Glossary
for Exploring GIS, by Nicholas Chrisman
Index to glossary entries (in alphabetic order)
A | B | C |
D | E | F |
G | H | I |
J | K | L |
M | N | O |
P | Q | R |
S | T | U |
V| W
The book has a running glossary with many terms
defined at the bottom of the page. This resource collects all
of those definitions and adds other definitions that appear in
the body of the text.
Page numbers refer to the printed version of the text.
These terms are © Copyright, 1997 John Wiley and Sons. Use
of these definitions should credit Exploring
Geographic Information Systems by Nicholas Chrisman.
A
- Absolute measurement p.16
- a level of measurement higher than
ratio where the unit of measurement is not
an arbitrary decision, so the numbers cannot be rescaled and
retain their meaning. Probability is an example of an absolute
scale.
- Accuracy p. 27
- closeness of a measurement to a value thought to be true;
repeatability can be estimated by repeated measurement, measured
by variance for continuous measures; accuracy of classification
for categories can be summarized by a misclassification matrix
when compared to a survey of greater accuracy.
- Areal interpolation p. 222
- transformation of an attribute of one set of choropleth
zones to another set of choropleth zones.
- Attribute p. 6
- the range of possible values of a characteristic; an attribute
value is a specific instance of the characteristic associated
with a geographic feature.
- axiom p. 24
- a proposition accepted as true without proof; an assumption
that is formally recognized. (component of data
model)
- AVHRR p. 229
- Advanced Very High Resolution Radiometer, a satellite sensing
system operated by the US National Oceanic and Atmospheric Administration
(NOAA) with a resolution of about 1.1 km, twice-daily repeat
cycle, broad scenes (2400 km), and four bands of spectral data.
B
Bézier curve p. 62
- a smooth curve that passes through specified points with
a given direction (tangent) at those points.
- bilinear interpolation p. 214
- interpolation method where the value is obtained by linear
interpolation on the two axes (row and column). Uses four neighbors
from a raster representation of the surface, and averages out
the inconsistency between the four possible triangles.
- bit map p. 46
- a spatial data structure that records a binary value (presence
or absence of some attribute value) for a set of cells in a compact
representation.
- blunders p. 73
- errors, usually due to human operators, whose magnitude cannot
be modelled by regular statistical models.
- Boolean algebra p. 109
- system of operations applied to sets (and logical propositions);
Boolean variables are zero or one, hence strongly connected to
modern computing; originated by George Boole in 1847.
- breadth-first p. 200
- algorithms to traverse a tree that explore the structure
by enumerating each level totally before moving to a finer level;
as opposed to enumerating each path from the root to the 'leaves'
(depth-first).
- buffer p. 142
- a zone constructed outwards from an isolated object to a
specific distance.
C
- CAD p. 31
- computer-aided design, software packages designed to automate
drafting of mechanical drawings.
- cadastral p. 264
- pertaining to the legal register of land parcels,
particularly for land taxation or property transfer; more loosely,
related to property parcels.
- cartographic scale p. 82
- formally, the ratio between distance measurements on a map
and the same distance on the ground (for example 1:24000). In
a digital environment, this term refers to the complex (and often
unwritten) rules for selection, generalization
and representation used for a particular map series. [Not to
be confused with a scale of measurement.]
- cartographic spaghetti p. 78
- a cartographic data structure that represents each feature
as a string of coordinate pairs, and imposes no rules for relationships
or logical consistency.
- case p. 24
- In statistics, an individual unit of observation. Selecting
the unit of analysis (discrete unit of control) establishes the
measurement framework for statistically based studies.
- categorical coverage p. 38
- an exhaustive partitioning of a two-dimensional region into
arbitrarily shaped zones that are defined by membership in a
particular category of a classification system.
- cell p. 44
- a regular unit of spatial sampling. Usually applied to area-based
measurement frameworks that treat a system of cells as an exhaustive
coverage, rather than a set of points. Often functionally identical
to pixel.
- CGIS p. 71
- Canada Geographical Information System, perhaps the earliest,
certainly one of the most ambitious, prototypes of a modern GIS.
(Tomlinson 1967) CGIS created a digital coverage of land suitability
maps for the areas of Canada with agricultural potential.
- chain p. 34
- a directed set of non-intersecting line segments with nodes
at each end and reference to left and right polygons.
- choropleth framework p. 52
- measurement framework whose spatial
units (derived from a categorical coverage
of named objects) serve as control for
attribute measurement (e.g. census tabulation).
- choropleth map p. 13
- a thematic mapping technique that displays a quantitative
attribute using ordinal classes applied
as uniform symbolism over a whole areal feature. Sometimes extended
to include any thematic map based on symbolism applied to areal
objects.
- cluster analysis p. 203
- a procedure that groups points in a multidimensional space
(attribute measurements) into clusters that minimize the distance
from cluster centers (or some other objective
function).
- Cohen's kappa p. 122
- a measure of agreement between two classifications. Defined
as (observed accuracy - change agreement) / (1- chance agreement)
where the chance agreement is estimated by the cross-product
of marginal frequencies (statistical independence model).
- compression p. 67
- a software procedure that encodes a data structure so that
its storage occupies less space (under certain conditions); may
preserve all the information (loss-less) or deliberately simplify.
- connected coverage frameworks p. 31
- a general term for measurement frameworks
that involve relationships between distinct spatial objects;
includes network and categorical
coverage frameworks.
- contour p. 30
- a line connecting points of equal elevation on a topographic
surface. An control p. 27
- a mechanism of restraint on the variation of a system to
permit measurement of one component of a phenomenon while other
components only vary within the limits of the control.
- control p. 27
- a mechanism of restraint on the variation of a system to
permit measurement of one component of a phenomenon while other
components vary only within the limits of the control
- control point p. 72
- a feature whose location can be established in an external
spatial reference system (ideally, the
Geodetic Reference System) and on the source material to be digitized.
- coordinates p. 18
- a structured set of measurements related to a specific spatial reference system; usually applied
to pairs of distance measurements (X,Y) on independent axes of
a planar reference system or to angular measurements such as
latitude-longitude pairs on a spherical reference model.
- counts p. 16
- numerical measurements that aggregates the number of some
objects within a collection unit (for example population). Counts
depend on the definition of the objects counted, so they cannot
be rescaled arbitrarily.
- course p. 161
- line on a surface where slopes converge from two directions.
(See Topology of Topography.)
- cubic convolution p. 214
- interpolation method where the value is interpolated by fitting
a third-order equation to the sixteen grid points surrounding
the desired location.
- custodian p. 265
- an organization that takes responsibility to generate a particular
kind of information for a defined geographic region and agrees
to make it available to others.
D
- dasymetric p. 224
- a method proposed by John K. Wright to estimate densities
using areal interpolation and quantitative
puzzle-solving; more generally, a type of thematic map whose
boundaries are conditioned by some other distribution.
database p. 24
- structured collection of data with software to provide access
in different ways; has a data model,
a data structure and an implementation (representation).
- database schema p. 245
- logical arrangement of tables, attributes, and integrity
rules to structure a database. Involves definitions of entities
and their relationships.
- data dictionary p. 246
- detailed definitions of the codes employed for identifying
objects and for attribute values.
- data model p. 23
- In the database literature
1) general description of sets of entities and the relationships
between these sets of entities (Ullman 1982)
2) collection of object types, collection of operators on those
object types and a collection of integrity constraints (Codd
1981).
[Integrity constaints are the axioms of
a data model.]
In a GIS, composed of a measurement framework
and a scheme for representation.
- data structure p. 57
- arrangement of data entities that permits the construction
of relationships through software operations; implements a data model.
- Delaunay triangulation p. 153
- a network that connects each point in a set of points to
its nearest neighbors; topological 'dual'
of the Voronoi network.
- DEM p. 41
- a framework for recording spot elevations in a regular rectangular
grid (matrix); an acronym originally created from Digital Elevation
Model at US Geological Survey. To avoid ambiguity, DEM will be
used exclusively for a grid framework, so it can be read matrix.
- derived measurement p. 8
- numerical measurements constructed from the relationship
of other measurements (for example, a density is a ratio of weight
by volume). [As opposed to extensive
measurements.]
- diagonal p. 121
- cells in a square matrix whose row and column indices are
the same. In a transition matrix, they represent no change; in
an error matrix they represent no error.
- Digital Line Graph (DLG) p. 82
- a data format developed by US Geological Survey National
Mapping Division that uses a topological vector model.
- digitizer p. 70
- a manually controlled machine that records a spatial measurement
usually on the surface of a tablet.
- DIME p. 86
- Dual Independent Map Encoding, a digital database of streets
and other census boundaries developed to conduct the 1970 US
Decennial Census; an early and prominent implementation of a
topological data structure.
- discount rate p. 240
- economic factor that deflates a future sum to make it comparable
with current expenses. Reflects expectations of interest rates
and inflation.
- dynamic segmentation p. 55
- a method for referencing attribute information along a network
that does not divide each segment of the network wherever any
attribute changes.
E
- easement p. 236
- a legal agreement that grants partial rights over a portion
of a property, such as a utility easement to install overhead
wires or underground pipes. Also applied to rights-of-way for
roads.
- efficiency p. 235
- compares the resources expended to attain a given product
- effectiveness p. 235
- compares the nature of the products and the process of creating
them
- ellipsoid p. 60
- three dimensional object formed by rotating an ellipse around
its minor axis; an oblate ellipsoid approximates the shape of
the Earth (geoid), computed by the best
fit to geodetic observations. See Table 3-2 p. 76
- equal interval p.93
- a classification procedure that divides the total range of
attribute values by the number of classes; breakpoints are spaced
at equal intervals, whether the class has any member or not.
- extensive measurement p. 8
- numerical measurements along an axis where addition provides
the basic rule; the base units of SI are extensive,
as are units of monetary value (dollars, pounds sterling, francs,
euros).
F
- feature p. 31
- cartographic feature: an instance of a defined class of objects
that cannot be divided into objects of the same type. 31
- field p. 29
- an abstract construct of a mathematical relationship viewed
as a spatial structure. In physics: a region of space subject
to a force (as a magnetic field). In GIS, often used as a synonym
for surface.
- flexible production p. 257
- a system of economic organization characterized by rapid
changes in production plans, "just in time" delivery,
and other techniques that break from the economy of scale, mass
production approach characterized by the assembly line.
- foreign key p. 97
- item in a relational table that contains a value identifying
rows in another table; represents a relationship between two
elements of a relational database.
- fuzzy set theory p. 16
- an extension to set theory that permits an object to have
a degree of membership (usually represented as a number between
zero and one). Fuzzy membership values do not have to follow
the rules of probability.
- fuzzy tolerance p. 114
- a distance within which intersections and points will be
treated as coincident. To be processed correctly, the fuzzy tolerance
cannot be handled immediately (otherwise a point might be moved
twice and beyond its original tolerance). A 'cluster' of points
must be grouped so that no point is moved more than the tolerance.
G
- generalization p. 83
- In cartography, conversion of a geographic representation
to one with less resolution and less information content; traditionally
associated with a change in scale.
- geodetic p. 76
- related to the science of earth measurement (geodesy).
- geodetic control p. 29
- a set of surveyed monuments used to define a spatial
reference system for a particular project.
- geoid p. 6
- three-dimensional shape of the Earth defined by the surface
where gravity has the value associated with Mean Sea Level. [The
geoid is lumpy, while an ellipsoid is
smooth, representing the average elevation of the geoid to some
level of approximation.]
- geographic information system (GIS) p. 5
- 1) "a system of hardware, software, data, people, organizations
and institutional arrangements for collecting, storing, analyzing
and disseminating information about areas of the earth."
(Dueker and Kjerne, 1989, p. 7-8) [Commentary
on this accepted definition]
- 2) the organized activity by which people measure aspects
of geographic phenomena and processes; represent these measurements,
usually in the form of a computer database, to emphasize spatial
themes, entities and relationships; operate upon these representations
to produce more measurements and to discover new relationships
by integrating disparate sources; and transform these representations
to conform to other frameworks of entities and relationships.
These activities reflect the larger context (institutions and
cultures) in which these people carry out their work. In turn,
the GIS may influence these structures. [This definition in more
graphic form.]
- geostatistics p. 181
- branch of statistical estimation concentrating on the incorporation
of spatial measures, particularly distance and neighborhood,
into models.
- GIRAS p. 155
- Geographic Information Retrieval and Analysis System, a project
conducted by US Geological Survey in the 1970s; produced vector
interpretations of land use/land cover stored in an early topological
data structure.
- GPS p. 77
- Global Positioning System, a constellation of communications
satellites that broadcast timing signals that can be converted
into a distance measurement, permitting trilateration (surveying
by knowing the sides of triangles).
- graphic elements p. 15
- the characteristics of a symbol system that can be manipulated
to encode information (also called 'visual variables', graphic
dimensions, etc.). For cartography, these include size, shape,
hue, saturation, brightness, orientation and pattern. (See Robinson
and others, 1995).
- ground truth p. 121
- a determination of geographic attributes judged to be of
higher accuracy; usually applied to a
'point' classified into a land use/land cover category.
H
- 'heads-up' digitizing p. 71
- a digitizing station that provides a graphical user interface
on the screen of a workstation (hence sometimes called on-screen
digitizing). The operator uses a pointing device (mouse or trackball)
to navigate on the scanned image of the original source, without
having to look down at a digitizing tablet.
- heuristic p. 201
- a procedure that attacks a problem in a way directed towards
the goal, but not guaranteed to attain it exactly.
I
- indirect measurement p. 50
- a procedure that assigns attributes to spatial objects through
some attribute of those spatial objects (such as a soil class
or a political subdivision), not through direct measurement.
- information p. 8
- data (observations, measurements, etc.) placed in context
of a system of meaning (a set of relationships and assumptions
about those relationships). Information, built into larger context,
constructs knowledge. [Loosely defined as a difference that makes
a difference.]
- integrated survey p. 137
- an approach to land evaluation that combines the opinions
of many disciplines in producing a common representation of the
processes that form a landscape.
- interval p. 12
- a level of measurement that assigns
numerical values to objects, but does not provide a 'zero' value
to serve as a reference. Differences between interval values
have meaning as ratio measures.
- invariance p. 10
- properties that remain unchanged despite transformations
of the numbers used to represent a measurement.
- isolated object frameworks p.
30
- a general term covering those measurement
frameworks that use a specific attribute value as a control
to obtain spatial measurements; includes spatial
object and isoline frameworks.
- isoline p. 30
- a line connecting points of equal elevation on a surface
(See contour)
- isoline framework p. 30
- a measurement framework that establishes
control by a systematic set of slices through an attribute to
obtain lines that represent the surface.
J
- join p. 97
- procedure that attaches values from a database table to another
table based on matching a foreign key
to its primary instance.
K
- kriging p. 212
- a geostatistical technique for interpolation
that uses information about the spatial autocorrelation in the
vicinity of each point to provide 'optimal' interpolation (in
the sense of greater use of the information provided by the spatial
arrangement).
L
- Landsat p. 29
- a series of Earth-observing satellites. The early ones (first
launched in 1972) had 80m resolution; the more recent ones include
the 30m TM sensor.
- land system p. 247
- a unit of an integrated survey
where common processes (involving soils, vegetation and other
factors) have created a combination of features that will support
a particular group of uses.
- layer p. 18
- a collection of related geographic data. Depending on context,
a layer may have some specific relationship to other layers.
For raster imagery, a layer can represent one sensor in a multispectral
array. In some cases, a layer may contain a coverage, hence provide
an attribute value at all locations. In some cases, a layer simply
collects all features with a common graphic symbolism.
- least-squares p. 73
- an estimation procedure that minimizes the sum of squared
deviations between observations and a numerical model for those
observations; used in ordinary regression analysis and many other
statistical procedures.
- level of measurement p. 9
- a grouping of measurement scales based on the invariance
of certain properties. Measurement scales
at a common level of measurement can be transformed into another
scale at the same level without reducing the information content.
- LLRW p. 126
- Low Level Radioactive Waste; Low-level waste is generated
(on the order of 160,000 m3 for the US per year) by nuclear power
plants, hospitals and various other industries; excludes waste
from weapons construction and the spent fuel from nuclear reactors
(high-level waste).
M
- mandates p. 265
- organizing principles of purpose that drive an organization;
in a bureaucracy, the laws, administrative rules and regulations
that define the purpose and content of actions.
- measurement framework p. 23
- a scheme that establishes rules for control
of other components of a phenomenon that permit the measurement
of one component. In a geographic information system, there are
three components: time, space, and attribute.
- metadata
- information describing a collection of data; includes source,
access issues, format, schema and data quality.
N
- national grid p. 74
- a spatial reference system usually
adopted by legal or administrative procedures for use through
a given country. Typically consists of a projection
or a system of projection zones plus a geodetic
datum and ellipsoid.
- NDVI p. 220
- difference between two bands (near infrared minus visible
red) divided by the sum of the two bands; high values indicate
active vegetation growth; often applied to NOAA AVHRR data.
- network framework p. 35
- a measurement framework based on a
set of distinct spatial objects (usually instances of linear
objects like roads or rivers) that connect to form a network.
- node p. 34
- a zero-dimensional object that is a topological junction
(or end point) and a geometric location. (Definitions for topological
objects simplified from (Morrison 1987)).
- nominal p. 10
- a level of measurement based assigning
objects into discrete categories.
- North American Datum p. 76
- an adjustment of geodetic measurements
that provides the horizontal reference for North America. The
1927 Datum held Mead's Ranch, Kansas as a fixed point, while
the 1983 Datum performed a simultaneous adjustment of all measurements.
1927 uses the Clarke's ellipsoid, while 1983 uses the 1980 Geodetic
Reference System. The plural of a geodetic datum is 'datums',
despite the Latin origins.
- NP-Complete p. 203
- class of algorithms conjectured to exhibit worst case performance
that cannot be written as a polynomial equation of the number
of objects processed.
O
- objective function p. 201
- the formula giving the goal in an optimization problem.
- ordinal p. 10
- a level of measurement that orders
objects or categories, but does not provide further information
regarding the distance between items.
P
- parcel p. 226
- contiguous unit of the earth's surface defined by a common
collection of legal rights.
- peak p. 161
- point of local divergence on a surface; all neighboring points
are lower. (Topology of topography)
- pixel p. 29
- smallest resolvable unit in an image; an area (usually rectangular)
forming a part of a systematic, uniform division of a study area.
Contraction of picture element.
- planar graph p. 37
- an arrangement of nodes and lines such that the lines intersect
only at nodes, thus remaining embedded in a plane (or a surface
topologically transformable onto a plane).
- point-in-polygon p. 116
- procedure to determine which points fall into which polygons;
can attach attributes to either the points or the polygons.
- polygon p. 34
- an area (bounded continuous two-dimensional object) consisting
of an interior area, one outer ring and zero
or more non- intersecting non-nested inner rings.
- polygon overlay p. 108
- a procedure that calculates the geometric relationships between
two geographic representations (usually applied to vector models);
used to merge attribute information .
- primitives p. 31
- basic components that are sufficient to build a larger system;
the primitives of two-dimensional geometry are points, lines,
and areas.
- projection p. 60
- coordinate transformation that converts latitude longitude
measurements into planar coordinates. Projections can be based
on a developable surface (such as a plane, cylinder, or cone)
or on a mathematical function.
- proportional symbols p. 13
- a thematic mapping technique that displays a quantitative
attribute by varying the size of a symbol. Typically, proportional
symbols use simple shapes such as circles, and are scaled so
that the area of the symbol is proportional to the attribute
value. Proportional symbols are located at a point, even when
they represent information collected for an area.
- prototype p. 16
- an approach to categorization that defines a category by
identifying a particular object as the typical example. Other
objects assigned to this category may not not share all characteristics
with the prototype object. The degree of resemblance can be treated
as a distance in a taxonomic space.
Q
- quadtree p. 67
- a spatial data structure that organizes a hierarchical structure
of square cells through iterative division into four daughter
cells (Samet 1990).
- quantile p. 93
- a classification procedure that assigns an equal number of
objects into each class. The interval of each class will vary
unless the distribution is completely uniform.
R
- raster p. 65
- a spatial data model based upon a regular tessellation of
a surface into pixels or grid cells.
- ratio p. 12
- a level of measurement that includes
both extensive and derived
measurements. The numerical values of the measurement have a
meaningful zero reference and a fixed unit of
measure.
- ray-casting p. 187
- computational technique used to simulate a visual scene with
optical effects, variations in light sources and other effects;
usually developed for arbitrary 3D objects, not just a single
surface.
- recursive p. 83
- a programming procedure that invokes itself to subdivide
a problem.
- registration p. 72
- the process of connecting a spatial representation to a broader
spatial reference system.
- relational database p. 97
- a data model based on set theory.
Each set has elements that can be uniquely defined by a primary
key. A table (relation) stores all records for a set. Each record
in a table has the same columns for attribute values. Relationships
between tables are constructed by storing the key to a record
in the other table (a foreign key).
- representation p. 57
- a symbolic mechanism to encode data. Using computers, involves
a data model, a data structure and its
implementation.
- representationalism p. 9
- a philosophy of measurement that defines measurement as the
connection of numbers with entities which are not numbers. The
numbers are not seen as inherent but as a representation of a
defined aspect of the entity. [This philosophy is opposed to
the classical theory that measurement is the numerical expression
of one quantity relative to another; quantities were seen as
inherent.]
- resampling p. 214
- a transformation usually applied to convert raster
data into another arrangement of raster cells. May require interpolation
to determine the attribute at a position not sampled by the original
grid.
- resolution p. 27
- least detectable difference in a measurement; in a geographic
context, resolution applies to all components (time, space and
attribute) according to the measurement framework.
Not identical to accuracy.
- retraced line p. 63
- a technique applied to simple vector data structures to embed
inner rings within outer rings. A line that is repeated (drawn
twice) should not appear graphically.
- ridge p. 161
- line along which a surface diverges in two different directions.
(Topology of topography)
- ring p. 35
- a sequence of non-intersecting chains that close.
S
- scalar field p. 158
- a surface whose value can be represented
by a single number.
- scale (measurement) p. 10
- a system used to encode the results of a measurement; typically
a number line, but generalized to include a list of categories.
[Distinct from cartographic scale.]
- setback p. 142
- a zone inside a polygon constructed by a fixed distance from
the edge of the polygon; typically used to restrict building
or activities too close to the edge of a property parcel.
- SI p. 9
- Système International d'Unités; the system
of weights and measures established by international agreement
in 1875. The International Bureau of Weights and Measures in
Sèvres, France oversees the measurement standards. SI
defines seven base units from which many others can be derived:
meter-length, kilogram-mass, second-time, kelvin-temperature,
ampere- electric current, mole-chemical quantity, candela- intensity
of light.
- sliver p. 114
- an artifact of polygon overlay; usually
created by overlay of two sources with different accuracy, different
sources or different interpretations.
- slope p. 158
- a multicomponent property associated with a surface at each
point; the vertical component (gradient) expresses the local
rate of change, the horizontal component (aspect) gives the direction.
- soil inclusions p. 52
- a category of soil expected to occur inside the units mapped
as another category.
- spatial autocorrelation p. 181
- degree of correlation between a surface value and the values
of its neighbors; propensity of spatial data to vary smoothly.
(Core element of geostatistics.)
- Spatial Data Transfer Standard (SDTS) p. 34
- a specification of generic data formats and metadata content
to promote interchange of geographic information. Adopted by
the US Government as Federal Information Processing Standard
(FIPS) 173.
- spatial object framework p. 30
- a measurement framework that identifies
a particular category, then maps the location of that object
(as a point, a line or an area).
- spatial reference system p. 18
- a mechanism to situate measurements on a geometric body,
such as the Earth; establishes a point of origin, orientation
of reference axes, and geometric meaning of measurements, as
well as units of measure. [basis of coordinates.]
- spline p. 62
- a smooth curve that models the behavior of a thin spring
(with a given modulus of elasticity) constrained to pass through
specified points.
- SQL p. 93
- Structured Query Language; a standard interface for access
to a relational database through queries
that select records matching logical expressions.
- stand p. 237
- a contiguous area of a forest considered to be homogeneous
in its ability to support the intended forest crop. Used to direct
forest practices such as planting, thinning, and harvest.
- surface p. 29
- a spatial distribution which associates a single value with
each position in a plane; (technically a field of a single-valued
function) usually associated with continuous attributes.
- SYMAP p. 211
- SYnagraphic MAPping package, developed under the direction
of Howard Fisher at Harvard Laboratory for Computer Graphics
beginning in 1966.
T
- temporal reference system p. 17
- an agreed measurement scheme for time; involves a time to
start counting (an origin) and a unit. International conventions
establish a calendar and synchronized clocks based on Greenwich
Mean Time.
- tensor p. 158
- a multi-component surface of higher degree than a vector.
The theory of relativity requires tensors to handle space-time
and electromagnetic fields.
- TIFF p. 67
- Tagged Image File Format: a family of image encoding formats
which can vary the resolution and the number of bits used to
represent each cell.
- TIN p. 49
- Triangulated Irregular Network: a system of terrain representation
that builds triangular facets to connect point heights. The points
and triangles are chosen to represent a surface within some limits.
- TM p. 230
- Thematic Mapper, a satellite sensing system with resolution
of 30 meters, 16 day repeat cycle, 185 km scene width and seven
bands of spectral data; launched by NASA.
- topology p. 34
- a branch of mathematics concerned with those properties of
geometry which are independent of a distance metric and are unchanged
by any continuous deformation. In cartography, topology refers
to combinatorial topology. The other branch, point-set or algebraic
topology, "emerged in the twentieth century as a subject
that unifies almost the whole of mathematics" (Boyer and
Merzbach 1991, p. 622) and is used as the basis for relational
databases.
- topology of topography p. 161
- qualitative (ordinal) relationships in the structure of a
surface. Topography refers to the surface of the earth.
- transverse p. 75
- a projection oriented at right
angles to the equator. A transverse cylindric projection uses
a meridian of longitude as its central meridian.
- travelling salesman problem p. 203
- given a graph connecting a set of nodes, devise a route that
visits each node in the graph exactly once and minimizes the
total cost accumulated.
- trend surface p. 210
- result of a statistical fit of the observed values using
a selected spatial equation (from a plane up to higher order
shapes).
U
- unit of measure p. 94
- the reference value used to establish a ratio-scaled measure;
provides the value of 1 for a ratio scale.
- USLE p. 137
- Universal Soil Loss Equation; predicts average soil loss
(in tons) for a period of time (a storm event or a year) as the
product of six factors: Rainfall intensity, Erodability of the
soil, Length of slope, Slope gradient, Crop, and Practices.
- UTM p. 75
- Universal Transverse Mercator; a spatial
reference system using a set of transverse
Mercator projections six degrees wide
that cover the Earth (except for polar regions covered by two
Polar Stereographic projections).
V
- variable length list p. 59
- a data structure that can store a flexible number of elements
in an ordered sequence. Frequently implemented with a count followed
by the elements of the list.
- vector p. 62
- a spatial data model based on geometric primitives (point,
line, and area), located by coordinate measurements in a spatial
reference system; from mathematical term for a direction, or
a directed line segment.
- vector field p. 158
- a multi-component surface whose values have a quantity and
a direction in space. Newtonian physics can be expressed as vectors.
This mathematical term is the origin of the term 'vector'
applied to a geometric data structure, but the connection is
indirect.
- Voronoi network p. 153
- a set of lines that divides a plane into the area closest
to each of a set of points. The lines are perpendicular bisectors
of the lines connecting nearest points (Delaunay
triangulation).
W
- watershed p. 161
- area bounded by ridges that would converge (downhill) to
a single exit point. (see topology of topography)
- well-defined point p. 76
- a point-like (isolated) feature that can be distinguished
on the source and on the ground to sufficient accuracy;
in US National Map Accuracy Standards of 1947, implemented as
'plottable to .01 inch'.
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Version of 1 January 1997