ellipsoid, but not both at the same time.

**Datum (horizontal)**A base measuring point (“0.0 point”) used as the origin of rectangular coordinate systems for mapping or for maintaining excavation provenience. Two examples are the North American Datum of 1927 (NAD27) and the North American Datum of 1983 (NAD83). Earth-centered coordinate systems, such as WGS84, combine horizontal and vertical datums.

**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 data grid, generally a regular (implicit) one. DEM also refers to the five primary types of digital elevation models produced by the U.S. Geological Survey; Mapping Toolbox functions can read 30-meter and 10-meter DEMs as well as 3-second DEMs.

**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**.**Display Structure**A Mapping Toolbox data structure for mapped objects dating from Version 1 of the product. The structures can contain line, patch, text, regular data grid, geolocated data grid, and light objects; vector data always has coordinates in latitude and longitude. In Version 2 of the toolbox, display structures were superseded by

*geostructs*and*mapstructs*.*See***Geographic data structure**. Most vector display structures can be converted to geographic data structures.**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.

**DM**Degrees-minutes angle notation of the form ddd° mm'. There are 60 seconds in a minute, and 60 minutes in a degree. In DM notation, degrees are always integer, but minutes can be fractional. Certain Mapping Toolbox functions represent DM angles as column vectors,

`[degrees minutes]`

.**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 DMS notation, degrees and minutes are always integer, but seconds can be fractional. Certain Mapping Toolbox functions represent DMS angles as column vectors,

`[degrees minutes seconds]`

.**DTED**Digital Terrain Elevation Data; a raster data format used by various terrain data products, based on specifications originating in the United States Department of Defense. DTED

^{®}files sample terrain elevation in a geographic grid at specific “levels” of spatial resolution; sampling intervals are approximately one kilometer for Level 0, 100 meters for Level 1, 30 meters for Level 2, and so on. High level DTED files are not generally available to the public. Mapping Toolbox software imports all levels of DTED data.**Easting**The distance of a point eastward from the origin in the units of the coordinate system for the defined projection. Paired with

**Northings**.**Ellipsoid**When used to represent the Earth, a solid geometric figure formed by rotating an ellipse about its minor (shorter) axis. Also called spheroid.

**Ellipsoid vector**A vector describing a specific ellipsoid model. The ellipsoid vector has the form

ellipsvec = [semimajor-axis eccentricity]

**Ellipsoidal height**Elevation of a point above a reference ellipsoid, as measured along a normal to the ellipsoid.

**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.

**Equator**The great circle straddling a planet at a latitude of 0°, perpendicular to its polar axis and midway along it, dividing the northern and southern hemispheres.

**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****Equivalent projection****False easting**The value of the easting assigned to the projection origin. Easting values increase to the east.

**False northing**The value of the northing assigned to the projection origin. Northing values increase to the north.

**Flat-polar projection**A cylindrical projection on which, in normal aspect, the pole is shown as a line rather than as a point. For example, the Miller projection is flat-polar.

**Frame***See***Map frame**.**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.

**Geodesic**A minimum-distance curve on a curved surface, independent of the choice of a coordinate system. On a sphere a geodesic is equivalent to a great circle arc.

**Geolocated data grid**A data grid defined with separate latitude, longitude, and value matrices, allowing irregular sampling, nonrectangular shapes, and noncardinal orientations. Satellite imagery swaths are often represented as geolocated data grids.

*See***Data grid**,**Regular data grid**.**Geodata**Geospatial data.

*See***Geospatial**.**Geoid**The figure of the earth less its topography, defined as an equipotential surface with respect to gravity, more or less corresponding to mean sea level. It is approximately an oblate ellipsoid, but not exactly so because local variations in gravity create minor hills and dales. Empirically determined geoids are used to define

*datums*and to compute orbital mechanics.**Geometric projection****Geographic coordinates**Spherical 2-D coordinate tuples (latitudes, longitudes) that specify point locations for unprojected geodata. The analogous term for geodata projected to a rectangular coordinate system is

*map coordinates*.**Geographic data structure**A Mapping Toolbox data structure for vector data comprised of a MATLAB

^{®}structure array with one element per vector geographic feature. It includes a mandatory`Geometry`

field, at least two coordinate array fields. The field names are`X`

and`Y`

(for*mapstructs*), or`Lat`

and`Lon`

(for*geostructs*), and optional attribute fields.**Georeferencing**Identifying objects and locations by name, identifier, or coordinates to describe where they are located on the Earth's surface.

**Geospatial**Spatial data, concepts, and techniques that specifically refer to geographic space or phenomena, and not just to arbitrary coordinate systems or abstract space frames.

**Geostruct**A Mapping Toolbox geographic data structure for vector geodata with coordinates in latitude and longitude.

*See***Geographic data structure**.**GeoTIFF**An extension of the TIFF image file format with additional tags containing parameters for image georeferencing and projected map coordinate system definition.

**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 subset of the Earth's parallels and meridians (or plane coordinates) used as a reference grid on globes and maps. Generally synonymous with

*map grid*, except that many map grids are rulings at regular intervals in projected coordinates.*See***Map grid**,**National grid (U.S.)**, and**National grid (U.K.)**. The vertices of the graticule grid are precisely projected, and the map data contained in any grid cell is warped to fit the resulting quadrilateral. 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.

**Homalographic/homolographic 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 and drainage.

**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.

**Hypsographic tints**A graphic means of representing terrain or other scalar attributes using a sequence of colors or tints indexed to elevation.

**Hypsography**The scientific study of the Earth's topological configuration above sea level, especially the measurement and mapping of land elevation.

**Index map**A small-scale map used to help locate a map containing a region or feature of interest in a tiled geospatial database, map series, plat book, or atlas.

**Indicatrix**A circle or ellipse useful in illustrating the distortions of a given map projection. Indicatrices are constructed by projecting infinitesimally small circles on the Earth onto a map and giving them visible dimensions. Their axes lie in the directions of and are proportional to the maximum and minimum scales at their point locations. Often called a Tissot indicatrix after the originator of the concept. Mapping Toolbox Tissot indicatrices can be displayed using the

`tissot`

command, and indicatrices for all supported projections are provided.**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. No Mapping Toolbox projections are of this type, but the user can separate data into sections and project these independently to achieve this effect.

**Keyhole markup language (KML)**A file format and dialect of XML used to georeference geographic locations and describe their attributes and relations, including hyperlinks, for display in earth browsers.

**Large-scale mapping**Mapping at a scale larger than about 1:75,000, although this limit is somewhat flexible. Includes cadastral, utility, and some topographic maps.

**Latitude (astronomical)**The complement of the elevation angle of the celestial North Pole, which depends on normal to the Earth's equipotential surface (geoid) at a given point (positive if the point is north of Equator, negative if it is south). It can be thought of as the angle that a plumb line makes with the equatorial plane.

**Latitude (auxiliary)**Intermediate forms of latitude that are mathematically constructed (normally by transferring latitudes first from an ellipsoid to a sphere, and then to a plane) in order to achieve desired map projection properties. Types include

*conformal*(for constructing conformal maps),*authalic*(for constructing equal-area maps), and*rectifying*(for constructing equidistant maps).**Latitude (geocentric)**The angle at which a line connecting the surface of a sphere or reference ellipsoid to its center intersects the equatorial plane (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 (geodetic)**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). Also called

*geographic latitude*. One of the two common geographic coordinates of a point on the Earth.**Latitude of opposite sign****Legs**Line segments connecting waypoints.

**Legend***See***Map legend**.**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 data grid**A binary data grid consisting entirely of 1s and 0s. An example of a logical data grid can be created with the

`topo`

map by performing a logical test for positive elevations (`topo>0`

). Each entry in the data grid 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. Paired with

*Latitude*.**Loxodrome***See***Rhumb line**.**Map**A diagrammatic or pictorial representation of a planet's surface or part of it, showing the geographical distributions, positions, etc., of natural or artificial features such as roads, towns, relief, land cover, rainfall, populations, etc. Maps represent geospatial data visually.

**Map axes**An axes object augmented with additional properties, including a projection type, projection parameters, map latitude and longitude limits and so forth. Many Mapping Toolbox display functions require that a map axes first be defined. Others create a map axes if necessary (e.g.,

`worldmap`

and`usamap`

) or assume that your coordinate data are in a projected map coordinate system (`mapshow`

and`mapview`

).**Map coordinates**Orthogonal planar 2-D coordinate tuples that specify point locations for projected geodata. The analogous term for unprojected geodata is geographic coordinates. Also called grid coordinates and plane coordinates.

**Map frame**A projected rectangle or quadrangle enclosing a geographic data displayed on a Mapping Toolbox map axes.

**Map grid**A symbolized network of lines, or graticule, representing parallels and meridians or plane coordinates. Plane coordinate grids are almost always rectangular with uniform spacing. Azimuthal map grids are organized as polar coordinates.

*See***Graticule**.**Map layer**A vector or raster geographic data set read into the Map Viewer, for example, roads, rivers, municipal boundaries, topographic grids, or orthophoto images. Map layers are “stacked” from top to bottom, and can be reordered and hidden by the user.

**Map legend**A key to symbolism used on a map, usually containing swatches of symbols with descriptions, and can include notes on projection, provenance, scale, units of distance, etc.

**Mapstruct**A Mapping Toolbox geographic data structure for vector geodata with coordinates in projected (

*x,y*) coordinates.*See***Geographic data structure**.**Matrix map***See***Data grid**.**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. When the Earth is regarded as a sphere, this line is half a great circle; on the Earth regarded 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”).

**National grid (U.K.)**A metric grid based on the Transverse Mercator Projection developed by Ordnance Survey in 1936 for use in Great Britain. Sometimes abbreviated “OSGB36,” it is the de facto standard projection for display of UK based mapping.

**National grid (U.S.)**A metric grid based on the Transverse Mercator Projection, adopted by the Federal Geographic Data Committee (FGDC) in 2001 for use in the United States. It is an evolving standard intended to unify georeferencing across the U.S., but is not yet as widely used as other countries' national grids.

**Nominal scale**The stated scale at which a map projection is constructed. Scale is never completely constant across the extent of a map, although in some maps (especially at large scales) it can vary by minuscule amounts.

**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.

**Northing**The distance of a point northward from the origin, in the units of the coordinate system for the defined projection. Paired with

**Eastings**.**Numerical geodata**Geospatial data in which raster pixel values (or vector data attributes) are cardinal, ratio, or ordinal numeric measurements or computed values. For example, the

`topo`

data set contains numerical geodata. Each value in its data grid is an average elevation in meters for the geographic area covered by that cell.*See***Categorical geodata**.**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.

**Orthometric height**Elevation above a datum defined by a geoid representing mean sea level.

**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, which can be defined in several ways. The Equator (a great circle) is usually also treated as a parallel.

*See*entries for**Latitude**.**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.

**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 directly above a pole. This aspect is called

*transverse*for cylindrical or pseudocylindrical projections.**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.

**Projected coordinate system**A coordinate system defined for a particular map projection and associated parameters, which normally is planar with well-defined coordinate origin, handedness, nominal scale, and units of distance. While map scale can vary at different coordinate locations, a linear projected coordinate system has constant units of distance.

**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. For a list of Mapping Toolbox map projections, type

`maps`

.**Projection 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.

**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.

**Pseudocylindric-al 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.

**Raster geodata**A georeferenced array or grid of values corresponding to specific geographic points, usually regularly and rectangularly sampled in either geographic or map space. Values can be continuous or categorical. In the case of georeferenced multiband images, raster geodata can take the form of 3- and higher dimensional arrays.

**Reckoning**The determination of geographic position by calculation.

**Referencing matrix**A 3-by-2 matrix defining the scaling, orientation, and placement of raster map data on the globe or in planar map coordinates. The matrix specifies an affine transformation that ties (geolocates) the row and column subscripts of an image or regular data grid to 2-D map coordinates or to geographic coordinates (longitude and geodetic latitude).

*See***Referencing vector**.**Referencing vector**A three-component vector defining the geographic placement and unit cell size for raster map data. A referencing vector has the form

`[cells/degree north-latitude west-longitude]`

, with latitude and longitude limits specified in degrees.A referencing vector specifies an affine transformation with rows and columns aligned to latitude and longitude, respectively, and the same data spacing in both latitude and longitude. As such, it is more specific than a referencing matrix. Note that a referencing vector can always be transformed to a referencing matrix, but only certain referencing matrices can be transformed to referencing vectors.

*See***Referencing matrix**.**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 data grid**A data grid with equally spaced grid points in either latitude-longitude or map coordinates, defined with a referencing matrix or vector, and limited to a rectangular shape and cardinal orientation.

*See***Data grid**,**Geolocated data grid**,**Referencing matrix**.**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. A given region will appear smaller on a small-scale map than on a large-scale map.

**Scale factor**The ratio of the scale at a particular location and direction on a map to the nominal 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.

**Selector**A cell array in which the first element is a predicate function and the remaining elements list the names of attributes in a shapefile. Function

`shaperead`

has an option to screen out any feature in the shapefile for which a predicate returns false when applied to the subset of attributes corresponding to the list in the selector.**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.

**Shapefile**A widely used file format for vector geodata designed by Environmental Systems Research Institute. Shapefiles encode coordinates for points, multipoints, lines (polylines), or polygons along with tabular attributes.

**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. Mapping Toolbox great circles, including the Equator and all meridians, are treated as special, limiting cases of small circles. Mapping Toolbox functions generalize the concept of small circle with computations for two other types of curve: the locus of points on an ellipsoid at a given distance (as measured along a geodesic) from a central point, or the locus of points on a sphere or ellipsoid at a given distance from a central point, as measured along a rhumb line.

**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.

**Spatial Data Transfer Standard (SDTS)**A self-documenting geospatial file formatting standard adopted by the U.S. government and others. SDTS can encode locations, attributes, topological relationships, data quality, and other metadata. Note that Mapping Toolbox software can read the SDTS Raster Profile, but does not currently support SDTS vector data.

**Spheroid***See***Ellipsoid**.**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.

**State Plane**A set of commensurate coordinate systems commonly used for utility and surveying applications in the lower 48 United States. Each state contains one or more zones. Coordinates for zones elongated north-to-south are based on Transverse Mercator projections, while zones elongated east-to-west use Lambert Conformal Conic.

**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.

**Symbolization**In cartography, a mapping between geospatial objects or numerical or categorical values and cartographic symbols. The choice of graphic symbols, their size, density, shape, contrast, color, and pattern are principal aspects of symbolization.

**Symbolspec**(Symbol specification) A cell array structure that defines symbolism characteristics for points, lines, and polygons with respect to attributes and their values, or as a default symbolization regardless of attributes.

**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.

**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**.**Vector data set**Data representing geospatial objects as sequences of geographic or projected coordinate points that are implicitly connected if they represent linear or areal shapes. In Mapping Toolbox and other software, such geodata is often represented by two vectors, one with latitudes, another with longitudes. Objects can be segmented by inserting NaNs at the same locations in both vectors. Such pairs of coordinate vectors can also be represented as the

`Lat`

and`Lon`

or`X`

and`Y`

field values in a geographic data structure array.**Viewshed**The portion of a surface that is visible from a given point on or above it; derived from the concept of a watershed.

**Waypoints**Points through which a trip, track, or transit passes, usually corresponding to course or speed changes.

**WGS 72 (World Geodetic System 1972)**An Earth-centered datum, used as a definition of DMA (now NGA) DEMs. 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. This data was 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.

**World file**A small text file used to georeference different raster image formats, developed to incorporate imagery into ESRI's ArcView software.

**Zenithal projection***See***Azimuthal projection**.