GIS tool

absolute georeference

The referencing in space of the location of a point using a predefined coordinate system such as a national grid or latitude/longitude.2


Conforming to a recognizable standard. If applied to paper maps or map databases, degree of conformity with a standard or acceptable value.1 The statistical meaning of accuracy is the degree with which an estimated mean differs from the true mean. 2


A step by step procedure for solving a mathematical problem.1


Arc Macro Language


A line connecting a sequence of coordinate points. 2


A position facing a particular direction. Usually referred to in compass directions such as degrees.1

The American Standard Code for Information Interchange. A standard that maps commonly used characters such as the alphabet onto one byte long sequences of bits.


Non-geographic information associated with a point, line or area in a GIS.2 An attribute is a characteristic of a feature that contains a measurement or value for the feature. Attributes can be labels, categories, or numbers. Attributes can be dates, standardized values, or field or other measurements. Item for which data are collected and organized. A column in a table or data file.


Australian Surveying and Land Information Group


Advanced Very High Resolution Radiometer
AVHRR The Airborne Very High Resolution Radiometer, an instrument on the NOAA orbiting polar satellites, that returns 1 and 4 km resolution data about the earth in 4 wavelengths. Used extensively for large area land cover and vegetation mapping, and weather prediction.


Advanced Visible and Infrared Imaging Spectrometer



boolean algebra

Boolean Algebra is useful for performing operations on the attributes (which may be positional or descriptive) attached to geographic entities in a GIS. Boolean Logic is especially useful in computing (or modelling) new attributes in topological overlay processing for both vector and raster based systems, as they can be applied to all data types, be they Boolean, Ratio, Interval, Ordinal, or Nominal. Boolean algebra uses the logical operators AND, OR, NOT to determine whether a particular condition is true or false.Each attribute can be thought of as defining a set. Consider two sets (set A and set B). The AND operator () is the intersection of two sets - for example those entities that belong to both set A and set B ( A B). The OR operator ( ) is the union of two sets - for example those entities that belong to either set A or to set B ( A B). The NOT operator ( ) is the difference operator identifying those entities that belong to A but not B ( A B) These simple relations can be visualised through the use of Venn diagrams.

For a GIS related example of the use of these boolean logical operations, go to Boolean Logic - Simple Interactive Example


A zone of a given distance around a physical entity, such as a point, line or polygon.1Buffering can also be defined as the vector equivalent to distance analysis in raster Geographical Information Systems. Buffering involves the creation of a zone of a specified width around a point, line or area. The resulting buffer is a new polygon, which can be used to determine which entities occur within or outside the defined area.


cadastral map

A map showing the precise boundaries and size of land parcels.


A record of interests in land, including both the nature and extent of interests. Usually this means maps and other descriptions of land parcels as well as the identification of who owns certain legal rights to the land. Cadastral information often includes other descriptive information about land parcels.1

cartesian coordinate system

A system or two or three mutually perpendicular axes along which any point can be precisely located with reference to any other point, often referred to as x, y and z coordinates. Relative measure of distance, area and direction are constant throughout the system.1

cartographic modelling

Cartographic modelling is a general, but well-defined methodology that is used to address diverse applications of GIS in a clear manner. It is a technique used for both vector and raster based GIS, and as the term suggests, cartographic modelling involves models (ie. of geospatial information) represented in cartographic form (ie. as Maps). Cartographic modelling is used to simultaneously analyse both the spatial and thematic characteristics of geospatial information. The thematic component of geospatial information is analysed via statistical operations on the data (for example, taking the Average and Standard Deviation of the data), where the spatial characteristics of geospatial information are described through spatial analysis techniques (which are based on coordinate data). (see the Vector Overlay module)


The basic element of spatial information in a raster data set. Cells are always square. A group of cells forms a grid.1 (see pixel)


The process of assigning items to a group or set according to their attributes. 2

To spatially aggregate. To join together features with similar characteristics into a single feature.


coordinate geometry

composite map

A single map created by joining together two or more maps that have been digitised seperately. 2

The topological property of sharing a common link, such as a line connecting two points in a network.
contour interval
The vertical difference in measurement units such as meters or feet, between successive contour lines on a contour map.


A line connecting points of equal value (e.g. elevation), often in reference to a horizontal datum such as mean sea level.1


The position of a point n space with respect to a Cartesian coordinate system (x, y and/or z values). In a GIS, a coordinate often represents locations on the earth’s surface relative to other locations.1

coordinate system

A systems used to measure horizontal and vertical distances on a plan metric map. In a GIS, it is the system whose units and characteristics are defined by a map projection. A common coordinate systems is used to spatially register geographic data for the same area.1



database management system


Digital elevation model. A quantitative model of a topographic surface in digital form. Also known as a ’digital terrain model’ (DTM). The resolution, or the distance between adjacent grid points, is a critical parameter.


A means of converting or encoding map data that are represented in analog form into digital information of x and y coordinates.1


A device used to capture planar coordinate data, usually as x and y coordinates from existing analog maps for digital use.1


digital line graph

double precision

Refers to a level of coordinate accuracy based on the possible number of significant digits that can be stored for each coordinate. Whereas single precision coverages can store up to 7 significant digits for each coordinate and therefore retain a precision of +/- 5 meters in an extent of 1,000,000 meters, double precision coverages can store up to 15 significant digits per coordinate, and therefore retain the accuracy of much less than 1 meter at a global extent.1


dots per inch, a measurement if the density of dots used to print or scan and area with larger values representing more detail and a finer resolution. 2


digital terrain model, see DEM

digital file exchange format, a vector mode industry standard format for graphic file exchange.



The x-coordinates in a plane coordinate system.1

edge matching
The GIS or digital map equivalent of matching paper maps along their edges. Features that continue over the edge must be "zipped" together, and the edge dissolved. To edge match, maps must be on the same projection, datum, ellipsoid, and scale, and show features captured at the same equivalent scale.
end node
The last point in an arc that connects to another arc.






ground control point


To (1) reduce the number of points, or vertices used to represent a line; or (2) increase the cell size and resample data in a raster format.1The process of moving from one map scale to a smaller (less detailed) scale and changing the form of features by simplification, etc.

A location in geographic space converted into computer readable form. This usually means making a digital record of the point's coordinates.


The activity of defining the position of geographical objects relative to a standard reference grid. 2 The conversion of analog maps into computer readable form. The two usual methods of geocoding are scanning and digitizing.

Geodetical surveying

The determination of the position of points on the earth’s surface accounting for its curvature, rotation and gravitational field. 2

geographical data

Data that record the location and a value characterizing the phenomenon.2

geographic information system

An organized collection of computer hardware, software, geographic data and personnel designed to efficiently capture, store, update, manipulate, analyse, and display forms of geographically referenced information.1

An industry standard raster graphic or image format.


global positioning system, a set of satellites in geostationary earth orbits used to help determine geographic location anywhere on earth by means of portable electronic receivers. 2


A grid of parallels and meridians on a map.1


(1) a set of regularly spaced sample points; (2) in cartography, an exact set of reference lines over the earths surface. 2






The estimation of values of an attribute at unsampled points from measurements made at surrounding sites. 2

interval data

Measurements that represent quantities in terms of equal intervals or degrees of difference, but whose zero point (or point of beginning) is arbitrarily established. Used to quantify differences but not proportions and characterise relative positions in space, time or magnitude. Examples include latitudes, longitudes, compass directions, times of day and normalised scores.Interval input map data can be used for the following statistical operations: Sum, Average, Maximum, Minimum, Median, Majority, Minority, Diversity (Variety) and Range. (see the Neighbourhood Operations module)




Japanese Earth Resources Satellite




Land resource assessment satellite system, a series of earth resource scanning satellites launched by the USA. 2


A method of measuring the earth representing angles of a line extending from the center of the earth to the earth’s surface. Lines of latitude run from east to west and measure the number of degrees north or south of the Equator (which represents 0 degrees). Values range from the North Pole, at positive 90 degrees, to the South Pole which is located at negative 90 degrees. Lines of latitude are often called ‘parallels’.1 Each degree can be further subdivided into 60 minutes, each composed of 60 seconds.

A set of digital map features collectively (points, lines and areas) with a common theme in coregistration with other layers.


A set of ordered coordinates that represent the shape of a geographic entity too narrow to be displayed as an area (e.g. streams, contours).1

Line simplification algorithms have been developed over the years for the purpose of 'weeding out' redundant or unnecessary coordinate information from line features, whilst retaining the perceptual characteristics of the line. They generally work via the application of some geometric criterion to a line's coordinate pairs (such as distance between points or displacement from a centre-line). (for more on line simplification go to the Line Generalisation module)


A method of measuring the earth representing angles of a line extending from the center of the earth to the earth’s surface. A line extending from the north to the south pole through the Greenwich, England, represents 0 degrees. Each line of longitude runs north and south and measures the number of degrees east or west of the Prime Meridian. Values range from positive 180 to negative 180 degrees. Lines of longitude are often called ‘meridians’. 1

lookup table

An array of data values that can be quickly accessed by a computer program to convert data from one form to another. 2


map projection

A mathematical model for converting locations on the earth’s surface from spherical to planar coordinates, allowing flat maps to depict three-dimensional features. Some map projections preserve the integrity of shape, others preserve accuracy of area, distance or direction.1
All map projections distort shape, area, distance or direction to some extent.

map units

The coordinate units in which the geographical data are stored, such as meters, or degrees, minutes and seconds.1


A line running vertically from the north pole to the south pole along which all locations have the same longitude. The prime meridian (0 degrees) runs through Greenwich, England. Moving left or right of the prime meridian, measures of longitude are negative to the west and positive to the east, up to 180 degreees (half-way around the globe).1

The GIS or digital map equivalent of matching multiple paper maps along their edges. Features that continue over the edge must be matched, and the edge dissolved. A new geographic extent for the map usually has to be cut or clipped out of the mosaic. To mosaic, maps must be on the same projection, datum, ellipsoid, and scale, and show features captured at the same scale.


Multispectral scanner



Normalized difference vegetation index


(1) the beginning or end of a line; (2) the location where lines connect.1

nominal data

Values that represent qualities rather than quantities and do so without any reference to a linear scale (ie. 'measurement' in terms of names or designations of discrete units or categories). Examples include telephone numbers, post codes, or soil types.Nominal input map data can be used for the following statistical operations: Majority, Minority, and Diversity (Variety). (see the Neighbourhood Operations module)


The y-coordinates in a plane-coordinate system.1


ordinal data
Measurements that quantify differences by order (ie. in terms of values such as 'more' or 'less', 'larger' or 'smaller'), not magnitude - that is, the size of the intervals is not specified. Often used where quantitative differences are apparent but hard to measure (ie. when values represent a ranked order). Examples include the distinction between poor, moderate, and good quality agricultural land (gives no indication of exactly how much better/worse each is than the others). Ordinal input map data can be used for the following statistical operations: Maximum, Minimum, Median, Majority, Minority, Diversity (Variety) and Range. (see the Neighbourhood Operations module)


The process of stacking digital representations of various spatial data on top of each other so that each position in the area covered can be analysed in terms of these data. 2


That portion of a line digitized past its intersection with another line. 1



A series of techniques foe measuring position and altitude from aerial photographs or images using a stereoscope. 2


One picture element of a uniform raster or grid line. Often used synonymously with cell. 1


A single x, y coordinate that represents a geographical feature too small to be displayed as a line or area, e.g. a mountain peak.1


A vector representation of an enclosed region, described by a sequential list of vertices or mathematical functions.1


If applied to paper maps or map databases, it means the accuracy of definition; (2) if applied to data collection devices such as digitisers, it is the exactness of the determined value; (3) the number of significant digits used to store numbers.1
Note: precision is not the same as accuracy - a large number of significant digits doesn't necessarily indicate that the measurement is accurate.


A hand-held device for entering data from a digitiser which usually has a window with accurately engraved cross-hairs, and several buttons for entering associated data. 2



ratio data

Measurements that represent quantities in terms of equal intervals and an absolute zero point of origin. Expressed using numbers that can be transformed or combined with any mathematical function to generate meaningful results. Examples include measurements of characteristics such as age, frequency, physical distances and monetary value. Ratio input map data can be used for the following statistical operations: Sum, Average, Maximum, Minimum, Median, Majority, Minority, Diversity (Variety) and Range. (see the Neighbourhood Operations module)


A regular grid of cells covering an area. 2

raster data

Data where values stored for maps and images are organized sequentially by rows and columns. Each cell must be rectangular.1


The process of converting an image of lines and polygons from vector representation to a gridded representation. 2


The process by which and image or grid is converted form image coordinated to real-world coordinates. Rectification typically involves rotation and scaling of grid cells and thus requires resampling of values.1

relational database management system (RDBMS)

A database management system with the ability to access data organized in tabular files that may be related together by a common field (item). An RDBMS has the capability to recombine the data files from different files, providing powerful tools for data usage.1


A technique for transforming a raster image from one particular scale and projection to another. 2


(1) The size of the smallest feature that can be represented in a surface. (2)The accuracy at which the location and shape of map features can be depicted for a given map scale. In a large scale map (e.g. a map scale of 1:1) there is less reduction of features than those shown on a small scale map (e.g. 1:1,000,000). On a larger scale map feature resolution more closely resembles real-world features. As map scale decreases, resolution also diminishes as feature boundaries must be smoothed, simplified or not shown at all.1

Root Mean Square
Error (RMS)

The Root Mean Square (RMS) error represents the difference between original control points and new control point locations calculated by the transformation process (e.g. during digitising). The transformation scale indicates how much the map being digitized will be scaled to match the real-world coordinates.

x  is the error in one dimension of a point
n  is the number of points in the sample

The statistic is calculated for each dimension (Eastings and Northings).
The vector error is computed by combining these results:

E   is the error in Eastings of a point
N   is the error in Northings of a point
n is the number of points in the sample




The relationship existence between distance on a map and the corresponding distance on the earth. It is usually expressed in the following form 1:10,000, meaning that 1 unit of measurement on the map represents 10,000 of the same units on the earth’s surface.1 A ‘ large’ scale map is one in which a given part of the Earth is represented by a large area on the map. Large scale maps generally show more detail than small scale maps because at a large scale there is more space on the map in which to show features. Large scale maps are typically used to show site plans, local areas, neighborhoods, towns etc. 1:2,500 is an example of a large scale. A ‘small’ scale map is one in which a given part of the Earth is represented by a small area on the map. Small scale maps generally show less detail than large scale maps, but cover large parts of the Earth. Maps with regional, national, and international extents typically have small scales, such as 1:1,000,000. Large scale maps typically show more detail than small scale maps, whereas on smaller scale maps there is simply not enough room to show all the available detail, so features such as streams and roads often have to be represented as single lines, and area features like cities, have to be shown as points. This is called generalization.4


A process by which information originally in hard copy format can be rapidly converted to digital raster form.1

single precision

A lower level of coordinate accuracy based on the possible number of significant digits which can be stored for each coordinate. Single precision numbers can store up to 7 significant digits for each coordinate and therefore retain a precision of +/- 5 meters in an extent of 1,000,000 meters, double precision coverages can store up to 15 significant digits per coordinate (typically 13 to 14 significant digits) and therefore retain the accuracy of much less than 1 meter at a global extent.1


A narrow gap between two lines created erroneously by digitising or by the vectorisation software of a scanner. 2


A process to generalize data and remove smaller, erratic variations.


Satellite Pour l’Observation de la Terra




The process of dividing an area into smaller, contiguous tiles with no gaps between them.2


Topologically integrated geographic encoding and referencing


Thematic Mapper


A term used to refer to the continuity of space and spatial properties, such as connectivity, that are unchanged after distortion. In GIS, this terms refers to the way in which geographical elements are linked together.2 For example the topology of a line includes all of its to- and from- nodes, and its left and right polygons. Topology is useful in GIS because some spatial modeling operations do not require coordinates, only topological information.


The process of converting data from one coordinate system to another through translation, projection, rotation and scaling. 1, 2

traingulated irregular
network (TIN)

A representation of a surface derived from irregularly spaced sample points and breakline features. The TIN data set includes topological relationships between points and their proximal 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 that can be used to represent a surface.1
The Triangulated Irregular Network model is an alternative to the regular raster of a DEM, and has been adopted in numerous GISs and automated mapping and contouring packages.

topological map

Topological map overlay creates new features and attribute relations by overlaying the features from two input map layers. Features from each input layer are combined to create new output features. Attributes of each input feature are combined from the two input layers to describe each new output feature, thus creating new attribute relationships. Often, the manipulation of multiple data layers is required to achieve the objective of the overlay operation. This is done in a stepwise fashion - two input layers are combined to form an intermediate layer, this intermediate layer is then combined with a third layer to form another intermediate layer, and so forth until the desired resultant map layer is achieved. (see the Vector Overlay module)




A digitised line that does not quite reach a line that it should intersect.1

Universal Transverse
Mercator (UTM)

A widely used planar coordinate system, extending from 84 north to 80 south latitude and based on a specialised application of the Transverse Mercator projection. The extent of the coordinate system is broken into 60, 6 degrees (longitude) zones. Within each zone, coordinated are usually expressed as meters north or south of the equator and east from a reference axis. For locations in the Northern Hemisphere, the origin is assigned a false easting of 500,000 and a false northing of 0. For locations in the Southern Hemisphere, the origin is assigned a false easting of 500,000 and a false northing of 10,000,000.1




The representation of spatial data by points, lines and polygons.2

vector data

A coordinate-based data structure commonly used to represent map features. Each liner feature is represented as a list of ordered x, y coordinated. Attributes are associated with the feature (as opposed to a raster data structure, which associates attributes with a grid cell). Traditional vector data structures include arc-node models vertex one point along a line.1


The conversion of point, line and area data from a grid to a vector representation.2


Those parts of a landscape that can be seen from a particular point.2










The elevation value of a surface at a particular x, y location. Also known as the spot value or spot elevation.1

1 Berry, J.K. (1995) Beyond Mapping: Concepts, Algorithms and Issues in GIS. GIS World Books, Fort Collins, USA.
  2 Burrough, P.A. (1986) Principles of Geographic Information Systems for Land Resource Assessment. Monographs on Soil and Resources Survey No. 12, Oxford Science Publications, New York.
  3 Steigeler, S.E. Ed. (1979) A Dictionary of Earth Sciences. Pan Books, London.
  4 ArcView GIS Version 3.1 (1998) Environmental Systems Research Institute.
  5 Ordnance Survey (1997) Consultation paper 3/1997 ‘Positional accuracy of large-scale data and products’
Developed and Maintained by:
Fiona Ellis:
Date Created: November 1999
Last Modified: 12 March 2001
Authorised by: Dr F. Escobar, Assoc Prof G. Hunter,
Assoc Prof I. Bishop, Dr A. Zerger
Webspace provided by: Department of Geomatics,
The University of Melbourne