GIS Glossary

Glossary of GIS Terms

Address Matching: A procedure that enables two or more data files to be related using a common address field. A procedure where objects in a non-geocoded database are geocoded using an address field such as a street address.
Aerial Photograph: A photograph, usually taken from an airplane, as a means of remotely recording ground level events. Not to be confused with satellite remote sensing which produces digital images, aerial photography provides black and white, color and infrared photographs on film. These can be taken at either vertical or oblique angles, depending on the phenomenon in question and the desired application. Aerial photography differs from satellite imagery in that the results are almost instantaneous and require only developing, as opposed to images which must undergo a great deal of processing before electromagnetic signals resemble real world features.
Annotation: Text or labels on a map such as a street or place name.
Attribute: A fact describing an entity in a relational data model, equivalent to the column in a relational table.
Base Map: A set of topographic data displayed in map form providing a frame of reference or contextual information to the user.
Buffer: A polygon enclosing an area within a specified distance from a point, line or polygon. Accordingly, there are point buffers, line buffers and polygon buffers.
CAD - Computer Aided Design: Software packages designed for high quality graphical output regarding the design of products. Automated functions allow for easy and fast output that can be utilized by a novice user as well as the specialist. CAD is used mainly within the fields of engineering and architecture.
COGO - Coordinate Geometry: A set of procedures for encoding and manipulating bearings, distances and angles of survey data into co-ordinate data. COGO is frequently a subsystem of GIS.
Cadastral Map: A map showing the boundaries of the subdivisions of land for purposes of describing and recording ownership and taxation.
Cartography: The organisation and communication of geographically related information in either graphic or digital form. It can include all stages from data acquisition to presentation and use.
Census Block: The smallest geographical area, bounded by visible boundaries, for which census data is collected.
Census Tract: A small, permanent statistical subdivision of an area, normally with homogeneous characteristics.
Centerline: A line digitized along the center of a linear feature.
Centroid: The geometric center of a polygon. In spatial information systems, the centroid is a point in a polygon to which attribute information about that specific area is linked.
Coordinate: Any of a set of two or more numbers used to determine the position of a point, line, curve, or plane in a space of a given dimension with respect to a system of lines or other fixed reference.
Coordinate System: A recognized reference system for the unique location of a point in space. The Cartesian co-ordinate system and the system of latitude and longitude of the earth are examples of coordinate systems based upon Euclidean geometry.
Contour Line: A set of points representing the same value of a selected attribute and forming an imaginary line. The terms contour or contour line is most commonly used for lines connecting points on the ground having the same elevation.
Control Point: A system of points with established horizontal and vertical positions which are used as fixed references in positioning and relating map features, aerial photographs or remotely sensed images.
DXF - Digital Exchange Format: A format for transferring drawings between Computer Aided Design systems, widely used as a de facto standard in the engineering and construction industries.
Data Capture: The encoding of data. In the context of digital mapping this includes digitizing, direct recording by electronic survey instruments, and the encoding of text and attributes.
Data Conversion: The translation of data from one format to another. Often, when data is moved from one system to another, some form of data conversion is required to convert the data to a format the receiving system can interpret. Sometimes it may be necessary to have an intermediate format such as DXF.
Database: A collection of data organized according to a conceptual structure describing the characteristics of the data and the relationships among their corresponding entities, supporting applications areas. For example, a GIS database includes data about the position and characteristics of geographical features.
Digital Data: Data represented in a computer compatible format.
Digital Geospatial Metadata – DGM: DGM was approved in June 1994 by the Federal Geographic Data Committee (FGDC). DGM describes the specifications for the content, quality, condition, and other characteristics of metadata. The standard provides a common set of terminology and definitions for the documentation of geospatial data. DGM establishes the names of data elements and groups of data elements to be used for these purposes, definitions of these data elements and groups, and information about the values that are to be provided for the data elements.
Digitizing: A method of data capture that involves the conversion of data in analog form, such as maps and aerial photographs, into a digital form that is directly readable by a computer. This is normally achieved manually by a human operator using a digitizer, although methods of automated digitizing and semi-automated digitizing also exist. The result of digitizing is a digital map in vector form.
Edge Matching: The process of ensuring that data along the adjacent edges of map sheets or some other unit of storage, matches in both positional and attribute terms.
Geographic Information System – GIS: A computer system for capturing, storing, checking, integrating, manipulating analyzing and displaying data related to positions on the Earth's surface. Typically, a GIS is used for handling maps of one kind or another. These might be represented as several different layers where each layer holds data about a particular kind of feature (e.g. roads). Each feature is linked to a position on the graphical image of a map. Layers of data are organized to be studied and to perform statistical analysis (i.e. a layer of customer locations could include fields for Name, Address, Contact, Number, Area). Uses are primarily government related, town planning, local authority and public utility management, environmental, resource management, engineering, business, marketing, and distribution.
Global Positioning System – GPS: A system for determining position on the Earth's surface by comparing radio signals from several satellites. When completed the system will consist of 24 satellites equipped with radio transmitters and atomic clocks. Depending on your geographic location, the GPS receiver samples data from up to six satellites, it then calculates the time taken for each satellite signal to reach the GPS receiver, and from the difference in time of reception, determines your location.
Geocoding: The cross-referencing between specifically recorded x,y co-ordinates of a location, relative to a standard reference grid such as the NC State Plane Grid, and non-geographic data such as addresses or Zip codes. In this way the accessing of the non-geographic data allows locations to be accurately mapped.
Georeference: To establish the relationship between page coordinates on a planar map and known real-world coordinates.
Ground Control Point – GCP: Any point which is recognizable on both remotely sensed images, maps and aerial photographs and which can be accurately located on each of these. This can then be used as a means of reference between maps or, more commonly, between maps and digital images. Often used in the geometric correction of remotely sensed images and surveying.
Image: A graphic representation or description of a scene, typically produced by an optical or electronic device. Common examples include remotely sensed data (satellite data), scanned data, and photographs. An image is stored as a raster data set of binary or integer values that represent the intensity of reflected light, heat, or other range of values on the electromagnetic spectrum.
Import: A process of loading data into a computer system or application from another system or application, often involving some form of data conversion.
Interpolation: The estimation of values of a surface at an unsampled point based on the known values of surrounding points.
Island: A closed area which lies completely within another, larger area. Islands, or island polygons are common in spatial data, for example an island may represent features such as enclosed areas of water such as lakes, or parks within urban areas.
Join: The relating of two or more tables in a relational database on the basis of a common item or field. Rows from these tables are compared, and based upon certain specified criteria, rows may be retrieved, updated or deleted from the database. One of the most common forms of join operation is the equijoin, where the values in one table must match the values in another. SQL is a common language employed for performing join operations using relational databases.
Label: A textual description of a geographic feature or object placed on a map.
Layer: A usable subdivision of a dataset, generally containing objects of certain classes, for example rivers, roads or buildings.
Line: A set of ordered co-ordinates that represent the shape of geographic features too narrow to be displayed as an area at the given scale (contours, street centrelines, or streams), or linear features with no area (county boundary lines).
Map: A graphic representation of features of the earth's surface or other geographically distributed phenomena. Examples are topographic maps, road maps, weather maps.
Map Projection: A method of representing the earth's three-dimensional surface as a flat two-dimensional surface. This normally involves 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 the map in two dimensions. Therefore such representations distort some parameter of the earth's surface, be it distance, area, shape, or direction.
Map Units: The coordinate units in which a geographic data set is stored. Map units can be inches, centimeters, feet, meters, or decimal degrees.
Metadata: Data that gives specific information about a map layer or file. This file may include at what scale the data was captured, what coordinate system it is in or where the file originated.
NAD - North American Datum: The official reference ellipsoid used for the primary geodetic network in North America.
Node: The beginning and ending locations of a line on a digital map. A node is topologically linked to all lines that meet at the node.
One-to-many Relationship: A relation in which one record in a table is related to many records in another table.
Orthophotograph: A modified copy of a perspective photograph of the earth's surface with distortions due to tilt and relief removed.
Overlay: The process of superimposing two or more maps, through registration to a common co-ordinate system, such that the resultant maps contain the data from both maps for selected features.
Point: An object represented by a single X,Y co-ordinate. A point normally represents a geographic feature too small to be displayed as a line or area; for example, the location of a building or a fire hydrant.
Polygon: A feature used to represent areas. A polygon is defined by the lines that make up its boundary and a point inside its boundary for identification. Polygons have attributes that describe the geographic feature they represent.
Polyline: A line made up of a sequence of line segments.
Query: A statement expressing a set of conditions that forms the basis for the retrieval of information from a database. Queries are often written in a standardized language such as SQL.
Raster Data: An abstraction of the real world where spatial data is expressed as a matrix of cells or pixels, with spatial position implicit in the ordering of the pixels. With the raster data model, spatial data is not continuous but divided into discrete units.
Rubber Sheeting: A procedure to adjust the coordinates of all the data points in a data set to allow a more accurate match between known locations and a few data points within the data set. Rubber sheeting, also known as rubber banding, preserves the interconnectivity or topology, between points and objects through stretching, shrinking or re-orienting their interconnecting lines.
SQL - Structured Query Language: 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.
Scale: The ratio of the distance measured on a map to that measured on the ground between the same two points. For example, as 1:50,000, which represents a scale of 1 inch on the map would equal 50,000 inches on the Earth's surface. In addition, this type of ratio can also be represented as 1" = 4167' ( 12" in a foot, so 50,000 is divided by 12 to convert the scale).
Spatial Analysis: Analytical techniques associated with the study of locations of geographic phenomena together with their spatial dimensions and their associated attributes. Spatial analysis is useful for evaluating suitability, for estimating and predicting, and for interpreting and understanding the location and distribution of geographic features and phenomena.
Symbol: A graphic pattern used to represent a geographic feature on a map. For example, line symbols represent linear features; various marker symbols can represent points; shade symbols can represent areas; and text symbols, annotations. Many characteristics define symbols, including color, size, shape, angle, and pattern.
Thematic Map: A map depicting selected kinds of information relating to one or more specific themes. Examples are soil type, land classification, population density and rainfall maps.
Topography: The study of the relief of a given area on the Earth's surface, usually on a large scale, including both natural and man-made features.
Topology: The relative location of geographic phenomena independent of their exact position. In digital data, topological relationships such as connectivity, adjacency and relative position are usually expressed as relationships between nodes, links and polygons. For example, the topology of a line includes its from- and to-nodes, and its left and right polygons.
Vector: One method of data type, used to store spatial data. Vector data is comprised of lines or arcs, defined by beginning and end points, which meet at nodes. The locations of these nodes and the topological structure are usually stored explicitly. Features are defined by their boundaries only and curved lines are represented as a series of connecting arcs. Vector storage involves the storage of explicit topology, which raises overheads, however it only stores those points which define a feature and all space outside these features is 'non-existent'.
Vertices: One of a set of ordered X,Y co-ordinates that constitute a line.
Z-value: The value of a surface at a particular X,Y location, for example, elevation. The Z-value usually refers to 3D features.