Gis

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Attribute

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A characteristic of a geographic feature, typically stored in tabular format and linked to the feature in arelational database. The attributes of a well-represented point might include an identification number, address, andtype

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Base Layer

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A primary layer for spatial reference, upon which other layers are built. Examples of a base layer typically used are either the parcels, or street centerlines.

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Buffer

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A zone of a specified distance around a feature.

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Computer Aided Design (CAD)

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An automated system for the design, drafting and display of graphically orientedinformation.

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Coordinate

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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.

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Database

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

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Digital Elevation Model (DEM)

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Terrain elevation data provided in digital form.

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Digitize

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

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Geocode

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The process of identifying a location by one or more attributes from a base layer.

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Geographic Information System (GIS)

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An organized collection of computer hardware, software, geographicdata, and personnel designed to efficiently capture, store, update, manipulate, analyze, and display all forms ofgeographically referenced information.

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Global Positioning System (GPS)

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A satellite based device that records x,y,z coordinates and other data. Groundlocations are calculated by signals from satellites orbiting the Earth. GPS devices can be taken into the field torecord data while walking, driving, or flying.

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Layer

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A logical set of thematic data described and stored in a map library. Layers act as digital transparencies thatcan be laid atop one another for viewing or spatial analysis.

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Line

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Lines represent geographic features too narrow to be displayed as an area at a given scale, such as contours,street centerlines, or streams.

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Metadata

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Information about a data set. It may include the source of the data; its creation date and format; itsprojection, scale, resolution, and accuracy; and its reliability with regard to some standard.

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Ortho Imagery

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Aerial photographs that have been rectified to produce an accurate image of the Earth by removingtilt and relief displacements, which occurred when the photo was taken.

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Point

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A single x,y coordinate that represents a geographic feature too small to be displayed as a line or area at thatscale

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Polygon

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A multisided figure that represents area on a map. Polygons have attributes that describe the geographic feature they represent.

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Scale

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The ratio or relationship between a distance or area on a map and the corresponding distance or area on theground.

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Spatial Analysis

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The process of modeling, examining, and interpreting model results. Spatial analysis is useful for evaluating suitability and capability, for estimating and predicting, and for interpreting and understanding.

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Structured Query Language (SQL)

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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 databasemanagement systems.

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Theme

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An ArcView theme stores map features as primary features (such as arcs, nodes, polygons, and points) andsecondary features such as tics, map extent, links, and annotation. A theme usually represents a single geographiclayer, such as soils, roads, or land use.

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Feature Classes

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Feature classes are homogeneous collections of common features, each having the same spatial representation, such as points, lines, or polygons, and a common set of attribute columns, for example, a line feature class for representing road centerlines. The four most commonly used feature classes in the geodatabase are points, lines, polygons, and annotation.

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Feature Dataset

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In ArcGIS, a collection of feature classes stored together that share the same spatial reference; that is, they share a coordinate system, and their features fall within a common geographic area. Feature classes with different geometry types may be stored in a feature dataset.

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Geodatabase

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A collection of geographic datasets for use by ArcGIS. There are various types of geographic datasets, including feature classes, attribute tables, raster datasets, network datasets, topologies, and many others.

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Projection

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A method by which the curved surface of the earth is portrayed on a flat surface. This generally requires a systematic mathematical transformation of the earth's graticule of lines of longitude and latitude onto a plane. Some projections can be visualized as a transparent globe with a light bulb at its center (though not all projections emanate from the globe's center) casting lines of latitude and longitude onto a sheet of paper. Generally, the paper is either flat and placed tangent to the globe (a planar or azimuthal projection) or formed into a cone or cylinder and placed over the globe (cylindrical and conical projections). Every map projection distorts distance, area, shape, direction, or some combination thereof.

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Polygon

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A set of many-sided area features that represent the shape and location of homogeneous feature types such as states, counties, parcels, soil types, and land-use zones.

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Raster

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A spatial data model that defines space as an array of equally sized cells arranged in rows and columns, and comprised of single or multiple bands. Each cell contains an attribute value and location coordinates. Unlike a vector structure, which stores coordinates explicitly, raster coordinates are contained in the ordering of the matrix. Groups of cells that share the same value represent the same type of geographic feature.

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Shapefile

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A vector data storage format for storing the location, shape, and attributes of geographic features. A shapefile is stored in a set of related files and contains one feature class.

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TIN

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Acronym for triangulated irregular network. A vector data structure that partitions geographic space into contiguous, nonoverlapping triangles. The vertices of each triangle are sample data points with x-, y-, and z-values. These sample points are connected by lines to form Delaunay triangles. TINs are used to store and display surface models.

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Digitizing

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The process of converting the geographic features on an analog map into digital format using a digitizing tablet, or digitizer, which is connected to a computer. Features on a paper map are traced with a digitizer puck, a device similar to a mouse, and the x,y coordinates of these features are automatically recorded and stored as spatial data.

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Geoprocessing

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A GIS operation used to manipulate GIS data. A typical geoprocessing operation takes an input dataset, performs an operation on that dataset, and returns the result of the operation as an output dataset. Common geoprocessing operations include geographic feature overlay, feature selection and analysis, topology processing, raster processing, and data conversion. Geoprocessing allows for definition, management, and analysis of information used to form decisions.

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Georeferencing

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Aligning geographic data to a known coordinate system so it can be viewed, queried, and analyzed with other geographic data.

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Query

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A request to select features or records from a database. A query is often written as a statement or logical expression.

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Absolute accuracy

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

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Attribute accuracy

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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.

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Cell

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individual picture elements in a raster image.

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Currency

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

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Data creators

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companies that develop their own or enhance existing geographic data.

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Data integrators

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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.

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Data packagers

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

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Datum

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

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Easting

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

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Hierarchical

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

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Hydrography

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

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Hypsography

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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.