Accuracy: The difference between a set of representative values
and the actual values. The accuracy of a point location
would be the difference between the point's coordinates in the GIS and
the coordinates accepted as existing in the real world.
Ancillary documentation: Information that describes how
the data were created or how they can be used.
Animated GIF: A GIF is a bitmap file
format often used on the World Wide Web. An animated GIF is a series of
individual GIF frames joined together to create an animation.
It is perhaps the easiest way to create and view simple animations.
Animation: A collection of static images joined together and
shown consecutively so that they appear to move.
Arc: See line.
ArcInfo: Was the market leading GIS software package when GIS
computing was workstation-based. Is now available for NT but has in some
ways been superseded by desktop solutions such as its sister product ArcView,
and MapInfo. MapInfo and ArcView are produced by
Environmental Systems Research Institute (ESRI).
ArcView: A commonly used desktop GIS software package
produced by Environmental Systems Research Institute (ESRI). Its sister
product ArcInfo provides more functionality but
is harder to use.
Area cartogram: These are choropleth
maps that have been distorted so that the size of the polygons
is not proportional to the polygon's area, but is instead proportional
to another of the polygon's variables such as its total population.
Areal interpolation: The process by which data from one
set of source polygons are re-districted onto
a set of overlapping but non-hierarchical target polygons.
Areas: See polygons.
Attribute data: Data that relate to a specific, precisely
defined location. The data are often statistical but may be text, images
or multi-media. These are linked in the GIS to spatial
data that define the location.
Attribute querying: A query that
extracts features from a layer based on the value
of its attribute data: for example, 'select polygons
with an unemployment rate greater than 15%' would be an attribute
AVI: A video file format that can be used to publish
Blunder: The introduction of error
Buffering: A buffer is a polygon
that encloses all areas within a set distance of the spatial features.
Points, lines, and polygons
can all have buffers placed around them. For example, if a user is interested
in all areas within 1km of a church, a buffer would be placed around all
the points representing churches. This would create a new layer
consisting of polygons representing those areas within 1km of a church.
Capture: See data capture.
Cartogram: See area cartogram.
Centroid: A point at the geometric centre of a polygon.
This can be used to represent a polygon as a point.
Choropleth maps: Maps of quantitative data that show patterns
by using different colours or different shading for polygons
classed in some way. For example, a map of polygon-based unemployment
rates (expressed as percentages) might sub-divide rates into 0-5, 5-10,
10-15 and 15-20 and shade the polygons accordingly.
Coordinate pair: An x and y coordinate used to represent a location
in two-dimensional space, for example (6,4).
Correlation: A form of statistical modelling that attempts to
summarise how one dataset will vary in response to another. A correlation
coefficient of +1.0 means that where there are high values in one set
there will be high values in the other, while a correlation coefficient
of -1.0 means that where there are high values in one set there will be
low values in the other. A correlation coefficient of 0.0 means that there
is no discernible relationship between the two sets. This is a form of
global analysis as it only provides a single
summary statistic for the entire study area.
Coverage: See layer.
Dangling node: A node that should join with
another node to join two or more lines together, but which does not join.
This will result in holes in topology.
Data capture: The process by which data are taken from the real-world
(primary source), or from a secondary source
such as a paper map, and entered into GIS software.
From primary data this is usually through the use of Global
Positioning Systems or remote sensing.
For secondary data it is usually through digitising
Database Management Systems: Software systems specifically designed
to store attribute data.
Date stamping approach: A way of handling time in GIS where time
is treated as an attribute. Each feature has date stamps attached that
define the times that it was in existence.
DBMS: See Database Management
DEM: See Digital Terrain Model.
DGPS: See Differential GPS.
Diachronic analysis: A form of analysis drawn from systems theory
in which change over time is examined by comparing a large number of states,
none of which are assumed to be in equilibrium.
Differential GPS: A way of collecting Global
Positioning Systems data with increased accuracy. It involves using
a fixed base station at a known position to help find the location of
a roving receiver.
Digital Elevation Model: See Digital
Digital Terrain Model: A data model that attempts to provide
a three dimensional representation of a continuous surface. Often used
to represent relief.
Digitising: In GIS this has a more precise meaning than in other
disciplines. It usually refers to extracting coordinates from secondary
sources such as maps to create vector data.
Digitising table: A flat table with a fine mesh of wires
under the surface used to allow accurate digitising
of paper maps through the use of a puck.
Digitising tablet: Similar to a digitising
table only smaller.
Dissolve: An operation in which adjacent polygons
are merged if a selected feature of their attribute
data are the same. An example might be merging polygons representing
fields to create a new layer containing crop type.
Drape: Involves laying features over a digital
terrain model to provide information on features that lie on the terrain.
The terrain model provides the shape of the terrain. Draped features may
then include a satellite image of the terrain
to show land use, and vector data to show
features such as roads.
DTM: See Digital Terrain
Ecological fallacy: The mistake of assuming that where
relationships are found among aggregate data, these relationships will
also be found among individuals or households.
Edge-matching: See rubber-sheeting.
Error: In the context of GIS this
means the difference between the real world and its digital representation.
Error propagation: As layers of data are integrated through
overlays the error present on the output layer
will become the cumulative total of the error present
on all the input layers.
Exploratory analysis: Statistical or visualisation techniques
that attempt to produce a good summary of the data or the patterns with
Fly-through: Often used to view digital
terrain models. In a fly-through a user is given the functionality
to allow him or her to move through the terrain in what appears to be
three dimensions, thus giving the illusion of flying. It is an effective
way of exploring a virtual landscape from different directions.
Gazetteer: Often used to standardise place names or to
locate place names within a hierarchy. These are often stored in a Relational
Database Management System.
GDA: See Geographical
Geary's coefficient: A statistical technique that measures
the degree of spatial autocorrelation
present in the data. It is a form of global
Geary's Gi: This is a local analysis form of Geary's
coefficient that produces a measure of spatial
autocorrelation for each location in the dataset.
Geographical Data Analysis (GDA): A way of analysing
data that explicitly incorporates information about location
as well about attribute. This term may be used almost interchangeably
with spatial analysis.
Geographical Information Science: Methods of exploring
and analysing spatially referenced data that take account of the benefits
and limitations of such data.
Geographical Information System: A computer system that
combines database management system
functionality with information about location.
In this way it is able to capture, manage, integrate, manipulate, analyse
and display data that is spatially referenced to the earth's surface.
Geographically weighted regression (GWR): A form of regression
modelling that explicitly incorporates the role of location. This is a
form of local analysis.
Geo-referencing: The process of proving a coordinate
system to a layer of data. This often involves converting
to a real-world coordinate system such as the British National Grid.
GIF: Graphics Interchange Format. A bitmap graphics format from CompuServe which stores screen images economically and aims to maintain their correct colours even when transferred between different computers. GIF files are limited to 256 colours and like TIFFs, they use a lossless compression format but without requiring as much storage space..
GIS: See Geographical
GIS data: Data stored in a GIS are
represented in two ways: attribute data says
what the feature is, and spatial data says
where it is using points, lines,
polygons, or pixels.
Global analysis: Forms of statistical analysis that provide
an average measure of a relationship or relationships across the study
area. Traditional correlation and regression
techniques do this. They are flawed in that they do not allow for any
geographical variations in the pattern so local
analysis techniques are seen as more relevant in a GIS
Global Positioning Systems (GPS): A system based on satellites
that allows a user with a receiver to determine precise coordinates for
their location on the earth's surface. These are a primary
source of spatial data.
GPS: See Global Positioning
Graphic primitive: The basic representations of spatial
features used in GIS. These are usually points,
lines, polygons, or pixels.
GWR: See Geographically
Head-up digitising: The process by which vector
data are extracted from raster scans
using a cursor on-screen.
Idrisi: A raster based
GIS software package produced by Clark Labs, Clark
Interpolation: A method of reallocating attribute data
from one spatial representation to another. A simple example is to reallocate
data from sample points to polygons
using Thiessen polygons. Kriging
is a more complex example that allocates data from sample points to a
Isolines: A line joining points
of equal value. The most common example is the contour line on a map.
Isobars showing lines of equal pressure on weather maps are another example.
Java: A computer programming language often used to create
JPEG: (Joint Photographic Experts Group), A digital image file format designed for maximal image compression. JPEG uses "lossy" compression in such a way that, when the image is decompressed, the human eye won't find the loss too obvious. The amount of compression is variable and the extent to which an image may be compressed without too much degradation depends partly on the image and partly on its use.
Key: In the context of Relational
Database Management Systems this refers to a common field that can
be used to join two or more tables.
Key dates approach: A way of handling time in a GIS
where the situation at different times is represented by different layers.
Kriging: A form of statistical modelling that interpolates
data from a known set of sample points to a continuous
Latitude: The angle of a location
on the earth's surface from the equator expressed in degrees north or
south. The Arctic Circle, for example, is at approximately latitude 66°
Layer: The GIS data model represents the world by sub-dividing
features on the earth's surface according to a specific theme.
Each theme is then georeferenced. Examples
of layers for a study area might include: roads, railways, urban areas,
coal mines, etc. A layer usually consists of both spatial
and attribute data.
Line: A spatial feature that is given a precise location
that can be described by a series of coordinate
pairs. In theory a line has length but no width.
Local analysis: Forms of statistical analysis that allow
relationships to vary across a study area by providing summary statistics
for many locations. The results are usually best presented in map form.
Examples of this type of technique include Geary's
Gi and Geographically
Weighted Regression. The opposite approach is global
analysis where only a single summary statistic is provided for the
average relationship across the study area.
Location: The position of a feature on the earth's surface.
In GIS this is usually explicitly defined in terms
of precise coordinates.
Location-allocation models: Models that attempt to find
the optimum location for a feature based on information
about other features. An example might be to find the best location for
an industrial plant based on information about the transport network and
the locations of raw materials and markets.
Longitude: The angle of a location
on the earth's surface usually expressed in degrees east or west of the
Greenwich Meridian. New York, for example, is at approximately 74°
Map algebra: A form of overlay
used with raster data. In it the values
for pixels on the output layer
is calculated by performing a mathematical operation on the pixels from
the input layers. The calculation may be arithmetic (addition, subtraction,
multiplication, etc) or Boolean (and, or, not, etc).
MapInfo: A commonly used desktop GIS
software package produced by the MapInfo Corporation.
MAUP: See Modifiable
Areal Unit Problem.
Metadata: Data that describe a dataset to allow others
to find and evaluate it.
Modifiable Areal Unit Problem (MAUP): Where data are
published using totals for arbitrary areas such as administrative units,
the patterns that they show may be simply the effect of the administrative
units rather than genuine patterns among the underlying population.
Moran's coefficient: A form of statistical modelling
that measures the degree of spatial
autocorrelation present in the data.
MPEG: A video file format that can be used to publish animations.
Network: A topological GIS data
structure that uses a series of lines to describe, for example a transport
or river network.
Network analysis: Usually used to analyse flows along
a network. For example, to find the shortest path
between two locations on a road network perhaps taking into account the
different speeds and different fuel costs on different types of roads.
Node: The start or end point of
a line segment. As such a node is often the point
at which lines intersect.
Non-spatial data: See attribute
Object-orientated approach: A way of modelling the world
that allocates entities to hierarchical classes.
Overlay: A formal geometric intersection between two
or more layers of spatially referenced data. A layer
produced by an overlay will contain both the spatial
data and the attribute data from the
Pixels: The small units that sub-divide space to make
up a raster surface.
They are usually small grid squares.
Points: Spatial features that are given a precise location
that can be described by a single coordinate
pair. In theory a point has neither length nor width.
Polygons: Spatial features that are areas
or zones enclosed by precisely defined boundaries.
The boundaries of a polygon are formed from one or more lines.
Polyline: A term for a line used by some GIS
Precision: The number of decimal places to which a value
is given. This usually far exceeds its accuracy.
For example, a GIS might give the coordinate of a point
location for building to ten decimal places providing
a value that is precise to fractions of a centimetre. In reality this
value may only be accurate to the nearest ten meters.
Primary source: In GIS terms this
usually means a digital data source that is derived directly from the
real world such as through Global
Positioning Systems or remote sensing.
Projection system: A method by which features on a curved
earth are translated to be represented on a flat map sheet. This involves
converting from longitude and latitude
to x and y coordinates.
Proximity measure: Usually an n by n
matrix that gives a measure of the influence each location
i has on each other location j. This is often expressed
as a weighting Wij.
Puck: A hand held device used with a digitising
table or tablet. It is used to point
to an exact location in order to capture its coordinate.
Quadrat analysis: Analysis where the study area is sub-divided
into regular grid squares and the number of occurrences of a phenomenon
in each square is counted. The resulting pattern can then be mapped. Quadrat
analysis is not a particularly satisfactory technique as the results are
too reliant on the size and position of the grid squares. Better techniques
such as kernel estimations are described in the literature.
Quadtree: A way of encoding raster
data that attempts to reduce storage requirements by avoiding sub-dividing
homogeneous areas rather than storing values for every pixel.
Quality: In the context of GIS data,
quality usually refers to how fit the data are for a particular purpose.
Querying: The process by which data are retrieved from
a database in order to gain information from it.
Raster data model: A way of representing the earth's
surface by sub-dividing it into small pixels, usually
square cells. Each pixel has values attached to it providing attribute
data about the pixel.
Raster-to-vector conversion: The process by which vector
features (points, lines and
polygons) are automatically extracted from raster
data. This usually requires a large amount of user input and is often
RDBMS: See Relational
Database Management Systems.
Reference points: A small number of points
used to georeference a layer.
Often the four corners of the layer are used. Once the layer has been
digitised we know the coordinates of the reference
points in inches from the bottom left hand corner of the digitising
table or tablet. We also know their
locations in real-world units from the map. This
allows us to convert the entire layer's coordinates from digitiser inches
to real-world coordinates.
Regression: A form of statistical modelling that attempts
to evaluate the relationship between one variable (termed the dependent
variable) and one or more other variables (termed the independent variables).
It is a form of global analysis as it only
produces a single equation for the relationship thus not allowing any
variation across the study area. Geographically
Weighted Regression is a local analysis
form of regression.
Relational Database Management Systems: Software systems that
store data in such a way that tables can be joined together by linking
on a common item of data, termed a key.
Relational join: The way by which two or more tables
from a Relational Database
Management System can be joined together based on one or more common
items or keys.
Remote sensing: The process by which satellite
images are created by scanning the earth's surface using sensors on
RMS Error: See Root Mean Square
Root Mean Square Error (RMS): A measure of the average error
across a map. It is used in digitising to give
an approximate measure of the difference between the real-world coordinates
and the registration points on the digital layer.
Rubber-sheeting: The process by which a layer
is distorted to allow it to be seamlessly joined to an adjacent layer.
Often this has to be done when layers created from adjacent map sheets
are joined together. It is a process that inevitably introduces some error.
Run-length encoding: A way of encoding raster
data that reduces storage requirements by creating linear groups of
identical pixels rather than storing the values
of each pixel individually.
Satellite images: Raster models
of the earth's surface produced from sensors on satellites.
Scanning: The process by which raster
data is captured from paper maps.
Segments: See lines.
Sliver polygons: Small polygons
formed as a result of overlaying two or more layers
of vector data. These are formed due to
small differences in the way that identical lines
have been digitised.
Space: In a GIS context this means position
on the earth's surface. Its meaning is very similar to location.
Space-time composite: A way of handling time in GIS
that preserves topology by sub-dividing space
into a small set of areas that can then be re-aggregated into the arrangement
that existed at different dates.
Spans: A raster-based
GIS software package produced by PCI-Geomatics
Spatial analysis: A way of analysing data that explicitly
incorporates information about location as well
about attribute. This term may be used almost
interchangeably with geographical
Spatial autocorrelation: The degree to which a set of
features tend to be clustered together (positive spatial autocorrelation)
or be evenly dispersed (negative spatial autocorrelation) over the earth's
surface. This is often measured using either Geary's
coefficient or Moran's coefficient.
When data are spatially autocorrelated the assumption that they are independently
random is invalid, so many statistical techniques are invalidated.
Spatial data: Data that define a location.
These are in the form of graphic primitives
that are usually either points, lines,
polygons or pixels.
Spatial querying: A query that extracts
features from a layer based on their location,
for example, clicking on a point and listing its
attribute data is a spatial query.
SQL: See Structured
Structured Query Language (SQL): A language used by many Relational
Database Management Systems to manipulate their data.
Surfaces: A surface is a way of modelling space
that attempts to treat it as continuous rather than sub-dividing it into
discrete features such as polygons. Surfaces are
usually modelled either as raster data
or digital terrain models.
Synchronic analysis: A form of analysis drawn from systems
theory in which change over time is examined by comparing the situation
at two points in time when the system is assumed to be in equilibrium.
Temporal data: Data that explicitly refer to time.
Tessellation: A sub-division of space
into discrete elements. Raster surfaces
sub-divide space into regular tessellations such as pixels.
Polygons are examples of irregular tessellations.
Theme: See layer.
Thiessen polygons: A method of allocating space
to the nearest point. The input layer
will contain a set of points. The output layer, containing the Thiessen
polygons, will contain polygons whose boundaries
are lines of equal distance between two points.
TIN: See Triangular Irregular
Topology: The description of how spatial features are connected
to each other.
Travelling Salesman Problem: A form of network
analysis that attempts to find the shortest or cheapest route between
a number of locations on a network.
Triangular Irregular Network: A data structure that produces
a continuous surface from point
data. Often used to create a digital terrain
Uncertainty: A measure of the amount of doubt or distrust with
which the data should be used.
Vector data model: Divides space into discrete
features, usually points, lines
Vector-to-raster conversion: The process by which vector
data are converted to rasters. This
is usually automated.
Voronoi diagrams: See Thiessen polygons.
Web-based mapping: Maps created for use on the Internet so they
often have some interactive functionality. Web-based mapping is not well
developed with vector file formats.
Whole-map analysis: See
Zones: See polygons.