GIS Glossary – A through G

Listed here are definitions of GIS and geographic terms starting with A through G (from Accuracy through GPS).  Also see:  GIS Acronyms.  To submit a definition email editor@gislounge.com.

GIS / Geospatial Glossary:
A through G |     H through P |     R through Z

- A -

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

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

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

ASCII
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.

Aspect
Direction that a surface faces.

Atlas
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.

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

- 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.

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

Buffer
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 -

Cadastral
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.

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

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

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.

Coordinates
X,Y input to denote a location on earth.

Coverage
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.

- 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.

Database
Set of data that is stored in a given file.

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

Datum
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.

Digitizing
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.

Draping
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.

- E -

Entities
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.

- F -

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

- G -

Generalization
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.

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

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

Geoid
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.

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

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

Geostationary
Satellites that maintain a stationary position relative to the earth.

Geostatistics
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).

References:

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, http://www.geovista.psu.edu/sites/geocomp99/Gc99/044/gc_044.htm

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.