GIS has
generated
its own jargon-rich language, which I'm afraid we all fall into at
times.
So to aid you, below is a comprehensive guide to the jargon of
GIS.
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.
CAD
Computer Aided Design. Software programs for the
design,
drafting and presentation of graphics. Originally designed for
manufacturing
drawing, now also widely used for mapping.
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.
