absolute location: A location in geographic
space given with respect to a known origin and standard measurement system,
such as a coordinate system.
accuracy: The validity of data measured
with respect to an independent source of higher reliability and precision.
attribute: A numerical entry that reflects
a measurement or value for a feature. Attributes can be labels, categories,
or numbers; they can be dates, standardized values, or field or other measurements.
An item for which data are collected and organized. A column in a table or
data file.
azimuthal: A map projection in which
the globe is projected directly on a flat surface. Only one "side"
of the globe can be shown at a time.
cartography: The science that deals
with the principles, construction, and use of maps.
compromise: A map projection that is
neither area preserving nor shape preserving. An example is the Robinson projection.
conformal: A type of map projection
that preserves the local shape of features on maps. On a conformal projection,
lines on the graticule meet at right angles, as they do on a globe.
conic: A type of map projection involving
projecting part of the earth onto a cone-shaped surface that is then cut and
unrolled to make it flat.
continuity: The geographic property
of features or measurements that gives measurements at all locations in space.
Topography and air pressure are examples.
coordinate pair: An easting and northing
in any coordinate system, absolute or relative. Together these two values,
usually termed (x, y) describe a location in two-dimensional
geographic space.
coordinate system: A system with all
the necessary components to locate a position in two- or three-dimensional
space: that is, an origin, a type of unit distance, and axes.
cylindrical: A type of map projection
involving projecting part of the earth onto a cylinder-shaped surface that
is then cut and unrolled to make it flat.
data: A set of measurements or other
values, such as text for at least one attribute and at least one record.
database: A collection of data organized
in a systematic way to provide access on demand.
datum: A base reference level for the
third dimension of elevation for the earth’s surface. A datum can depend on
the ellipsoid, the earth model, and the definition of sea level.
dimensionality: The property of geographic
features by which they are capable of being broken down into elements made
up of points, lines, and areas. This corresponds to features being zero-,
one-, and two-dimensional. A drill hole is a point, a stream is a line, and
a forest is an area, for example.
distortion: The space distortion of
a map projection, consisting of warping of direction, area, and scale across
the extent of the map.
easting: The distance of a point in
the units of the coordinate system east of the origin for that system.
edge matching: The GIS or digital map
equivalent of matching paper maps along their edges. Features that continue
over the edge must be "zipped" together and the edge dissolved.
To edge-match, maps must be on the same projection, datum, ellipsoid, and
scales and show features captured at the same equivalent scale.
equal area: A type of map projection
that preserves the area of features on maps. On an equal-area projection,
a small circle on the map would have the same area as on a globe with the
same representative fraction. See also equivalent.
equatorial radius: The distance from
the geometric center of the earth to the surface, usually averaged to a single
value for a sphere.
equirectangular: A map
projection that maps angles directly to eastings and northings. A cylindrical
projection, made secant by scaling the height-to-width ratio. The nonsecant
or equatorial version is called the Plate Carree. Credited to Marinus of Tyre,
about A.D. 100.
equivalent: A type of map projection
that preserves the area of features on maps. On an equal area projection,
a small circle on the map would have the same area as on a globe with the
same representative fraction. See also equal area.
field variable: A geographic value
that is continuous over space.
file: Data logically stored together
at one location on the storage mechanism of a computer.
flattening (of an ellipsoid): The ratio
of the length of half the short axis of the ellipse to half the long axis
of the ellipse, subtracted from 1. The earth’s flattening is about 1/300.
geocode: A location in geographic space
converted into computer-readable form. This usually means making a digital
record of the point’s coordinates.
geodesy: The science of measuring the
size and shape of the earth and its gravitational and magnetic fields.
geographic coordinates: The latitude
and longitude coordinate system.
geographic property: A characteristic
of a feature on earth, usually describable from a map of the feature, such
as location, area, shape, distribution, orientation, adjacency, and so on.
geography: (1) A field of study based
on understanding the phenomena capable of being described and analyzed with
a GIS. (2) The underlying geometry and properties of the earth’s features
as represented in a GIS.
geoid: A complex earth model used more
in geodesy than cartography or GIS that accounts for discrepancies over the
earth from the reference ellipsoid and other variations due to gravity, and
so on.
globe: A three-dimensional model of
the earth made by reducing the representative fraction to less than 1:1.
GPS (Global Positioning System): An
operational, U.S. Air Force–funded system of satellites in orbits that allow
their use by a receiver to decode time signals and convert the signals from
several satellites to a position on the earth’s surface.
graticule: The latitude and longitude
grid drawn on a map or globe. The angle at which the graticule meets is the
best first indicator of what projection has been used for the map.
GRS80 (Geodetic Reference System of 1980):
Adopted by the International Union of Geodesy and Geophysics in 1979 as
a standard set of measurements for the earth’s size and shape. The length
of the semimajor axis is 6,378,137 meters. Flattening is 1/298.257.
information: The part of a message
placed there by a sender and not known by the receiver.
interval: Data measured on a relative
scale but with numerical values based on an arbitrary origin. Examples are
elevations based on mean sea level, or coordinates.
latitude: The angle made between the
equator, the earth’s geometric center, and a point on or above the surface.
The south pole has latitude –90 degrees, the north +90 degrees.
level of measurement: The degree of
subjectivity associated with a measurement. Measurements can be nominal, ordinal,
interval, or ratio.
link: The part or structure of a database
that physically connects geographic information with attribute information
for the same features. Such a link is a defining component of a GIS.
location: A position on the earth’s
surface or in geographic space definable by coordinates or some other referencing
system, such as a street address or space indexing system.
longitude: The angle formed between
a position on or above the earth, the earth’s geometric center, and the meridian
passing through the center of the observing instrument in Greenwich, England,
as projected down onto the plane of the earth’s equator or viewed from above
the pole. Longitudes range from –180 (180 degrees West) to +180 (180 degrees
East).
map: A depiction of all or part of
the earth or other geographic phenomenon as a set of symbols and at a scale
whose representative fraction is less than 1:1. A digital map has had the
symbols geocoded and stored as a data structure within the map database.
map millimeters: A coordinate system
based on the dimensions of the map rather than those of the features represented
on the earth itself, in metric units.
map projection: A depiction of the
earth’s three-dimensional structure on a flat map.
mean sea level: A local datum based
on repeated measurements of sea level throughout all of its normal cycles,
such as tides and seasonal change. The basis for elevations on a map.
meridian: A line of constant longitude.
All meridians are of equal length on the globe.
metric system: A system of weights
and measures accepted as an international standard as the Systeme International
d’Unites (SI) in 1960. The metre (meter in the United States) is the unit
of length.
military grid: A coordinate system
based on the transverse Mercator projection, adopted by the U.S. Army in 1947
and used extensively for world mapping.
mosaicing: The GIS or digital map equivalent
of matching multiple paper maps along their edges. Features that continue
over the edge must be "zipped" together and the edge dissolved.
A new geographic extent for the map usually has to be cut or clipped out of
the mosaic. To permit mosaicing, maps must be on the same projection, datum,
ellipsoid, and scale, and show features captured at the same equivalent scale.
NAD27 (North American Datum of 1927): The
datum used in the early part of the national mapping of the United States.
The Clarke 1866 ellipsoid was used and locations and elevations were referenced
to a single point at Meade’s Ranch in Kansas.
nominal: A level of measurement at
which only subjective information is available about a feature. For a point,
for example, the name of the place.
northing: The distance of a point in
the units of the coordinate system north of the origin for that system.
oblate ellipsoid: A three-dimensional
shape traced out by rotating an ellipse about its shorter axis.
oblique: A map projection in which
the centerline of the map is not at right angles to the earth’s geographic
coordinates, following neither a single parallel nor a meridian.
ordinal: A level of measurement at
which only relative information is available about a feature, such as a ranking.
For a highway, for example, the line is coded to show a Jeep trail, a dirt
road, a paved road, a state highway, or an interstate highway, in ascending
rank.
origin: A location within a coordinate
system where the eastings and northings are exactly equal to zero.
parallel: A line of constant latitude.
Parallels get shorter toward the poles, becoming a point at the pole itself.
perfect sphere: A three-dimensional
figure traced out by all possible positions of an arc of a fixed radius about
a point. A good approximation of the shape of the earth.
polar radius: The distance between
the earth’s geometric center and either pole.
precision: The number of digits used
to record a measurement or which a measuring device is capable of providing.
prime meridian: The line traced out
by longitude zero and passing through Greenwich, England. The prime meridian
forms the origin for the longitude part of the geographic coordinates and
divides the eastern and western hemispheres.
ratio: A level of measurement at which
numerical information is available about a feature, based on an absolute origin.
For land parcels, for example, the assessed value in dollars would be an example,
the value zero having real meaning.
record: A set of values for all attributes
in a database. Equivalent to a row of a data table.
relative location: A position described
solely with reference to another location.
representative fraction: The ratio
of a distance as represented on a map to the equivalent distance measured
on the ground. Typical representative fractions are 1:1 million, 1:100,000,
and 1:50,000.
scale: The geographic property of being
reduced by the representative fraction. Scale is usually depicted on a map
or can be calculated from features of known size.
scaleless: The characteristic of digital
map data in abstract form of being usable and displayable at any scale, regardless
of the scale of the map used to geocode the data.
secant: A map projection in which the
surface used for the map "cuts" the globe at the map’s representative
fraction. Along this line there is distortion-free mapping of the geographic
space. Multiple cuts are possible, for example, on a conic projection.
state plane: A coordinate system common
in utility and surveying applications in the lower 48 United States and based
on zones drawn state by state on transverse Mercator and Lambert conformal
conic projections.
standard parallel: A parallel on a
map projection that is secant and therefore distortion- free.
transverse: A map projection in which
the axis of the map is aligned from pole to pole rather than along the equator.
UTM (Universal Transverse Mercator): A
standardized coordinate system based on the metric system and a division of
the earth into sixty 6-degree-wide zones. Each zone is projected onto a transverse
Mercator projection, and the coordinate origins are located systematically.
Both civilian and military versions exist.
value: The content of an attribute
for a single record within a database. Values can be text, numerical, or codes.
WGS84 (World Geodetic Reference System
of 1984): A higher precision version of the GRS80 used by the U. S. Defense
Mapping Agency in world mapping. A common datum and reference ellipsoid for
hand-held GPS receivers.
zone (of a coordinate system): The
region over which the coordinates relate with respect to a single origin.
Usually, some part of the earth or a state.
