GIS Dictionary

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The GIS dictionary contains definitions of GIS, cartography, remote sensing, and geographic terms.  Geographic concepts are defined here and are arranged alphabetically.

Browse the GIS dictionary by alphabet

– A –

The closeness or nearness of the measurements to the true or actual value of the quantity being measured. Usually represented as a percentile.

The cooling of air as it rises over the terrain. Conversely, the warming of air as it descends.

A set of characters containing either letters or numbers. For example, an alphanumeric password could be “2xbe14y”.

Acronym for American Standard Code for Information Interchange. ASCII is a code for representing English characters as numbers, with each character assigned a number from 0-127. An ASCII file is a text file in which each byte represents one character according to the ASCII code. ASCII files are sometimes called plain text files.

Direction that a surface faces.

An atlas is a collection of maps, usually bound into book form. As well as geographic features and political boundaries, many often feature geopolitical, social, religious and economic statistics. The name “atlas” derives from the custom of adorning the cover or title page of such collections with a picture of the Atlas of Greek mythology holding the Earth on his shoulders.

This usage of the term dates from Gerardus Mercator’s Atlas Sive Cosmographicae, (Atlas, or Description of the Universe) of 1585-1595. The earliest atlas was Ptolemy’s Geography of c. 150 AD.

Information about a spatial feature. For example, attributes about a school may be the name, level of education, and number of students.

An expression of distance, measure in degrees.

GIS Acronyms Starting with A

ACSM – American Congress on Surveying and Mapping

ASCII – American Standard Code for Information Interchange

ASPRS – American Society for Photogrammetry and Remote Sensing

Regional Organizations

AGRC – Utah Automated Geographic Reference Center

– B –

Base Map
The base map is the most accurate spatial database within a data system. Base maps tend to be streets, parcels and other fundamental layers. Because base maps tend to serve as the point of reference when creating other spatial databases, they tend to have the highest level of accuracy.

The direction of a line expressed in degrees and cardinal points.

Abandoned, lightly contaminated properties often found in economically distressed areas.

A zone of a specified distance around features in a geographic layer. Buffers can be set at constant or variable distance based on feature attributes. The resulting buffer zones form polygonal coverages.

– C –

Relating to land boundaries and subdivisions, parcels of land suitable for transfer of title. The legally recognized registration of the quantity, value and ownership of land parcels.

Cardinal Points
The four directions of north, south, east, and west.

Cartesian Coordinate System
In this system features on the earth’s surface are referenced to map locations using an XY coordinate system.

A type of map that uses a qualitative attribute to over or under-inflate the geographic depiction of an area.  See: Area Cartograms Explored.

Change Detection
A GIS technique for comparing the changes in a specific geographic area between two time periods.  See: Change Detection in GIS.

An instrument for measuring slopes or grades, heights, and distance.

Clip Analysis
Clip analysis is the function of using the extent of one geographic layer to clip the extent of another geographic layer. See: Clip Analysis using ArcGIS Desktop

A magnetic needle, mounted in a case or box, pivoted at its center, that always points to a general locality known as the magnetic north pole.

Conformal Projection
Projection which preserves the original shape of the area of interest but not the area or distance

Contour Line
An irregular curved line on a map joining all points of any given elevation, or a line all points of which are at the same elevation. See: Overview of Elevation Data.

Contour lines corresponding to elevation points on a hill slope.
Contour lines corresponding to elevation points on a hill slope.

Convergence of Information
The principle of using multiple indicators to deduce information.

Coordinate Geometry – COGO
A method of defining geometric features through the input of bearing and distance measurements. Coordinate Geometry (COGO) functions are typically used by land surveyors to enter traverses around spatial features such as parcels, to calculate precise locations and boundaries sing distances and bearings from reference points, and to define curves using a point location, radius, arc-length, and so on.

Coordinate System
The system used to measure horizontal and vertical distances on a planimetric map. A common coordinate system is used to spatially register geographic data for the same area.

X,Y input to denote a location on earth.

A spatial dataset containing a common feature type. Coverages are also referred to as layers or themes.  Within ESRI’s ArcInfo 7.x GIS software, coverages are a specific type of geographic file type.

GIS Acronyms Starting With C

CAD – Computer-Aided Design

CADD – Computer Aided Drafting and Design

– D –

Data Dictionary
Document containing information about a series of datasets. A Data Dictionary is a form of metadata or “data about the data.” Information found in a data dictionary typically are names of the data, information about the attributes, and information about any relational tables to the spatial datasets.

Data Integrity
Refers to the consistency and accuracy of data that is stored in a database.

Data Mining
The practice of searching through datasets to find patterns and trends. See: Spatial Data Mining.

Set of data that is stored in a given file.

Database Management System (DBMS)
System providing for the input, storage and retrieval of data.

A datum is a mathematical surface on which a mapping and coordinate system is based.

Digital elevation model (DEM)
A topographic surface arranged in a data file as a set of regularly spaced x, y, z coordinates where z represents elevation.

Digital Line Graph (DLG)
The digital format standards published by USGS for exchange of cartographic data files and for which Digital Line Graph data sets are delivered by USGS. Digital Line Graph files from the U.S. Geological Survey. These files include digital information from the USGS map base categories, such as transportation, hydrography, contours and public land survey boundaries.

Method of converting information from one format to another using a trace methology. Traditionally, digitizing has meant the creation of a spatial dataset from a hardcopy source such as a paper map or a plan. On-screen digitizing is the creation of a spatial dataset by tracing over features displayed on a computer monitor with a mouse. In both cases, the newly created dataset picks up the spatial reference of the source document.

The process of applying another data set over a shaded relief image.

Drawing Exchange Format (DXF)
An exchange format for CAD files. A format for storing vector data in ASCII or binary files; used by AutoCAD and other CAD software and convertible to ARC/INFO coverages.

GIS Acronyms Starting with D

DEM – Digital Elevation Model

DO(Q)Q – Digital Ortho(quarter)quads

DRG – Digital Raster Graphic

DSS – Decision Support System

DTM – Digital Terrain Model

– E –

Individual spatial features which include trees, lakes, grasslands and so forth.

Equal Area Projection
Projection which preserves the area of the area of interest shown but does not preserve the real-world distance or shape.

GIS Acronyms Starting with E

EIA – Environmental Impact Analysis

EIR – Environmental Impact Report

EOS – Earth Observing System

EPA – Environmental Protection Agency

ERP – Enterprise Resource Planning

ESRI – Environmental Systems Research Institute

– F –

Field of View
The field of view analysis in GIS is a polygon file that shows the portions of an area that are visible versus no visible across a terrain starting from a given point.

Field of view analysis in GIS.  The yellow areas are visibile from the starting point at the top of the screen.
Field of view analysis in GIS. The yellow areas are visibile from the starting point at the top of the screen.

Fuzzy Boundary
A boundary that is treated as band of uncertainty.

GIS Acronyms that start with F

FGDC – Federal Geographic Data Committee

FOSS4G – Free and Open Source Software for Geospatial

– G –

Removal of detail from a data layer to make processing or visualization easier at smaller scales.

Geocentric Datum
A geocentric datum is a datum, which has its origin at the Earth’s center of mass.

Geographic Coordinates
Location of features on the earth’s surface using a coordinate system of degress of latitude and longitude. More: Latitude and Longitude

Geographic Information System (GIS)
A computerized information system for storing, manipulating and analyzing spatially indexed information.  See:What is GIS?

Geography is the study of the surface of the Earth.  See What is Geography?

A close representation, physical model, of the figure of the Earth. According to C.F. Gauss, it is the “mathematical figure of the Earth”, in fact, of her gravity field. It is that equipotential surface (surface of fixed potential value) which coincides on average with mean sea level. The geoid surface is more irregular that the ellipsoid of revolution often used to approximate the shape of the physical Earth, but considerably more smooth than the Earth’s physical surface. While the latter has excursions of +8000 m (Mount Everest) and -11000 m (some deep ocean trenches), the geoid varies by only approx. ±100 m about the reference ellipsoid of revolution.

Because the force of gravity is everywhere perpendicular to the geoid (being an equipotential surface), sea water if left to itself would assume a surface equal to it. Similarly if sea water would be allowed to freely penetrate the continental masses, e.g., through tunnels. In reality it is not, of course; still, geodesists are able to derive the heights of continental points above this imaginary, but physically defined, surface by a technique called spirit levelling.

When travelling by ship, one does not notice the undulations of the geoid; the local vertical is always perpendicular to it, and the local horizon tangential to it. Only a GPS receiver on board may show the height variations relative to the (mathematically defined) reference ellipsoid, the centre of which coincides with the Earth’s centre of mass, the centre of orbital motion of the GPS satellites.


Geoportals are e-commerce sites that allow browsing, viewing and procuring spatial databases and orthophotography online.

To establish the relationship between page coordinates on a planar map and real-world coordinates.


Geostatistics is an application of the theory of random functions for estimating natural phenomena. It was traditionally used in geo-sciences. Methods of geostatistics are used in petroleum geology, hydrogeology, meteorology, oceanography, geochemistry, forestry, environmental control, landscape ecology, agriculture (esp. in precision farming) etc. The basic concept of geostatistics is that of scales of spatial variation. Data which is spatially independent show the same variability regardless of the location of data points. However, spatial data in most cases is not spatially independent. Data values which are close spatially show less variability than data values which are farther away from each other. The exact nature of this pattern varies from data set to data set; each set of data has its own unique function of variability and distance between data points. This variability is generally computed as a function called semivariance.

Spatial autocorrelation can be analyzed using correlograms, covariance functions and variograms (=semivariograms).


Galli, A., Wackernagel, H.: Multivariate geostatistical methods for spatial data analysis. 1987
Sharov, A: Quantitative Population Ecology, 1996,

Shine, J.A., Wakefield, G.I.: A comparison of supervised imagery classification using analyst-chosen and geostatistically-chosen training sets, 1999,

Global Positioning System (GPS)
Method for identifying locations on earth using triangulation calculations of satellite positions. Originally created by the United States Military, it has since found numerous commercial applications. More: What is GPS?

Graphical Scale
Graphical representation of the scale of a map with the numbers representing the real world units and the length of the bar the corresponding distance representation on the map. Also known as a bar scale.  More: Understanding Scale.   

Example of a bar scale used on a map.
Bar scale showing graphically the ratio of map units to ground units. The top scale shows the comparable ground measurement in kilometers (km) and the bottom bar scale shows the comparable length in miles.

GIS Acronyms that start with G

GOES – Geostationary Operational Environmental Satellite

GIS – Geographic Information Systems

GITA – Geospatial Information and Technology Association

GML – Geography Markup Language

GOES – Geostationary Operational Environmental Satellites

GUI – Graphical User Interface

GPS – Global Positioning Systems

– H –

Database representing the location of water bodies and flow lines.

Hyperlocal Cartography
Mapping at the macro scale.  For example, a map of a neighborhood.

Hyperspectral Imagery
Useful for classifying material types on the Earth’s surface – beneficial in agriculture and forestry management, mineral exploration, environmental monitoring and national security activities.

Lines connecting points of equal elevation on the surface of the earth. From the Greek word “Hypso” meaning height.

– I –

The process of estimating values for missing points based on nearby known values.

Amount of time that passes before the satellite scans the same point of the globe.

GIS Acronyms that start with I

IEEE – Institute of Electrical and Electronics Engineers Inc.

IGUG – International Intergraph Graphics User Group

IT – Information Technology

Regional Organizations

IGIC – Iowa Geographic Information Council

– K –

GIS Acronyms that start with K

KML – Keyhole Markup Language

– L –

Land Information Systems (LIS)
Specialized GIS systems most commonly found among municipal agencies geared toward legal, administrative and economic spatial analysis. Also known as Land Records Information System (LRIS).

The latitude of any place is its angular distance north or south of the equator.   The parallels of latitude are a series of imaginary circles passing around the earth, parallel to the equator.  The angular distance from the equator to the poles is 90 degrees. More: Latitude and Longitude

A spatial dataset containing a common feature type. Layers are also referred to as coverages or themes.

Line of Sight
The unimpeded view or access from one point to another point across a terrain or surface. See: Line of Sight in GIS

Lines of longitude are called meridians and are imaginary great circles that pass through both the north and south poles and perpendicular to the equator.  The prime meridian passes through Greenwich, England.  The angular distance, measured along the equator from the meridian of Greenwich to the meridian passing through any other place, is called the longitude of that place. More: Latitude and Longitude

Government lot or subdivision lot representing the boundary of a legally conveyable unit of land identified on a record document. A lot may or may not be coterminous with an Assessor parcel.

GIS Acronyms that start with L

LBS – Location-based Services

LiDAR – Light Detection and Ranging

– M –

Magnetic Declination
The angle between magnetic north as shown by the needle of a compass and true north.

A two-dimensional representation of a piece of three-dimensional space.

Map Gallery
A collections of maps sometimes grouped by a common theme. Map galleries are a popular method of showcasing. You’ll often find them at Geography related conferences. Ever increasing are online map galleries.

Map Projection
A method used to represent the 3-dimensional surface of the earth or other round body on a 2-dimensional plane in cartography (mapmaking). This process is typically, but not necessarily, a mathematical procedure (some methods are graphically based).

Map Reading
The art of interpreting or understanding the information conveyed by a map.

The use of maps and cartography to promote a specific (and often political) agenda.  See the post on Maptivism.

Two or more images taken simultaneously, but each image taken in a different part of the electromagnetic spectrum.

GIS Acronyms that start with M

MAPPS – Management Association for Private Photogrammetric Surveyors

MIS – Management Information System

Regional Organizations

MERLIN – Maryland’s Environmental Resources and Land Information Network

– N –

National Map Accuracy Standards (NMAS)
Standards established in 1941 by the U.S. Bureau of the Budget to set accuracy standards for all federally produced maps.

GIS Acronyms that start with N

NEPA – National Environmental Policy Act (1969)

NMAS – National Map Accuracy Standards

NOAA – National Oceanic and Atmospheric Administration

NSDI – National Spatial Data Infrastructure

NSPS – National Society of Professional Surveyors

NTF – Neutral Transfer Format (formerly National Transfer Format), which is used by the Ordnance Survey in Britain.

– O –

The act of circling the earth. The type of orbit describes the path the satellite takes as it circles the earth.

Digital imagery in which distortion from the camera angle and topography have been removed, thus equalizing the distances represented on the image. More: Orthophotos and GIS

GIS Acronyms that start with O

OGC – Open Geospatial Consortium

OS – Operating System

– P –

Imagery taken of all wavelengths within the visible spectrum, though not uniformly.

A mechanical or electronic device that calculates the area of a map feature.

An area bounded by a closed loop of straight-line arc segments. More: Types of GIS Data Explored: Vector and Raster

Positional Error
The difference between the true location and map location of a point.

The closeness with which the measurements agree with each other. More: GIS Data: A Look at Accuracy, Precision, and Types of Errors

A mathematical calculation transforming the three dimensional surface of the earth to a two-dimension plane. More: Common Map Projections

GIS Acronyms that start with P

PARC – Public Access Resource Center

PDA – Personal Digital Assistants

– Q –

GIS Acronyms that start with Q

QA/QC – Quality Assurance/Quality Control

– R –

Relational Database
A collection of data organized in two-dimensional tables consisting of named columns and (usually unique) rows.

Relief Map
A model of the ground as a it actually exists, showing the topography of the geographic area mapped.

Remote Sensing
The technique of collecting information from a distance. Most common mediums used are aerial photography and satellite imagery. Before the advent of photography, remote sensing was accomplished in such ways as information collection from air ballons. See: Remote-Sensing Technologies.

Remotely Sensed Data
Data collected from a distance usually acquired through remote sensing technologies such as satellite imagery.

Representational Fraction
Also referenced as R.F.  Scale indicated by a fraction and written with a numerator of unity.   For example, an R.F. of 1/10,000 means that one unit on the map equals 10,000 units on the ground. More: Understanding Map Scale

The amount of detail found in one pixel of the image. For example an image with one meter resolution means that each pixel in the image represents one square meter on the ground.

Right of Way
Land area providing legal right of passage, i.e., street right-of-way, railroad right-of-way, etc.

Root Mean Square Error
Determined by first calculating the Mean Square Error (MSE) which is achieved by: (1) calculating the square of the deviations of points from their true position, (2) summing up the measurement, (3) and then dividing by the total number of points.  To get the Root Mean Square Error, calculate the square root of the MSE.

GIS Acronyms that start with R

RDBMS – Relational Database Management Systems

ROI – Return On Investment

– S –

1. The relationship between the size of the map and the corresponding size of the real world. 2. A level of resolution or zoom. See: Understanding Map Scale

Editable spatial database format developed by Esri. More: What is a Shapefile?

Simultaneous Localization and Mapping
Referred to as SLAM,  this is a technique used by robots and autonomous vehicles to build up a map within an unknown environment (without a priori knowledge), or to update a map within a known environment (with a priori knowledge from a given map), while at the same time keeping track of their current location.  The concept was recently extended to humans by MIT reseachers, see: wearable cartography.

Rate of change in elevation measured as the angle between the ground and the horizontal.

Slope Board
A board for measuring slope.   Attached to the board is a cord with a weight on the bottom.  Holding the board in a  vertical plane, the sketcher sights along the edge to the point whose slope or grade from the sketcher’s position is desired.  While sighting, the sketcher clamps the cord against the edge of the board and takes the reading.

Slope Board
Slope Board

Spatial Data Transfer Standard (SDTS)
A comprehensive transfer standard for Earth-referenced data which was used to transfer all types of spatial data between dissimilar computer systems. Created by the USGS.

State Plane Coordinate System (SPCS)
The State Plane Coordinate System is not a projection; rather it is a system for specifying positions of geodetic stations using plane rectangular coordinates. This coordinate system that divides all fifty states of the United States, Puerto Rico and the U.S. Virgin Islands into over 120 numbered sections, referred to as zones. More: State Plane Coordinate System

Statistical Abstract
Compiled from various U.S. Government and private sources such as the Census Bureau, Bureau of Labor Statistics, Bureau of Economic Analysis, the Statistical Abstract is the mother lode of statistics on many aspects of American life. Covering a range of such subjects as politics, economics, life expectancy, and marital status, the Statistical Abstract has been capturing an annual snapshot of the United States since 1878.

Statistical Surface
A statistical surface is any geographic entity that can be thought of as containing a Z value for each X,Y location. More: Statistical surfaces.

Sun-Synchronous Orbit
An orbit that always passes over the earth at the same local sun time.

Amount of ground covered lengthwise in the passing of the satellite.

GIS Acronyms that start with S

SDK – Software Development Kit

SGI – Silicon Graphics Inc.

SPOT – Système Pour l’Observation de la Terre

– T –

A spatial dataset containing a common feature type. Themes are also referred to as layers or coverages.

A numerical value representing the acceptable error range a feature will have from its actual point found on earth.

Topology stores the relationships of one spatial element with respect to another

Transactional Geography (Definition provided by Di-Ann Eisnor of Platial)
Transactional cartography is the movement from mapproviding entertainment/information to map as enabling action. This shift in mapmaking is possible due to the merging of crowd-sourced information and access to real time data. Both bring location to vast amounts of complex data and allow for new forms of analysis and visualization.

In the past we emphasized mapping (relatively) static objects like rivers and roads, it evolved to mapping people and their movements and now continues to accelerate to mapping complex processes. Transactional cartography and the utilization of these new sources and abundance of data can actually impact decisions and actions such as a when/where to purchase a product, when/where and with whom to barter, when/where most effective routes for emergency response, when/where to build new roads or public facilities, when/where to buy a home, change a route or itinerary based on real-time circumstances and real world situation. In this realm, time is inextricably linked to location which removes historic obstacles to actionable or live mapping.

Triangulated Irregular Network (TIN)
A surface representation derived from irregularly spaced points and breakline features. Each sample point has an x, y coordinate and a z value or surface value.

Triangulation Station
A point on the Earth, the position of which is determined by triangulation. Also called trig point.

Triangulation Station in the Santa Monica Mountains, California.
Triangulation Station in the Santa Monica Mountains, California.

GIS Acronyms that start with T

TEC – Topographic Engineering Center, U.S. Army Corp of Engineers

TIFF – Tagged Image File Format

TIN – Triangular Irregular Networks

– U –

Universal Transverse Mercator (UTM)
The Universal Transverse Mercator is an international plane (rectangular) coordinate system developed by the U.S. Army. The UTM divides the world into 60 zones of 6 degrees longitude. Each zone extends 3 degrees east and west from its central meridian and are numbered consecutively west to east from the 180-degree meridian. Transverse Mercator projections may then be applied to each zone.

GIS Acronyms that start with U

UGIC – Utah Geographic Information Council

UMAC – Upper Midwest Aerospace Consortium

URISA – Urban and Regional Information Systems Association

USGS – United States Geological Survey

UTM – Universal Transverse Mercator

– V –

Vertical Interval
The difference between elevation between two adjacent contours.   In the example below, the vertical elevation is 20 meters.  Therefore, each contour represents a change of 20 meters in elevation as compared to its nearest neighboring contour.

Contour lines corresponding to elevation points on a hill slope.
Contour lines corresponding to elevation points on a hill slope.

Visibility Analysis
A line showing the visibility from one point to another as a result of line of sight analysis.See: Line of Sight in GIS

Line of sight analysis showing the obstructed (red) and visible (green) points across a terrain.
Line of sight analysis showing the obstructed (red) and visible (green) points across a terrain.

GIS Acronyms that start with V

VRS – Virtual Receiving Station

– W –

A navigation GPS fix. Usually a destination or point of reference.

Weird Polygons
Weird polygons are defined as polygons with missing nodes. The most common occurr

– X –

GIS Acronyms that start with the letter X

XML – eXtensible Markup Language

– Z –

Z Value
The elevation value of a surface at a particular x,y location. Also, often referred to as spot values or spot elevations

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