Address Matching
A geocoding process which matches
the street address of property to its location. This usually
involves the matching of two database files, one containing the
addresses of interest, the other a list of addresses and their
co-ordinates. Address matching is central to many applications in
direct marketing.
AM/FM
Automated Mapping/Facilities
Management. This is a specific application of GIS to the management
and production of maps of plant such as cables, pipes, valves etc.
It is currently the most widely used application of GIS, and
particularly relevant to local authorities and utilities.
ASCII
American Standard Code for
Information Interchange. A standard set of codes which represent
alphanumeric characters stored as a single byte value. For example,
using the ASCII code, a byte containing the value 69 would represent
the letter E. Because of its simple nature, ASCII text is one of the
best ways of transferring information between different programs and
platforms. Attribute An item of text, a numeric value or an image
that is a characteristic of a particular spatial entity.
Buffer
A zone of user-specified distance
around a point, line or area. The generation of buffers to establish
the proximity of features is one of the most common forms of GIS
analysis. For example, it may be used to find all areas of industry
less than 5km from a reservoir.
Cadastre
A data set containing information
related to land ownership and rights. This usually takes the form of
maps and descriptions of uniquely identifiable land parcels. For
each parcel, legal information such as ownership, easements and
mortgages are recorded. Cell The basic element within a grid or
raster data set.
Centroid
The centre point of a polygon,
often used to attach attribute information to an area such as a
census ward. The centroid may be mathematically derived (such as the
centre of gravity) or may be user defined. It must always be placed
inside the polygon.
COGO
CO-ordinate GeOmetry. Algorithms
for handling basic two and three dimensional vector entities built
into all surveying, mapping and GIS software.
Co-ordinate
Numbers representing the position
of a point relative to an origin. Cartesian co- ordinates express
the location in two or three dimensions as the perpendicular
distances from two or three orthogonal axes.
Data Model
A generalised, user-defined view of
data representing the real world.
DEM
Digital Elevation Model (or Terrain
Model). A data model used to represent a topographic surface, often
based on a grid with a height value for each cell, or on a set of
irregular triangles (see TIN).
Digitising
Conversion or encoding of existing
maps from an analogue form (paper) into digital information, usually
in the form of Cartesian co-ordinates. This may be via a digitising
table or tablet with a hand-held cursor, or via a scanner.
DXF
Digital eXchange Format A data
format defined by Autodesk originally for the transfer of data
between CAD systems. Due to its simplicity, it is now widely used in
the transfer of data between GIS, despite a number of
limitations.
Gazetteer
A list of spatial entities held in
computer form, such as properties or streets, which allows for rapid
search and query. The gazetteer often forms the core of larger
GIS-based applications such as LIS.
Geocode
The element in a database used to
identify the location of a particular record, for example a
postcode. The process of geocoding is similar to that of address
matching, in that a data file is compared against a file of geocode
and their associated co-ordinates.
Geodetic Datum
A set of parameters defining
co-ordinate systems for all or parts of the earth. These datums have
been refined and revised over time. NAD 27 is the North American
datum for 1927, for example. ED50 is the European datum for 1950,
and WGS is the World Geodetic System for 1984. Varying datums are
used to produce better local ëfití of a spheroid (a ësquashedí
sphere) to the actual shape of the earth - the geoid.
GPS
Global Positioning Systems. A
position-finding system which uses radio receivers to pick up
signals from four or more special satellites (there are 24 in orbit)
and compute WGS co- ordinates for the receiver. Accuracy depends on
the sophistication of processing and the time available for
reception. Real-time navigation using GPS on aircraft and ships can
be to better than 100m. Processed data from several hoursí
observation can provide relative positions accurate to a few
centimetres.
GUI
Graphical User Interface.A method
of interaction with a computer which uses pictorial buttons (icons)
and command lists controlled by a mouse. It is generally regarded as
simpler and easier to learn than command line interfaces, where
commands have to be typed. Examples include MS WINDOWS for PCs, Open
Look or MOTIF for workstations and System 7 for Macintosh.
Grid Data
A data structure composed of square
cells of equal size arranged in columns and rows.
LIS
Land Information System. A subset
of the geographic information industry that is dedicated to the
management, analysis and presentation of information relating to
land, including ownership and legal rights. Often an automated
development of the Cadastre.
Latitude-Longitude
A spatial reference system for the
Earth's surface. Latitude is an angular measurement N or S of the
equator, longitude is an angular measurement E or W of the meridian
at Greenwich, UK.
Macro
A series of program commands or
instructions which are stored in a file and can be recalled when
necessary. Macros are commonly used to customise high-end GIS
toolkits for individual applications.
Map Projection
A mathematical model used to
convert three dimensional reality into two dimensions for
representation on a map, or within a two dimensional GIS database.
All map projections have particular strengths, some preserve shape,
other preserve distance, area and direction. All projections have
limitations, however, of which you should be aware.
Map Scale
The measure of reduction between
the representation and the reality, be it a map or a spatial
database. Scale is usually represented as a representative fraction
of distance e.g. 1:50,000, one unit of distance on the map
representing fifty thousand units in reality. The nominal scale of a
spatial data set has considerable influence over the possible
application of the data set, and you should always be aware of any
such implications. For example, it would not be sensible to compare
the shape of a road represented in a 1:625 000 scale data set with
one of 1:1250. Theoretically, a dataset does not have a scale
(unlike a map) but the terms Scale is usually used as a metaphor for
resolution and content.
Multispectral
Remote sensing in two or more
spectral bands.
Network
A model representing the
interconnected elements through which some form of resource can be
transmitted or will flow. In GIS this is represented as a series of
nodes connected by arcs, each or which has attributes representing
flow characteristics e.g. a road or pipeline network.
Node
A basic spatial entity within the
vector data model which represents the beginning or end of a
segment. Also, a node may be formed when a number of segments join.
For example a node might be represented in a road network as a
highway intersection.
Operating System
A series of computer programs which
control the operation of the computer itself. Application programs
such as GIS software run under an operating system. Examples of
operating systems include UNIX, VMS, DOS and OS/2.
Peripheral
A hardware component which is
connected to a computer to perform specialist functions. Common GIS
peripherals include plotters, digitising tables, and printers. When
selecting GIS software it is important to ensure that it is
compatible with any existing peripherals you use.
Pixel
A picture element of a raster image
as displayed on a screen or raster plot.
Point
A spatial entity that represents
the simplest geographical element. Represented in the vector data
model as a single x,y co-ordinate, and in the raster as a single
cell. The point may have associated attributes which describe the
element it is representing; the telephone number of a public call
box, for example.
Polygon
A representation of an enclosed
region defined by an arc or a series of arcs that make up its
boundary. Polygons may have attributes describing the region they
represent, such as the population of a census ward.
Quadtree
A data structure that subdivides
any given space into four quadrants and continues to subdivide each
quadrant in a similar way until they are uniform or the basic
resolution of the data is reached. It is mostly used to compress
raster data.
Raster
A data structure composed of a grid
of cells. Groups of cells represent geographical features; the value
in the cell represents the attribute of the feature.
Relational Database
A database which structures data in
the form of tables. Each table contains information relevant to a
particular feature, and is linked to other tables by a common value.
For example, two attribute tables could be linked to a spatial data
table via a geocode, such as the postcode.
Remote Sensing
The science of acquiring
information about the earth using instruments which are remote to
the earthís surface, usually from aircraft or satellites.
Instruments may use visible light, infrared or radar to obtain data.
Remote sensing offers the ability to observe and collect data for
large areas relatively quickly, and is an important source of data
for GIS.
Resolution
The resolution of a digital dataset
expresses the size of the smallest object which can be depicted. The
term is most commonly associated with the raster data model where
the resolution of a raster or grid is equal to the size of the cell
in the real world. For example, the resolution of a remotely-sensed
image may be 10m (each cell representing 10mx10m on the ground).
Increased resolution leads to larger storage requirements, increased
processing and higher costs for a given area.
Rubber Sheeting
A process which adjusts the
relative positions of features within a data set in a non-linear, or
non-uniform way. It is used to transform the co-ordinates of maps
with different scales, orientation or co-ordinate systems.
Run-length Encoding
A data compression technique which
encodes a digital data stream in terms of the number of successive
digital data elements of the same value, rather than repeating every
data value.
Scanning
A data capture technique which
digitises information from paper or film hard copy into digital
raster data. The process is rapid, but the resulting raster data set
only has colour, grey scale or black and white attributes associated
with it, and may not have the intelligence necessary for GIS
analysis. In effect, the result of scanning is a raster image of the
original source material. Segment One of the basic spatial entities,
and a basis for spatial models. Formed from a set of ordered
co-ordinates (vertices) that represents the shape of a geographic
object. An arc begins and ends in a node.
Spatial Analysis
Spatial analysis is the process of
applying analytical techniques to geographically-referenced data
sets to extract or generate new geographical information. Spatial
analysis may be used to model complex geographical interactions, and
is useful for investigating site suitability and predicting future
events. Although the overall analytical technique may be complex, it
is usually a combination of simple techniques applied in the
appropriate order.
SQL
Structured Query Language. A
language developed by IBM in the 1970s for defining and manipulating
relational databases. It has since become the industry standard, and
is often used to enable GIS toolkits to access the data held in
existing corporate databases.
Thematic Map
A map which communicates a single
theme or subject. For example, a population density map and
political boundary map are both thematic maps. This contrasts with a
topographical map which is a general purpose map containing
landscape features such as rivers, roads, landmarks and
elevation.
TIN
Triangulated Irregular Network. A
method of creating a surface from point data in the vector data
model. The TIN is created from an arbitrary distribution of points
joined to form triangles. Each point has an x and y co-ordinate and
one or more attributes (e.g. height). Attribute values for a point
anywhere in the model can then be interpolated .
Topology
The relationships in spatial terms
between connected or adjacent geographical objects. Topology is used
to apply intelligence to data held in the vector data model. For
example, topological information stored for an arc might include the
polygon to its left and right, and the nodes to which it is
connected.
Vector Data
A data model based on the
representation of geographical object by Cartesian co- ordinates,
commonly used to represent linear features. Each feature is
represented by a series of co- ordinates which define its shape, and
which can have linked information. More sophisticated vector data
models include topology.
|
