Glossary

Arc - term used by Arc/Info to describe a line formed by a string of points (see line).

Area- a closed figure bounded by lines. See polygon

Attribute- data that describes geographic features in a GIS database (e.g. the attributes of a field might include soil type, crop cover, insect population, etc).

Base map - a map containing geographic features used for locational reference.

Boolean expression- an expression for which the results are either true or false (e.g. "and" / "or" expressions).

C/A code - Clear/Acquisition code (also referred to as Civilian Code) transmitted by GPS NavStar satellites. The code used by most civilian GPS receivers.

Coordinate- an x,y location in a Cartesian coordinate system or an x,y,z location in a three dimensional matrix. Coordinates represent locations in relation to other locations.

Coordinate system- a system used to measure horizontal and vertical distances on a map. Data in GIS databases are assigned locations based on this system.

Coverage- a collection of associated thematic maps represented as one digital map. Coverages are composed of the geographical spatial features (represented as points, lines, and polygons), and attribute features which describe the spatial features. (This definition is particular to Arc/Info although it is used in some other GIS packages, this concept may have other names in other GIS software).

Data conversion- translation of data from one format to another (i.e. importing DLG, TIGER, and DEM data into a GIS).

DEM- Digital Elevation Model data. A database of elevation data represented by a regularly-spaced set of x,y,z locations.

Differential Correction- a method of obtaining greatly enhanced precision from GPS navigation. Generally involves a correction signal transmitted from a reference GPS receiver, the exact location of which is known, (called the base station) to the GPS receiver being used to take measurements (called the 'rover')

Digitize- to transform map features into digital form. Usually accomplished with a digitizing table, or by creating a digital version and georectifying an image.

DLG- Digital Line Graph files are a format standard used by USGS for converting and exchanging datafiles.

Georectify - see georeference

Georeference- to establish the relationship between page coordinates (i.e. x,y) of a planar map of image with known real-world coordinates (i.e. longitude/latitude, UTM, etc). Also referred to as registering.

Geographic Information System (GIS) - an organized collection of computer hardware, software, geographic data and personnel designed to capture, manipulate, analyze and display geographically referenced data.

Global Positioning System (GPS) - portable, satellite based system which will establish the real-world location (position) of the GPS receiver.

Line- a spatial data feature defined by a string of points.

Longitude/Latitude- spherical reference system used to measure locations on the earth's surface.

Layer- a collection of thematic data of the same type. One or more layers compose a map. (This definition is particular to Arc/Info although it is used in some other GIS packages, this concept may have other names in other GIS software).

Multi-channel receiver - a GPS receiver that can track more than one satellite at the same time.

P-Code - Precise (or Protected) code transmitted by GPS NavStar satellites. The code used (in addition to the C/A code) by very accurate survey and military GPS receivers.

Point - a single x,y coordinate representing a geographic (spatial) feature. Often a point is a feature too small to be represented by a line or a polygon on a small scale map.

Polygon- a geographic (spatial) area enclosed by a series of lines. Also called 'area'

Precise Positioning Service (PPS) - the most accurate dynamic positioning possible with a GPS. Uses dual frequency P-code.

Remote sensing- acquiring information about an object without physically coming into contact with it.

Selective Availability (S/A) - Spatial analysis- the process of extracting or creating new information about geographic features by combining and intersecting existing maps. Also refers to statistical analyses conducted on spatial data (e.g. measures of aggregation or dispersal, interpolation of data between known sample points, krieging surfaces).

Spatial data- data regarding the location and shape of geographic features.

Standard Positioning Service (SPS) - normal civilian positioning accuracy obtained with the C/A code.

Thematic data- data describing the characteristics of geographic features.