Almanac
Information describing the orbit of each GPS satellite. Including clock corrections and
atmospheric delay parameters. An almanac is used by a GPS receiver to facilitate
rapid satellite acquisition and is also required by Trimble’s Mission Planning
software.
Alphanumeric
Consisting of alphabetic and numeric characters.
Altitude (Alt)
Altitudes are specified relative to either mean sea level (MSL) or an ellipsoid (HAE).
Altitudes above an ellipsoid are distance from a precise mathematical model,
whereas altitudes above Mean Sea Level are distances from a surface of gravitational
equipotential which approximates the statistical average level of the sea.
Attribute
A characteristic of a feature in a Geographic Information System (GIS) . Each
identifiable feature has attributes. One common attribute of all geographic features
is its position. Other attributes depend on the type on the type of feature.
Example a road may have a name or designation number, pavement type, width, number of
lanes, etc. Each attribute has a range of possible values called its domain.
Attribute value
A particular value for an attribute of a feature, chosen from the domain of that
attribute. For example, ‘Pavement type’ is an attribute while
‘Gravel’ is an attribute value for the ‘Pavement type’ attribute.
Barcode wand
An optical device for reading barcodes.
Base Station
Also called a GPS reference station. An antenna and GPS receiver set up on
a known location specifically to collect data which will be used to differentially correct
rover files. Trimble’s GPS Pathfinder Community base Station and Universal
Reference Station Systems are examples of dedicated GPS base stations, with the Pathfinder
Basic Plus, Pathfinder Professional and Pathfinder ProLite Systems can all be operated in
base rover files, but some base rover files, but some base stations can also output RTCM
differential corrections in real time. A telemetry link can be used to transmit
these real time corrections to rovers in the field.
Baud
A unit of data transfer speed used in digital communications. One baud
generally equates to a transfer speed of one bit per second, for serial (RS232)
communications.
Bearing
A direction from your current position to some other point of interest.
Bearings are measured in degrees (360 in a full circle), clockwise from either true or
magnetic north. See also Heading.
Central meridian
A zone constant used when defining a map projection. A meridian is an
imaginary circle around the world that crosses both of the geographic poles. A
particular meridian is defined by the angle it forms east or west of the meridian which
runs through Greenwich, England which has a value of 0 degrees.
CEP
See Circular Error Probability
Circular Error Probability
A statistical measure of precision defined as the 50th percentile value of the
two-dimensional position error statistic. A CEP value determines the radius of a
circle within which ½ of the GPS positions measured over a period of time will fall.
Constellation
1) A specific set of satellites used in calculating positions 3 satellites for 2D
fixes, 4 satellites or more for 3D fixes. 2) All of the satellites visible to a GPS
receiver at one time. The optimum constellation is the constellation with the lowest
PDOP.
Coordinate System
Any three dimensional reference frame that can be used to describe the location
of objects in space .
Cursor
A movable point which identifies a specific position on a computer screen.
Characters which you type on the keypad will be inserted at the cursor position.
Data capture
The collection of attributes and position information for geographic features.
Data dictionary
A description of the features and attributes relevant to a particular project or
job. This description includes feature names, data type classifications (point,
line, area), attribute names, attribute types and attribute values. After being
created in PFINDER, a data dictionary can be transferred to a data logger and used to
control the capture of features and attributes.
Data file
Capture feature and attribute data is stored in a data file, together with GPS
and ESP (external sensor) data.
Data logger
Also called a data recorder. A hand-held, light weight data entry computer.
Data loggers used by Trimble are the Omnidata Polycorder, the CMT MC-V and the
TDC1.
Datum
See Geodetic Datum
Datum transformation
See Geodetic Datum and Transformation.
DGPS
See Differential correction
Differential Correction (DGPS)
The process of correcting GPS positions recorded at an unknown location using
data recorded simultaneously at a known location (by a GPS Base Station). A roving
GPS receiver’s location can be differentially corrected relative to a base station in
real time or by post processing.
Dilution of Precision (DOP)
An indication of the quality of a GPS position which takes account of each
satellite’s location relative to the other satellites in the constellation and their
geometry in relation to the GPS receiver. A low DOP value indicates a higher
probability of accuracy. Standard GPS applications are: PDOP – Position
(three coordinates), HDOP – Horizontal (two horizontal coordinates) VDOP- Vertical
(height only), TDOP – Time (clock offset only).
Domain
The domain of a GIS attribute determines the set of possible values for that
attribute. A domain can be set of numbers, characters, or strings. For example
the domain for the attribute ‘Pavement type’ may be the set of strings
{‘Gravel’, ‘Bitumen’, Concrete}.
DOP
See Dilution of Precision
Elevation angle
The angle of a satellite above the horizon, measured from 0 degrees (on the
horizon) to 90 degrees (directly overhead).
Elevation mask
The angle below which the GPS receiver will not track satellites. For rover
files this is normally set to 15 degrees to avoid interference problems caused by building
and trees and multipath errors.
Ellipsoid
A mathematical model of the earth formed by rotating and ellipse around its minor
axis. For ellipsoids which model the earth, the minor axis is the polar axis, and
the major axis is the equatorial axis. An ellipsoid is completely defined by
specifying the lengths of both axes, or by specifying the length of the major axis
and the flattening.
Ellipsoid height
The distance measured from a point in space and the surface of an ellipsoid,
along the normal to that ellipsoid.
Ephemeris
Orbit prediction information which is periodically transmitted by each GPS
satellite. The Ephemeris provides up-to-the-minute modifications to the almanac.
EPROM
Erasable Programmable Read Only Memory. A computer chip which can be
programmed with software. For instance the EPROM chips in the TDC1 data logger
can be
programmed with Asset Surveyor software.
Feature
In a GIS, a physical object or location of an event. Features can be points
(a tree or a traffic accident), lines (a road or river), or areas ( a forest or a parking
lot).
Geodetic datum
A mathematical model designed to fit part or all of the geoid (the physical
earth’s surface). Defined by the relationship between an ellipsoid and a point
on the topographic surface established as the origin of a datum. World geodetic
datums are typically defined by the size and shape of the ellipsoid and the location of
the center of the ellipsoid with respect to the center of the earth.
Geographic coordinates
Latitude, longitude, and height on an ellipsoid.
Geographic Information System (GIS)
A computerized system used to input, manage manipulate, analyze and display
geographic data in digital form.
Geoid
The surface of gravitational equipotential which closely approximates mean sea
level (MSL).
Geoid height
The distance from the geoid or mean sea level to a particular point in the
ellipsoid. Also called the geoid separation or geoid undulation.
GIS
See Geographic Information System.
GPS Control Segment
The component of the GPS system, currently administered by the US Air Force,
which controls the operation of the NAVSTAR satellites.
Great Circle Arc
An arc formed on the surface of the earth by the intersection of a plane that
passes through the center of the earth and two points on the surface of the earth,
constituting., the shortest distance on the earth’s surface between those two points.
Greenwich Mean Time (GMT)
See Universal Time Coordinated (UTC)
GRS-80
The ellipsoid used by the WGS-84 and NAD-83 datums.
Grid
A two-dimensional horizontal rectangular coordinate system, such as a map
projection.
HAE
Height above ellipsoid
Heading
The current direction in which one is moving. See also Bearing
Horizontal Dilution of Precision (HDOP)
See DOP and Position Dilution of Precision.
Ionosphere
The band of charged particles 130 to 190 kilometers above the earth’s
surface.
Ionosphere delay
The variable delay experienced by a wave propagating through the ionosphere.
The duration of the delay depends on the electron content of the portion of
ionosphere. The duration of the delay depends on the electron content of the portion
of ionosphere delay induces a variable error in GPS positions.
Kb (Kbytes)
Kilobyte (1024 bytes or characters of data).
Latitude (Lat)
The north/south component of a location on the surface of an ellipsoid.
Latitude is an angular measurement north or south of the equator. Traditionally
latitudes north of the equator are considered as positive and those south of the equator
as negative.
LCD
Liquid-crystal display. The screen of a data logger.
Log
To store data in the memory of a data logger, in the memory of a GPS receiver or
on a computer disk.
Longitude (Lon)
The east/west component of a location on the surface of the earth.
Longitude is usually measured as an angular value east or west of the Greenwich meridian
(London, England). Traditionally longitudes east of Greenwich are considered as
positive and those west of Greenwich as negative.
Major axis
See Ellipsoid.
Map projection
A rigorous mathematical expression of the curved surface of the ellipsoid on a
rectangular coordinate grid.
MB
Megabyte; Approximately 1 million bytes or characters of data.
MHz
Megahertz; millions of times per second.
Minor axis
See Ellipsoid.
MSL
Mean Sea Level. See Geoid.
Multipath
Interference similar to ‘ghosting’ on a television screen which occurs
when GPS signals arrive at an antenna having traversed different paths (e.g. after getting
reflected from buildings). The different paths yield different pseudo range
estimates and introduce errors into computed GPS positions.
NAD-27
North America datum of 1927. Technically, only a horizontal datum
employing the Clarke 1866 ellipsoid. Height values of this era are expressed
in terms of the NGVD (National Geodetic Vertical Datum) of 1929.
NAD-83
North America datum of 1983. Like NAD-27, technically only a horizontal
datum. Its geometric definition is almost identical to WGS-84. For GPS
purposes, the NAD-83 and WGS-84 datum are identical. Uses the GRS-80 ellipsoid.
NAVSTAR
The official name for the GPS satellites. NAVSTAR is an acronym for
‘NAVigaion Satellite Timing And Ranging.’
Nesting Features
The ability of the Asset Surveyor software to record a point feature while
already recording a line or area feature.
Office Support Module (OSM)
A device which is included with most Trimble 4000 Series GPS systems and also
with the Trimble ProLite system. An OSM is used to supply power to TDC1 data loggers,
to recharge camcorder batteries and for connection of a TDC2 to a PC when the TDC2’s
software is being upgraded.
Operating systems
The software controlling a computer.
Origin
The intersections of axis in a coordinate system.
Orthometric Height
The vertical linear distance between an observed elevation on the ground and the
geoid. Stated another way, it is the ellipsoid height minus the geoid height.
Overdetermined Position
A 3D GPS position which was computed using more than four satellites. The
extra satellite(s) in the solution provide a degree of mathematical redundancy in the
solution which can be used to eliminate a number of errors in the GPS system, thereby
producing more accurate positions.
Parity
A form of error checking used in RS232 serial data transfer. Parity can be
sent of even odd, or none. A digital message is composed of 1s and 0s.
The parity of a byte is the binary sum over the length of that byte. A parity error
results when one of the bits in a byte is changed so that the parity of the byte changes
during transmission.
PDOP
See DOP and Position Dilution of Precision.
PDOP Mask
The highest PDOP value at which a receiver will compute positions.
PDOP Switch
The PDOP value at which a receiver will switch from computing 2D positions (used
only in Auto 2D/3D mode).
Position Dilution of Precision (PDOP)
A unitless figure of merit expressing the relationship between the error in user
position and error in satellite positions. Geometrically PDOP is proportional to 1
divided by the volume of the pyramid formed by lines running form the receiver to four or
more satellites observed. Thus a small PDOP is associated with widely separated
satellites.
PRN
See Pseudo Random Noise and Pseudo Random Number.
Projection
See Map Projection.
Pseudo Random Noise
(PRN)
A sequence of digital 1s and 0s which appears to be randomly distributed like
noise, but which can be exactly reproduced.
Pseudo Random Number
(PRN)
Each GPS satellite (SV) has unique identifying PRN.
RAM
Random Access Memory
ROM
Read Only Memory
Rotation
In transformations, an angle through which a coordinate axis is moved around the
coordinate system origin.
Rover
Any mobile GPS receiver and data logger collecting data in the field. A
roving receiver’s position can be differentially corrected relative to a
stationary base GPS receiver.
RS232
A protocol for serial communications between computers and other types of
hardware.
RTCM
Radio Technical Commission for Maritime Services. A commission established
to define a differential data link for real time differential correction of roving GPS
receivers. There are two types of RTCM differential correction messages: all Trimble
GPS receivers use the newer ‘Type 2 RTCM protocol; S/A, See Selective
Availability.
Satellite Geometry
The current position and movement of GPS satellites relative to each other and to
a particular GPS receiver.
Scale factor
A map projection parameter which is used to convert ellipsoid distances to
projection (grid) distances, and vice versa.
Selective Availability (SA)
A US Department of Defense program to control the accuracy of GPS pseudo range
measurements, whereby each GPS receiver computes a false pseudo range which is in error by
a controlled amount. In essence, this amount to artificial degradation of the
satellite signal by the US Department of Defense. The error in position caused by SA
can be up to 100 meters, but differential GPS techniques can eliminate most of the errors
introduced by SA.
Semimajor axis
One-half of the major axis or an ellipse.
SEP
See Spherical Error Probability.
Signal to Noise Ratio (SNR)
Also called Signal Level. An arbitrary unit used to describe the strength
of a satellite signal. SNR ranges from 0 (no signal) to around 35.
Higher-elevation satellites have SNRs in the high teens to low 20s. SNRs lower than
4 are considered unusable. See also SNR Mask.
SNR
See Signal-to-Noise Ratio.
SNR Mask
The lowest signal strength at which a receiver will use a satellite for
positioning.
Spherical Error Probability (SEP)
A statistical measure of precision defined as the 50th percentile value of the
three-dimensional position error statistics. Thus, half of the results are within a
3-D SEP value. See CEP.
Spheroid
See Ellipsoid.
State Plane Coordinates
Special definitions of Transverse Mercator and Lambert Conformal map
projection adopted by statute in the USA. There is one set of such zones for the
NAD-27 datum and another for NAD-83.
SV
Abbreviation for Satellite Vehicle or Space Vehicle. Each GPS satellite is
an SV.
Track
To make a pseudo range measurement of a GPS satellite.
Universal Time Coordinated (UTC)
The local solar mean time at the Greenwich Meridian. UTC is synonymous with
GMT. There is a variable offset from UTC to GPS time, which in 1993 is 8 seconds.
URA
See User Range Accuracy.
US Survey Foot
Defined as 1200/3937 meters. The official unit of linear measure for the
NAD-27 datum.
User range accuracy
(URA)
The contribution to the range-measure error from an individual error source
assuming that the error source is uncorrelated with all other error sources. Values
less than 6 are preferred. A value above 16 is an indication that Selective
Availability is active on that satellite.
UTM
The Universal Transverse Mercator Map Projection. A special case of the
Transverse Mercator project. Abbreviated as the UTM Grid it consists of 60
north-south zones, each 6 degrees wide in longitude.
Waypoint
A waypoint is a 3D point which can be given a name and location either prior to
field work or in the field. Waypoints are useful for navigation purposes an
can be used as a reference for a particular site. This makes returning for a
particular site. This makes returning to the same site possible at a latter date.
WGS-84
World Geodetic System (1984); the mathematical ellipsoid used by GPS since
January 1987.
This glossary was compiled by Nielsen Communications, Inc.