Accuracy
A measure of how close a recorded location is to its actual position.
Allocation
The process of partitioning a network into areas that satisfy one or more criteria.
For example, a public transportation authority might wish to establish bus routes and stops
such that every likely commuter resides within five minutes' walking time of the nearest stop.
Attribute
A non-spatial, quantitative or qualitative characteristic of an object or phenomenon. For
example, population is one attribute of a county or census tract.
Attributes
Quantities or qualities which represent the non-spatial properties of real-world objects.
Buffering
A GIS procedure by which zones of specified radius or width are defined surrounding
selected vector features or raster grid cells.
Computer-Aided Design
Digital drafting tools for engineering, architectural, facilities management, and graphic design
applications. CAD packages allow users to precisely specify sizes, shapes, and positions of
vector elements, and to assign elements to separate layers. Only graphic attributes
(such as
line width, color, and polygon shading) are encoded. Linkage of vector elements and
attribute databases is not directly supported. Therefore, CAD packages do not provide
analytical capabilities such as polygon overlay and spatial searches.
Cartogram
A type of thematic map in which the areas of spatial features are
distorted in proportion to the value of an attribute.
Centroid
A point feature centrally located within, and used to identify, an area feature.
Choropleth Map
A type of thematic map in which the colors (or shades of gray) of area
features represent categories of attribute values.
Coordinate System
A juxtaposition of two or more measurement scales which together define spatial locations in
two or more dimensions.
Data Model
A conceptual framework for representing the locations and characteristics of real-world
phenomena in a geographic database.
Database
Data are symbolic representations of phenomena.
A database can be thought of as a "population" of data.
Each data element is stored in a field.
All data fields related to an individual member of the "population" are stored as a unique record.
Database systems provide a means to select and organize data in response to a question, and thereby to produce information.
Datum
A network of precisely-surveyed control points used as a reference for measurements of
other horizontal or vertical positions. For global-scale measurements, the WGS 84 ellipsoid
also serves as a datum. Reference: Peter Dana,
The Geographer's Craft
Declination
The angular difference between true north and magnetic north.
Differential Correction
A technique for improving the accuracy of GPS positioning. Errors introduced by selective
availability are recorded by a stationary receiver at a well-defined location. Errors detected
by the stationary receiver can be used to cancel out errors recorded at the same time by a
mobile receiver in a different location.
Electromagnetic Energy
Radiant energy emitted by all matter whose temperature is greater than absolute zero (0° K).
The warmer the radiant object, the shorter the frequency of electromagnetic waves it emits.
Types of electromagnetic waves includes electric currents, heat, radio waves, microwaves,
infrared radiation, visible light, ultraviolet radiation, x rays, gamma rays, and cosmic rays.
Ellipsoid
A three-dimensional mathematical model which approximates the shape of the geoid. Many
different ellipsoids have been developed for continents or individual countries to minimize
local deviations from the geoid. The standard global ellipsoid is the World Geodetic System
of 1984 (WGS 84). Reference: Peter Dana,
The Geographer's Craft
Feature
A set of positions that specifies the location and extent of an object or phenomenon.
Features
Representations of real-world objects using geometric primitives (points, lines, and areas).
Geographic Coordinate System
A system for specifying positions on the globe. North-south positions are specified in
degrees of latitude, and east-west positions are specified in degrees of longitude.
Geographic coordinates may be expressed either in degrees, minutes, and seconds, or in decimal degrees.
To convert -89.40062° from decimal degrees to degrees, minutes, seconds:
To convert 43° 4' 31" from degrees, minutes, seconds to decimal degrees:
Geocoding
The process of assigning geographic locations to attribute data.
For example, the U.S. Census Bureau surveys population characteristics by household. The population data can be assigned to locations by matching household addresses to street address ranges encoded in the Bureau's TIGER Line Files.
Geoid
A complex three-dimensional figure used as a basis for extremely precise surveys of
positions on the surface of the Earth. The geoid is the surface on which sea level gravity is
everywhere equal. Gravity at mean sea level varies due to local differences in topography
and the density of materials in the Earth's interior.
Geographic Information System
Information is data which has been selected and organized in response to a question.
Information systems are computerized tools that assist people in transforming data into information.
Geographic information systems (GIS) enable people to integrate geographic data sets produced by different technologies and organizations, to interrogate the data, and finally to produce information, often in the form of a map.
Graticule
The grid formed by lines of latitude (parallels) and longitude (meridians).
Reference: Matching the Map
Projection to the Need
Great Circle
A line which divides the Earth into two hemispheres. The shortest path between two
locations on the globe.
Map Projection
A systematic transformation of locations on the globe to locations on a plane.
Reference: Matching the
Map Projection to the Need
Desktop Mapping
A class of software that supports creation of thematic maps for diverse applications,
including public health, business, and social science. Desktop mapping packages provide
rudimentary linkages between attribute databases and vector features or
raster grid cells. Display capabilities are usually limited to pre-existing data.
Unlike geographic information systems, desktop mapping packages do not include
operations for generating new data from existing data.
Metadata
Documentation of the characteristics of a data set, such as sources, creation date, ownership,
datum, ellipsoid, projection type and parameters, and data model.
Network
In the context of geographic data, a matrix of interconnected nodes representing a system of
paths through which people, objects, or data move.
Map Overlay
A GIS procedure by which the features and attributes of two
or more data layers are combined. When performed on vector data, the
operation is called polygon overlay. The merger of two or more raster
data layers is called grid overlay. In both cases, data layers
must be spatially registered prior to overlay.
Positioning
The processes and technologies used to specify positions in space. Positions are specified
with reference to a coordinate system, a datum, and an ellipsoid.
Precision
A measure of how exactly a position is defined. For example, the precise of geographic
coordinates specified as degrees, minutes, and seconds is (360°)(60')(60") / 40,000 km, or
about 30 meters.
Proportional Symbol Map
A type of thematic map in which the areas of symbols are varied in
proportion to the value of an attribute.
Reference Map
A graphic display which shows the geographic distribution of many different attributes.
Registration
Alignment of multiple superimposed spatial data layers. Data layers which are "in registry"
are alike with regard to scale, datum, ellipsoid, and projection.
Remote Sensing
The collection of technologies and procedures by which electromagnetic radiation can be
recorded from a distance.
Resolution
In general, the least detectable difference in a measurement.
Selective Availability
Intentional degradation of the accuracy of the Global Positioning System.
Standard Line
In the context of map projections, the line or lines of tangency between the globe and a
projection surface (cylinder, cone, or disk). A parallel, meridian, or other line along which
there is no distortion.
Reference: Matching the Map
Projection to the Need
Thematic Map
A graphic display that highlights the geographic distribution of a particular attribute, or
relationships among several selected attributes.
Topology
Spatial relationships among features, including adjacency and connectivity. Relative
position, as opposed to absolute position specified by coordinates, angles, and distances.
Courtesy: David DiBiase