Spatial Information Clearinghouse  Glossary
Absorption
 the process by which radiant energy is absorbed and converted
into other forms of energy
Aliasing
 Unwanted visual effects caused by insufficient sampling resolution
or inadequate filtering to completely define an object; most commonly
seen as a jagged or stepped edge along object boundaries or along
lines.
Base Station
 carefully surveyed receiver that collects satellite data then
calculates error vectors for correcting field data
Cartography
 The organisation and communication of geographically related information
in either graphic or digital form. It can include all stages from
data acquisition to presentation and use.
Coordinate
System  A recognised
reference system for the unique location of a point in space. The
Cartesian coordinate system and the system of latitude and longitude
of the earth are examples of coordinate systems based upon Euclidean
geometry. A coordinate system is usually defined by a map
projection, a spheroid of reference, a datum, one or more standard
parallels, a central meridian, and possible shifts in the X and
Y directions to locate X,Y positions of point, line, and area features.
A more complete
explanation is available in the Standards
Section.
Datum
 Any point, line or surface used as a reference for a measurement
of another quantity. A model of the earth used for Geodetic
calculations. A more complete
explanation is available in the Standards
Section.
Differential
Correction  differential correction is the method used, via
base station, to calculate error vectors that coincide with field
data to correct the field data error
Digital
Image Processing (DIP)– the use of the computer on raster image
files, to carry out such operations as image enhancement, image
compression, image analysis, mapping, georeferencing, etc.
Digitizing
 A method of data capture that involves the conversion of data
in hard copy or raster form, such as maps and aerial photographs,
into a digital vector format. This is usually done by a human operator
using on a digitizing tablet or sitting at a computer screen with
a mouse, drawing over top of a raster image. Methods of automated
digitizing and semiautomated digitizing also exist.
Electromagnetic
Spectrum  the ordered array of known electromagnetic energies,
extending from the shortest cosmic ray, through gamma rays, xrays,
ultraviolet radiation, and including microwave and all other wavelengths
of radio energy.
Ellipsoid
 Athreedimensional ellipse which is used to represent the
shape of the surface of the earth. A
more complete
explanation is available in the Standards
Section.
Enhanced Thematic
Mapper Plus (ETM+)  the primary sensor system aboard Landsat
7
Features
 A set of points, lines or polygons
in a spatial database that represent a realworld entity. The terms
feature and object are often used synonymously.
Geodesy 
The science of measuring the shape and
size of the earth, together with the determination of the exact
position of particular points on its surface by taking the earth's
curvature into account.
Geographic
Information System  A
computer system for capturing, storing, checking, integrating, manipulating,
analyzing and displaying data related to positions on the earth's
surface. Typically, a Geographical Information System (or Spatial
Information System) is used for handling maps of one kind or another.
These might be represented as several different layers where each
layer holds data about a particular kind of feature. Each feature
is linked to a position on the graphical image of a map. Layers
of data are organised to be studied and to perform statistical analysis.
Geoid  An
imaginary shape for the earth defined by mean sea level and its
imagined continuation under the continents at the same level of
gravitational potential. A more complete
explanation is available in the Standards
Section.
Georeferencing
– the process of taking an image and assigning it geographic coordinates
Global
Positioning System  A
satellitebased navigational system allowing the determination of
any point on the earth's surfacewith a high degree of accuracy given
a suitable GPS receiver. The network of satellites is owned by the
US Department of Defense. Error in the accuracy of GPS derived positions
can be introduced through the nature of local conditions. These
errors can be greatly reduced using a technique known as differential
GPS.
Metadata
 Data about data and usage aspects of it. This information will
often include some of the following: What it is about, Where it
is to be found, Who one needs to ask to get it, How much it costs,
Who can access it, In what format it is available, What is the quality
of the data for a specified purpose, What spatial location does
it cover and over what time period, When and where the data were
collected and by whom and what purposes the data have been used
for, by whom and what related data sets are available, etc. A more
complete explanation
is available in the Standards
Section.
Network
analysis  A data processing
method using topologically linked data such as street maps or river
networks with the purpose of determining routes between geographic
locations, and other analyses requiring the consideration of path
and direction. Such analyses might include finding the most efficient
travel route, generating directions, or defining service coverage
based on travel time.
Orthophotography
 A modified copy of a perspective photograph of the earth's surface
with distortions due to tilt and relief removed.
Photogrammetry
 the art or science of obtaining reliable measurements by means
of photography.
Pixel 
A contraction of the words picture element.
Pixel refers to the smallest unit of information available in an
image or raster map that can be independently assigned attributes
such as colour and intensity.
Position Dilution
of Precision (PDOP)  regulates amount of interference the rover
unit accepts and still records data; the lower, the better.
Projection
 A method of representing the earth's threedimensional surface
as a flat twodimensional surface. This normally involves a mathematical
model that transforms the locations of features on the earth's surface
to locations on a twodimensional surface. Because the earth is
threedimensional, some method must be used to depict the map in
two dimensions. Therefore, such representations distort some parameter
of the earth's surface, be it distance, area, shape, or direction.
There are a variety of map projections, but all are generally of
three basic types; these are the Azimuthal, conical and cylindrical
projections. For example, the Transverse Mercator Projection is
a variant of the cylindrical projection. A more complete
explanation is available in the Standards
Section.
Pseudo
Random Code  means by which time error is corrected between
the receiver and the satellite via a code that the receiver matches
with the satellite's
Radiometric
Resolution  Radiometric
resolution, or radiometric sensitivity refers to the number of digital
levels used to express the data collected by the sensor. In general,
the greater the number of levels, the greater the detail of information.
The number of levels is normally expressed as the number of binary
digits needed to store the value of the maximum level, for example
a radiometric resolution of 1 bit would be 2 levels, 2 bit would
be 4 levels and 8 bit would be 256 levels. The number of levels
is often referred to as the Digital Number, or DN value.
Raster Data
 An abstraction of the real world where spatial data is expressed
as a matrix of cells or pixels, with spatial position implicit in
the ordering of the pixels. With the raster data model, spatial
data is not continuous but divided into discrete units. This makes
raster data particularly suitable for certain types of spatial operations,
for example overlays or area calculations. Unlike vector data however,
there are no implicit topological relationships.
*Reference
Ellipsoid  A geometric model of the earth, required
for accurate range and bearing calculations over long distances.
Ellipsoidal models define an ellipsoid with an equatorial radius
and a polar radius. The best of these models can represent the shape
of the earth over the smoothed, averaged seasurface to within about
onehundred meters. A more complete
explanation is available in the Standards
Section.
Reflection
 Electromagnetic Radiation neither absorbed nor transmitted is
reflected. Reflection may be diffuse, when the incident radiation
is scattered upon being reflected from the surface, or specular
when all or most angles of reflection equal the angle of incidence
Refraction
 the bending of Electromagnetic radiation rays when they pass from
one medium to another having a different index of refraction or
dielectric coefficient.
Remote
Sensing  the measurement or acquisition of information of some
property of an object or phenomenon, by a recording device not in
physical or intimate contact with the object or phenomenon under
study
Resolution
 A measure of the ability to detect quantities. High resolution
implies a high degree of discrimination but has no implication as
to accuracy. Resolution is a term that is used often within remote
sensing.
Rover Unit
 also termed: field unit or data logger  The mechanism used
in the field that receives GPS satellite signals from outer space
Scale  The
ratio of the distance measured on a map to that measured on the
ground between the same two points. In Britain, most map scales
are now metric and are shown, for example, as 1:50,000, which represents
a scale of 1cm = 50,000 cm (or 500 metres). Often, the difference
between large and small map scales is confused. The larger the ratio,
the smaller the map scale. Therefore, a map of the world, would
have a very small scale, whereas a map of a town centre, will have
a large scale. A more complete
explanation is available in the Standards
Section.
Scanning
 A method of data capture whereby a hard copy image, map, or even
text is converted into digital raster form by using a specialized
high quality digital camera called a scanner.
Spatial Data
 Any information about the location and shape of, and relationships
among, geographic features. This includes remotely sensed data as
well as map data.
Spatial
Resolution  Spatial resolution refers to the area on the
ground that an imaging system, such as a satellite sensor, can distinguish.
There are many measures of spatial resolution, the most common include
the Instantaneous Field of View (IFOV), and the Effective Instantaneous
Field of View (EIFOV).
Spectral
Resolution  The term spectral resolution refers to the
width of spectral bands that a satellite imaging system can detect.
Often satellite imaging systems are multispectral meaning that
they can detect in several discrete bands, it is the width of these
bands that spectral resolution refers too. The narrower the bands,
the greater the spectral resolution.
Spheroid 
A solid that resembles a sphere in geometry. One of the terms used
to describe the shape of the earth.
Stereoscope
 a binocular optical instrument for assisting the observer to view
two properly oriented photographs or diagrams to obtain the mental
impression of a threedimensional model
Stereoscopic
Vision  binocular vision which enables the observer to view
an object simultaneously from two different to obtain the mental
impression of a threedimensional model
Tabular
or Attribute Data  A trait, quality or property describing
a geographical feature. A fact describing an entity in a relational
data model, equivalent to the column in a relational table.
*Temporal
Resolution  The frequency of obtaining imagery of a particular
area that a sensor/platform system maintains.
Thematic Map
 A map depicting selected kinds of information relating to
one or more specific themes. Examples are soil type, land classification,
population density and rainfall maps.
Topographic
Map  A map whose principal purpose is to portray the features
of the earth's surface. These features might include the cultural
landscape, but normally refer to the terrain and its relief.
Topology 
The relative location of geographic phenomena independent of their
exact position. In digital data, topological relationships such
as connectivity, adjacency and relative position are usually expressed
as relationships between nodes, links and polygons. For example,
the topology of a line includes its from and tonodes, and its
left and right polygons. Topology is useful in GIS because
many spatial modelling operations don not require coordinates, only
topological information. For example, to find an optimal path between
two points requires a list of the lines or arcs that connect to
each other and the cost to traverse each line in each direction.
Coordinates are only needed for drawing the path after it is calculated.
Transmission
 the passage of electromagnetic energy through a material such
as air, glass, or water.
Triangulation
 refers to the calculation of receiver position by way of at least
four satellites
Vector Data
 An abstraction of the real world where positional data is
represented in the form of coordinates. In vector data, the basic
units of spatial information are points, lines and polygons. Each
of these units is composed simply as a series of one or more coordinate
points, for example, a line is a collection of related points, and
a polygon is a collection of related lines. Vector data may
or may not possess topological relationships.
Most
Definitions are From the AGI Online GIS Dictionary
*Definitions
From The Vitual Geography Department
Department of Geography, University of Colorado at Boulder
