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.