GIS Glossary

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Glossary of common GIS terms

AAT: old term for Arc Attribute Table. A table containing attributes for a line coverage such as streets or streams. 

accuracy (absolute): The accuracy of a map in representing the geographic location of an object relative to its true location on the surface of the Earth. Absolute accuracy is based on geographic coordinates. 

accuracy (relative): The accuracy of a map in representing the geographic location of an object relative to the locations of other objects. 

AM/FM: Automated Mapping and Facilities Management. The management of mapping and facilities management using integrated computer software. 

AML: the old ARC Macro Language. A high-level language that provides full programming capabilities and a set of tools for building menus to tailor user interfaces for specific applications. 

annotation: Descriptive text used to label coverage features. 

arc: A string of x,y coordinate pairs (vertices) that begin at one location and end at another. Connecting the arc's vertices creates a line.

attribute: A characteristic of a geographic feature described by numbers or letters, typically stored in tabular format and linked to the feature in a relational database. The attributes of a well-represented point might include depth, location and permit number. 

base map: A map containing visible surface features and boundaries, essential for locating additional layers, or types, of georeferenced information. 

buffer: A zone of a specified distance around coverage features. Both constant and variable width buffers can be generated for a set of coverage features based on each features attribute values.

CAD: Computer Aided Design. An automated system for the design, drafting and display of graphically oriented information. 

cadastre: Public record of the extent, value and ownership of land within a district for purposes of taxation. 

Cartesian Coordinate System: A two-dimensional coordinate system in which x measures horizontal distance and y measures vertical distance. An x,y coordinate defines every point on the plane. 

clip: The spatial extraction of those features from one coverage that reside entirely within the boundary defined by features in another coverage. Clipping works much like a cookie cutter.

COGO: Abbreviation for the term COordinate GeOmetry. Land surveyors use COGO functions to enter survey data, to calculate precise locations and boundaries, to define curves, and so on.

contour line: An imaginary line joining points of equal elevation. 

control points: A set of points on the ground whose horizontal and vertical location is known. Control points are used as the basis for detailed surveys. 

coordinate: An x,y location in a Cartesian coordinate system or an x,y,z coordinate in a three-dimensional system. Coordinates represent locations on the Earth's surface relative to other locations. 

coverage: A digital version of a map forming the basic unit of vector data storage in ARC/INFO. A coverage stores map features as primary features (such as arcs, nodes, polygons, and label points) and secondary features (such as tics, map extent, links, and annotation). Associated feature attribute tables describe and store attributes of the map features. A coverage usually represents a single theme, or layer, such as soils, roads or land use.

coverage units: The units (e.g., feet, meter, inches) of the coordinate system in which a coverage is stored. 

data conversion: The translation of data from one format to another. ARC/INFO supports data conversion from many different geographic data formats in addition to routines for converting paper maps. Those data formats include DLG, TIGER, DXF and DEM.

database: A logical collection of interrelated information, managed and stored as a unit. A GIS database includes data about the spatial location and shape of geographic features recorded as points, lines and polygons as well as their attributes. 

datum: A set of parameters and control points used to accurately define the three-dimensional shape of the Earth. The corresponding datum is the basis for a planar coordinate system. For example the North American datum, 1927, is the datum for coordinates used in Volusia County's GIS. 

DBMS: Data Base Management System. Software that manages, manipulates and retrieves data in a database.

DGPS: Differential Global Positioning System. A positioning procedure that uses two receivers, a rover at an unknown location and a base station at a known, fixed location. The base station computes corrections based on the differences between its actual and observed ranges to the satellites being tracked. 

digital map library: A series of directories and subdirectories designed to uniformly organize a collection of spatial data. Map libraries organize geographic data spatially as a set of tiles and thematically as a set of layers. Volusia County's digital map library is divided into large scale and small scale subdirectories containing several hundred tiles comprising 90 layers of information. 

Digital Elevation Model (DEM): Terrain elevation data organized by quadrangle and provided in digital form. 

Digital Terrain Model (DTM): A three-dimensional model of the Earth's surface, provided in digital form.

digitize: To encode map features as x,y coordinates in digital form. Lines are traced to define their shapes. This can be accomplished either manually or by use of a scanner.

dissolve: The process of removing boundaries between adjacent polygons that have the same values for a specified attribute.

DLG: Digital Line Graph files from the U.S. Geological Survey.

DXF: Data Exchange Format. A format for storing vector data in ASCII or binary files. Used by AutoCad or other CAD software and convertible to ARC/INFO coverages.

edge matching: An editing procedure to ensure that all features that cross adjacent map sheets have the same edge locations.

ethernet: A baseband protocol invented by the Xerox Corporation in common use as the local area network for UNIX operating systems interconnected by TCP/IP. Runs at 16 megabits per second.

feature attribute table: A table used by GIS  to store attribute information for a specific coverage feature class.

feature class: The type of feature represented in a coverage. Coverage feature classes include arcs, nodes, label points, polygons, tics, annotation, links, boundaries, routes and sections.

geocode: The process of identifying a location by one or more x,y coordinates from another location description such as an address. For example, an address can be matched against street centerline file to determine an x,y coordinate. 

GIS or Geographic Information System: An organized collection of computer hardware, software, geographic data and personnel designed to efficiently capture, store, update, manipulate, analyze and display all forms of geographically referenced information. 

Global Positioning System (GPS): A satellite-based device that records x,y,z coordinates and other data. GPS devices can be taken into the field to record data while driving, hiking or flying. Ground locations are calculated by signals from satellites orbiting the Earth.

intersect: The topological integration of two spatial data sets that preserves features that fall within the spatial extent common to both input data sets. 

item: In an attribute table, a field of information commonly displayed as a column. A single attribute from a record in an INFO data file. 

latitude-longitude: A spherical reference system used to measure locations on surface. Latitude measures angles in the north south direction and longitude measures angles in the east west direction.

layer: A logical set of thematic data described and stored in a map library. Layers organize a map library by subject matter, e.g., soils, roads, wells and extend over the entire geographic area defined by the spatial index of the map library. 

line-in-polygon: A spatial operation in which arcs in one coverage are overlaid with polygons in another to determine which arcs, or portions of arcs, are contained within the polygons. Polygon attributes are associated with corresponding arcs in the resulting line coverage. 

logical selection: The process of selecting a subset of features from a coverage using logical selection criteria that operate on the attributes of coverage features (e.g., area greater than 16,000 square feet). Only those features whose attributes meet the selection criteria are selected. Also known as feature selection by attribute.

many-to-one-relate: A relate in which many records in one table are related to a single record in another table. A goal in relational database design is to use one to many relates to reduce data storage and redundancy.

map extent: The rectangular limits (xmin,ymin,xmax,ymax) of the area of the Earth's surface you want to display using ARC/INFO. The geographic extent specified by the minimum bounding rectangle of a study area. 

map projection: A systematic conversion of locations on the Earth's surface from spherical to planar coordinates. Several of the more popular projections are: State Plane Coordinates (SPC) which uses feet for units of measure; Universal Transverse Mercator (UTM) which uses meters for units of measure; and latitude and longitude which uses degrees, minutes, and seconds of arc for units of measure.

map scale: The extent of reduction needed to display a representation of the Earth's surface on a map. A statement of a measure on the map and the equivalent measure on the Earth's surface, often expressed as a representative fraction of distance, such as 1:24,000 (one unit in the map equals 24,000 units on the ground). 

one-to-many-relate: A type of relate connecting a unique value in one file to many records (that have the same value) in another file. 

orthophotography: The process of aerial photographs that have been rectified to produce an accurate image of the Earth by removing tilt and relief displacements which occurred when the photo was taken. 

PAT or Point Attribute Table/Polygon Attribute Table: A coverage can have either a point attribute table or a polygon attribute table, but not both.

photogrammetry: The science of deducing the physical dimension of objects from measurements on photographs.

planimetric: The horizontal (x,y) locations of non-topographic features, such as rivers, lakes, buildings, roads, etc. 

point-in-polygon: A spatial operation in which points from one coverage are overlaid with a polygonal coverage to determine which points fall within the polygon boundaries. Points assume the attributes of the polygons within which they fall.

polygon: A multisided figure that represents area on a map. A feature defined by the arcs that make up its boundary. Every polygon contains one label point within its boundary. Polygons have attributes that describe the geographic feature they represent.

polygon overlay: A process that merges spatially coincident polygons from two coverages and their attributes to create a third coverage that contains new polygons and describes new relationships. 

quadrangle (quad): Typically refers to a map sheet published by the U.S. Geological Survey, a 7.5 minute quadrangle series or the 15 minute quadrangle series. Also known as a topographic or topo map.

raster: Data displayed as discrete picture elements (pixels).

relate: An operation that establishes a temporary connection between corresponding records in two tables using an item common to both. A relate gives access to additional feature attributes that are not stored in a single table. 

relate key: The common set of columns used to relate two attribute tables. 

remote sensing: Any of the technical disciplines for observing and measuring the Earth from a distance, including satellite imaging, Global Positioning Systems, RADAR, SONAR, aerial photography, etc. 

resolution: Measures the sharpness of an image. 

Selective Availability (S/A): A U.S. Department of Defense program to limit the accuracy of autonomous position fixes computed by civilian receivers. The error in position caused by S/A can be up to 100 meters.

slope: A measure of change in surface value over distance, expressed in degrees or as a percentage. For example, a rise of 2 meters over distance of 100 meters describes a 2% slope.

spatial analysis: The process of modeling, examining and interpreting model results. Spatial analysis is the process of extracting or creating new information about a set of geographic features. Spatial analysis is useful for evaluating suitability and capability, for estimating and predicting, and for interpreting and understanding. In GIS there are four traditional types of spatial analysis: spatial overlay and contiguity analysis, surface analysis, linear analysis, and raster analysis. 

spatial modeling: Analytical procedures applied with GIS. There are three categories of spatial modeling functions that can be applied to geographic data within a GIS: geometric models, such as calculating the distance between features, generating buffers, calculating areas and perimeters, and so on; coincidence modeling, such as polygon overlay; and adjacency modeling such as redistricting and allocation. 

SQL or Structured Query Language: A syntax for defining and manipulating data from a relational database. Developed by IBM in the 1970s, it has become an industry standard for query languages in most relational database management systems. 

State Plane Coordinates (SPC): A map projection that measures distance in feet. By providing an SPC easting (x) and northing (y), the state name, and the zone number, any location in the United States can be identified by a unique coordinate value. SPC Zone boundaries follow state and county boundaries. Florida, due to its size and shape is divided into three SPC zones, north, east, and west. Volusia County is entirely within the East Zone. State Plane Coordinates are admirably suited to the needs of the local land surveyor and are widely used for public works, land surveys, and for Geographic Information Systems. 

TIN or Triangulated Irregular Network: A series of triangles constructed using elevation data points taken from coverages. These triangles are used for surface representation and display. 

topography: Shape or configuration of the land surface; represented in map form by contour lines.

topology: The spatial relationships between connecting or adjacent coverage features.

transformation: The process that converts coordinates from one coordinate system to another through translation, rotation and scaling. 

triangulation: A method of surveying in the location of an object may be calculated from the known locations of two other objects. Creating a triangle from the three items, the angles and sides of the triangle can be measured and the location of the unknown object is calculated algebraically. 

vector: A geometric element, stored as a point with x,y coordinates within a computer database.