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Common GIS Terms

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Absolute accuracy: exact correspondence between the location of features in map data and their  actual positions on the earth.

Attribute accuracy: the difference between information recorded as digital map data or database tables and the real-world features represented. For example, for map data that includes street names as attributes, the percentage of correct names would be the measure of accuracy.

Attribute information: descriptive, non-graphic      information recorded as digital map data or an associated database table. Examples of attributes are street names, street types (highway, side street, etc.), and pavement types.

Cell: individual picture elements in a raster image.

Complete/Completeness: a measurement of whether map data includes the features a user would expect to be represented.

Control points: exact positions of often-surveyed geographic features used to register map sheets and transform coordinates.

Currency: measures how recently the map data was collected, usually expressed the revision date.

Data creators: companies that develop their own or enhance existing geographic data.

Data integrators: companies that gather digital map
data from a variety of public or private sources and adapt it for a specific mapping project and target software.

Data packagers: companies that repackage existing map data, with very little customization, for mass distribution.

Datum: a mathematical model that provides a smooth approximation of the earth’s surface.

Digital Chart of the World (DCW): a digital database published by the US Defense Mapping Agency that contains maps of the entire world, input at a scale of 1:1,000,000.

Digital elevation models (DEMs): a representation of the terrain in a given area, expressed as a rectangular array of regularly spaced elevation values. Also referred to as a Digital Terrain Model (DTM), or Digital Terrain Elevation Data (DTED).

Digitizing: the process of converting existing data from paper maps-as well as drawings and aerial photos into digital form by (1) manually tracing the maps on hardware that consists of a digitizer tables and a cursor with crosshairs and keys used to record map features as X,Y coordinates or (2) scanning the map and using automatic conversion software to translate the resulting raster file to vectors and storing it directly in the GIS.

Display information: information that describes how digital map features will appear visually. Display information includes color, fill pattern, linetype, and so on.

Easting: the east-west, X, coordinate in a rectangular coordinate system.

File format:the physical arrangement of digital data stored in a map file.

Geographic coordinate system: a system of numeric coordinates used to locate and record specific positions on the earth’s surface.

Geographic information: information in a digital map that records the physical position and shape of a map feature.

GIS (geographic information system): a computer-based technology for retreiving, storing, and   organizing data based on its location on a map.

Global Positioning System (GPS): satellite-based positioning technology that, with differential correction, can yield engineering-level accuracy.

Hierarchical: a logical structure that classifies    information in a series of steps, starting with broad, simple classifications, and proceeding, in stages, to narrow, precise classifications.

Hydrography: map data that describes the positions and characteristics of bodies of water.

Hypsography: map data that describes the exact shape of the earth’s surface, usually in the form of contour lines, digital elevation models, or color shadings.

Internal file format: the binary file format used      internally by a specific GIS platform.

Latitude: the first component of a spherical coordinate system used to record positions on the earth’s surface. Latitude indicates the angular    distance north of south of the earth’s equator measured through 90 degrees (see “Longitude”).

Layers: a means of organizing and managing map data by type. Hydrological features (such as floodplains), parcel maps, railroads, and so on can be contained on separate layers for easy map creation and maintenance.

Local Coordinate System: a system designed for a small geographic area. Local coordinate systems cannot be expanded to include large areas without loss of accuracy. Typically, local coordinate  systems are not projected systems; they are X,Y-coordinate systems (e.g., State Plane Coordinate System).

Longitude: the second component of a spherical coordinate system used to record east-west    positions on the earth’s surface, measured in degrees as the arc or position of the earth’s equator intersected between the meridian of a given place and the prime meridian, which runs through Greenwich, England. (See ”Latitude“)

Mercator projection: a map projection designed by Gerhardus Mercator, where the earth’s surface is drawn as it would appear if projected on a cylinder wrapped around the earth.

Metadata: information describing a data set. A    digital map’s metadata might state its scale,      revision date, author, accuracy standards, and other pertinent information.

Northing: the north-south, Y, coordinate in a        rectangular coordinate system.

Paper coordinates: coordinates measured directly from the paper on which a map is drawn, often recorded in inches or centimeters.
Planar coordinate system: a coordinate system drawn on a flat surface, or plane. Planar coordinates are usually expressed as pairs of rectangular X,Y values.

Prime meridian: the line of longitude that runs through Greenwich, England, used as the origin (zero point) for longitudinal measurements.

Projection: a system to portray all or part of the earth, which is an irregular sphere, on a planar, or flat, surface.

Raster: raster data takes an evenly spaced grid (like a piece of graph paper) and places a value in each square, or cell. Raster data is best suited for continuous data such as slope, rainfall, or the amount of light reflecting off the ground (as in a photograph).

Rectification: the process of assigning spatial coordinates to a map data image by warping it to fit known geographic control points.

Relative accuracy: the difference between how features on a map and those same features in the real world are positioned in relationship to each other. A measurement system may employ a “bias” or “systematic” error, with consequently inaccurate results, but still preserve local relationships.

Remote-sensing data: digital data collected by satellite and other airborne, electronic-[A1]imaging systems.

Resolution: the minimum distance that can be recorded by a measurement system. For example, if a map has a resolution of 10 meters, the map cannot accurately depict features smaller than 10 meters. Therefore, these features may be depicted as points, or they may not be depicted at all.

Rubber sheeting: an editing method that corrects errors by stretching maps to fit known control points.

Scale: the proportion or ratio between a map measurement and the corresponding measurement in the real world. Map scale is usually expressed as a ratio, such as 1:24,000, which means that a         measurement of one unit on the map represents 24,000 units in the real world.

Spherical coordinate system: a coordinate system measured on the surface of a sphere, usually expressed as angular distances.
State Plane Coordinate System: a set of rectangular mapping coordinate systems defined in the United States. Each state is divided into one or more zones, and a separate coordinate system is defined for each zone.

Transfer file formats: digital-data file formats specifically designed to transfer data between different systems such as DXF™ and SDTS.

Transportation data: digital map data that describes transportation features, such as roads, railroads, or pipelines.

Universal Transverse Mercator (UTM): a specific implementation of the transverse mercator projection, designed for common usage around the world. The UTM system divides the world into 60 east-west zones, each of which is six degrees of longitude wide. Each zone is projected individually.

Vector: vector data is the storage of X,Y,Z coordinates connected to form points, lines, areas, and volumes. Vector data is best suited to store    discrete, well-defined data that can clearly be delimited. Location of oil wells (points), street   centerlines (lines), timber stands (areas), and groundwater tables (volumes) are good  candidates for vector storage.

More GIS Terms

GIS Dictionary: This on-line dictionary of GIS terms is brought to you by the Association for Geographic Information and the University Of Edinburgh Department of Geography.

The dictionary includes definitions for 980 terms compiled from a variety of sources which either relate directly to GIS or which GIS users may come across in the course of their work. The dictionary is also supplemented by 52 diagrams

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