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Geographic Information Systems (GIS)

Glossary

annotation

• 1. Descriptive text used to label coverage features. It is used for display, not for analysis.
• 2. One of the feature classes in a coverage used to label other features. Information stored for annotation includes a text string, the location at which it is displayed, and a text symbol (color, font, size, etc.) for display.

area

• 1. A homogeneous extent of the Earth bounded by one or more arc features (polygon) or represented as a set of polygons (region). Examples: states, counties, lakes, land-use areas, and census tracts.
• 2. The size of a geographic feature measured in unit squares. ArcInfo stores an area measure for each polygon and region.

attribute

• 1. A characteristic of a geographic feature described by numbers, characters, images and CAD drawings, typically stored in tabular format and linked to the feature by a user-assigned identifier (e.g., the attributes of a well might include depth and gallons per minute).
• 2. A column in a database table. See also item.

attribute table

• An INFO or other tabular file containing rows and columns. In ArcInfo, attribute tables are associated with a class of geographic features, such as wells or roads. Each row represents a geographic feature. Each column represents one attribute of a feature, with the same column representing the same attribute in each row. See also feature attribute table.

basemap

• A map containing geographic features used for locational reference. Roads, for example, are commonly found on basemaps.

column

• The vertical dimension of a table. A column has a name and a data type applied to all values in the column.

coordinate

• A set of numbers that designate location in a given reference system, such as x,y in a planar coordinate system or an x,y,z in a three-dimensional coordinate system. Coordinates represent locations on the Earth's surface relative to other locations. See also vector and Cartesian coordinate system.

coordinate system

• A reference system used to measure horizontal and vertical distances on a planimetric map. A coordinate system is usually defined by a map projection, a spheroid of reference, a datum, one or more standard parallels, a central meridian, and possible shifts in the x- and y-directions to locate x,y positions of point, line, and area features. In ArcInfo, a system with units and characteristics defined by a map projection. A common coordinate system is used to spatially register geographic data for the same area.

database

• A logical collection of interrelated information, managed and stored as a unit, usually on some form of mass-storage system such as magnetic tape or disk. A GIS database includes data about the spatial location and shape of geographic features recorded as points, lines, areas, pixels, grid cells, or tins, as well as their attributes.

DCW

• "Digital Chart of the World." The first 1:1,000,000-scale digital basemap of the world. The DCW contains topologically based vector data digitized from the U.S. Defense Mapping Agency's Operational Navigation Charts.

digital elevation model

• 1. A digital representation of a continuous variable over a two- dimensional surface by a regular array of z values referenced to a common datum. Digital elevation models are typically used to represent terrain relief. Also referred to as 'digital terrain model' (DTM).
• 2. An elevation database for elevation data by map sheet from the National Mapping Division of the U.S. Geological Survey (USGS).
• 3. The format of the USGS digital elevation data sets.

digitize

• 1. To encode geographic features in digital form as x,y coordinates.
• 2. The process of using a digitizer to encode the locations of geographic features by converting their map positions to a series of x,y coordinates stored in computer files. Pushing a digitizer button records an x,y coordinate. A digitized line is created by recording a series of x,y coordinates.

digitizer

• 1. A device that consists of a table and a cursor with crosshairs and keys used to digitize geographic features.

field

• In a database, another term for column.

foreign key

• One or more table attributes that can uniquely identify a record in another table. A foreign key is the primary key of another table. Foreign key-primary key relationships define a relational join. See also relate.

geographic database

• A collection of spatial data and related descriptive data organized for efficient storage and retrieval by many users.

GIS

• 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

• A system of satellites and receiving devices used to compute positions on the Earth. GPS is used in navigation, and its precision supports cadastral surveying.

latitude-longitude

• A spherical reference system used to measure locations on the Earth's surface. Latitude and longitude are angles measured from the Earth's center to locations on the Earth's surface. Latitude measures angles in a north-south direction. Longitude measures angles in the east-west direction.

layer

• The visual representation of a geographic dataset in any digital map environment. Conceptually, a layer is a slice or stratum of the geographic reality in a particular area, and is more or less equivalent to a legend item on a paper map. On a road map, for example, roads, national parks, political boundaries, and rivers might be considered different layers.

legend

• 1. The reference area on a map that lists and explains the colors, symbols, line patterns, shadings, and annotation used on the map. The legend often includes the scale, origin, orientation, and other map information.

line

• 1. A set of ordered coordinates that represents the shape of geographic features too narrow to be displayed as an area at the given scale (e.g., contours, street centerlines, or streams), or linear features with no area (e.g., state and county boundary lines).

lookup table

• 1. A special tabular data file containing additional attributes for features stored in an associated feature attribute table. The table can be an external attribute table or an INFO table that describes coverage features.
• 2. A special lookup table in which numeric item values are classified into categories. For example, well depth can be recorded explicitly in the feature attribute table, but displayed and used as a set of classes, such as 0 to 250 feet, 251 to 500 feet, and so on. An INFO lookup table contains at least two items: the relate item and an item named either SYMBOL or LABEL.

many-to-one relate

• A relate in which many records in one table are related to a single record in another table.

map

• An abstract representation of the physical features of a portion of the Earth's surface graphically displayed on a planar surface. Maps display signs, symbols, and spatial relationships among the features. They typically emphasize, generalize, and omit certain features from the display to meet design objectives (e.g., railroad features might be included in a transportation map but omitted from a highway map).

map projection

• A mathematical model that transforms the locations of features on the Earth's surface to locations on a two-dimensional surface. Because the Earth is three-dimensional, some method must be used to depict a map in two dimensions. Some projections preserve shape; others preserve accuracy of area, distance, or direction. See also coordinate system. Map projections project the Earth's surface onto a flat plane. However, any such representation distorts some parameter of the Earth's surface be it distance, area, shape, or direction.

map query

• The process of selecting information from a GIS by asking spatial or logical questions of the geographic data. Spatial query is the process of selecting features based on location or spatial relationship (e.g., select all features within 300 feet of another; point at a set of features to select them). Logical query is the process of selecting features whose attributes meet specific logical criteria (e.g., select all polygons whose value for AREA is greater than 10,000 or select all streets whose name is 'Main St.'). Once selected, additional operations can be performed, such as drawing them, listing their attributes or summarizing attribute values.

map scale

• The 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 of distance on the map represents 24,000 of the same units of distance on the Earth). Map scale can also be expressed as a statement of equivalence using different units; for example, 1 inch = 1 mile or 1 inch = 2,000 feet.

map units

• The coordinate units in which a geographic data set (e.g., a coverage) is stored in ArcInfo. Map units can be inches, centimeters, feet, meters, or decimal degrees.

neatline

• A border line commonly drawn around the extent of a map.

null value

• The absence of a value. If a particular column of a row in a table is null, that means there is no value stored. Null is not the same as blank or zero.

one-to-many

• A relate in which one record in a table is related to many records in another table.

pan

• To move the viewing window up, down, or sideways to display areas in a geographic data set which, at the current viewing scale, lie outside the viewing window. See also zoom.

pathname

• The path to a file or directory located on a disk. Pathnames are always specific to the computer operating system.

pixel

• A contraction of the words picture element. The smallest unit of information in an image or raster map. Referred to as a cell in an image or grid.

point

• 1. A single x,y coordinate that represents a geographic feature too small to be displayed as a line or area; for example, the location of a mountain peak or a building location on a small-scale map.
• 2. A coverage feature class used to represent point features or to identify polygons. It is not possible to have point and polygon features in the same coverage. When representing point features, the x,y location of the label point describes the location of the feature. When identifying polygons, the label point can be located anywhere within the polygon. Attributes for points are stored in a PAT.

polygon

• A coverage feature class used to represent areas. A polygon is defined by the arcs that make up its boundary and a point inside its boundary for identification. Polygons have attributes (PAT) that describe the geographic feature they represent.

PostScript

• PostScript is a page-description computer language developed, marketed, and trademarked by Adobe Systems, Inc. PostScript is supported on most LaserWriter printers. PostScript is particularly useful in computerized typesetting applications and desktop publishing with graphics. PostScript files can be plotted on non-PostScript plotting devices by means of Raster Image Processor (RIP) software.

precision

• Refers to the number of significant digits used to store numbers, and in particular, coordinate values. Precision is important for accurate feature representation, analysis and mapping. ArcInfo supports single precision and double precision.

primary key

• One or more attributes whose values uniquely identify a row in a database table. See also foreign key.

raster

• A cellular data structure composed of rows and columns for storing images. Groups of cells with the same value represent features. See also grid.

record

• 1. In an attribute table, a single 'row' of thematic descriptors. In SQL terms, a record is analogous to a tuple.
• 2. A logical unit of data in a file. For example, there is one record in the ARC file for each arc in a coverage.

relate

• An operation that establishes a temporary connection between corresponding records in two tables using an item common to both (i.e., relate key). Each record in one table is connected to those records in the other table that share the same value for the common item. Compare with relational join.

relate key

• The common set of columns used to relate two attribute tables. See also relate, primary key and foreign key.

relational database

• A method of structuring data as collections of tables that are logically associated to each other by shared attributes. Any data element can be found in a relation by knowing the name of the table, the attribute (column) name, and the value of the primary key. See also relate, relate key, and relational join.

relational join

• The operation of relating and physically merging two attribute tables using their common item.

remote sensing

• Acquiring information about an object without contacting it physically. Methods include aerial photography, radar, and satellite imaging.

row

• 1. A record in an attribute table. The horizontal dimension of a table composed of a set of columns containing one data item each.

scale bar

• A map element that shows the map scale graphically.

scanning

• The process of capturing data in raster format with a device called a scanner. Some scanners also use software to convert raster data to vector data.

snapping

• The process of moving a feature to coincide exactly with coordinates of another feature within a specified snapping distance, or tolerance.

SQL

• 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.

symbol

• A graphic pattern used to represent a feature. For example, line symbols represent arc features; marker symbols, points; shades symbols, polygons; and text symbols, annotation. Many characteristics define symbols, including color, size, angle, and pattern. See also text symbol, marker symbol, shade symbol, and line symbol.

table

• A set of data elements that has a horizontal dimension (rows) and a vertical dimension (columns) in a relational database system. A table has a specified number of columns but can have any number of rows. A table is often called a relation. Rows stored in a table are structurally equivalent to records from flat files in that they must not contain repeating fields.

template

• 1. A coverage containing common feature boundaries, such as land-water boundaries, for use as a starting place in automating other coverages. Templates save time and increase the precision of topological overlays.
• 2. A map template containing neatlines, North arrow, logos, and other cartographic map elements for a common map series.
• 3. An empty tabular data file containing only item definitions.

theme

• A user-defined perspective on a coverage, grid, tin or image geographic data set specified, if applicable, by a coverage name and feature class or data set name, attributes of interest, a data classification scheme, and theme-specific symbology for drawing.

TIFF

• Tagged interchange (image) file format. An industry-standard raster data format. TIFF supports black-and-white, gray-scale, pseudocolor, and true-color images, all of which can be stored in a compressed or uncompressed format. TIFF is commonly used in desktop publishing and serves as an interface to numerous scanners and graphic arts packages. (See CCITT.)

tin

• Triangulated irregular network. A surface representation derived from irregularly spaced sample points and breakline features. The tin data set includes topological relationships between points and their neighboring triangles. Each sample point has an x,y coordinate and a surface, or z-value. These points are connected by edges to form a set of non-overlapping triangles used to represent the surface. Tins are also called irregular triangular mesh or irregular triangular surface model.

topographic map

• 1. A map containing contours indicating lines of equal surface elevation (relief), often referred to as topo maps.
• 2. Often used to refer to a map sheet published by the U.S. Geological Survey in the 7.5-minute quadrangle series or the 15-minute quadrangle series.

topology

• The spatial relationships between connecting or adjacent coverage features (e.g., arcs, nodes, polygons, and points). For example, the topology of an arc includes its from- and to-nodes, and its left and right polygons. Topological relationships are built from simple elements into complex elements: points (simplest elements), arcs (sets of connected points), areas (sets of connected arcs), and routes (sets of sections, which are arcs or portions of arcs). Redundant data (coordinates) are eliminated because an arc may represent a linear feature, part of the boundary of an area feature, or both. Topology is useful in GIS because many spatial modeling operations don't require coordinates, only topological information. For example, to find an optimal path between two points requires a list of the arcs that connect to each other and the cost to traverse each arc in each direction. Coordinates are only needed for drawing the path after it is calculated.

union

• A topological overlay of two polygonal spatial data sets which preserves features that fall within the spatial extent of either input data set; that is, all features from both coverages are retained. See also intersect and identity.

vector

• A coordinate-based data structure commonly used to represent linear geographic features. Each linear feature is represented as an ordered list of vertices. Traditional vector data structures include double-digitized polygons and arc-node models.

vertex

• One of a set of ordered x,y coordinates that constitutes a line.

z-value

• The value of a surface at a particular x,y location (e.g., elevation). Often referred to as spot values or spot elevations.

zoom

• To enlarge and display greater detail of a portion of a geographic data set.