NavCom RT-3020 Rev.A - Manual (Page 48)

RT-3020 User Guide
Glossary-46
minor axis. GPS positions are computed relative to the
WGS-84 ellipsoid . An ellipsoid has a smooth surface,
which does not match the earth’s geoidal surface
closely, so
GPS altitude measurements can contain a
large vertical error component. Conventionally surveyed
positions usually reference a
geoid , which has an
undulating surface and approximates the earth’s surface
more closely to minimize
altitude errors.
epoch literally a period of time. This period of time is
defined by the length of the said period.
geoid the gravity-equipotential surface that best
approximates
mean sea level over the entire surface of
the earth. The surface of a geoid is too irregular to use
for GPS readings, which are measured relative to an
ellipsoid . Conventionally surveyed positions reference a
geoid . More accurate GPS readings can be obtained by
calculating the distance between the geoid and
ellipsoid at each position and subtracting this from the
GPS altitude measurement.
GIS (Geographical Information Systems) a computer
system capable of assembling, storing, manipulating,
updating, analyzing and displaying geographically
referenced information, i.e. data identified according to
their locations. GIS technology can be used for scientific
investigations, resource management, and
development planning. GIS software is used to display,
edit, query and analyze all the graphical objects and
their associated information.
Global Positioning System (GPS) geometrically, there
can only be one point in space, which is the correct
distance from each of four known points.
GPS measures
the distance from a point to at least four satellites from
a constellation of 24 NAVSTAR satellites orbiting the