SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
36
3.4.8. GPS Terminology
This section describes some of the key GPS terms.
Latitude and Longitude
Latitude and longitude are two numbers or coordinates that are used to describe locations on
Earth. Lines of constant latitude circle the Earth East-West. Lines of constant longitude circle the
Earth North-South. Latitude and longitude are measured in degrees, minutes, and seconds.
Altitude
Altitude is the height of the GPS receiver above sea level. The receiver is assumed to be at a con-
stant elevation above sea level for the entire transect. A significant change in altitude may sug-
gest that the GPS data may be unreliable.
Number of Satellites
The number of satellites is the number of visible satellites used in the computation of the re-
ceiver position and is reported in the GGA output string. Four satellites are required for the com-
putation to produce valid results.
Number of Invalid (Satellites)
Each time the Number of Satellites falls below four for a single profile, the Number of Invalid
Satellites will increment by one. The Number of Invalid Satellites will be displayed in red for
any value greater than zero.
Number of Satellite Changes
Each time the number of visible satellites changes, the Number of Satellite Changes will incre-
ment by one. A significant number of satellite changes may suggest that the GPS data may be
unreliable.
HDOP – Horizontal Dilution of Precision
The HDOP is the degree of uncertainty in the horizontal positions (Latitude and Longitude) re-
ported by the GPS receiver. A Delta HDOP value greater than one may suggest that the GPS
data may be unreliable.
Magnetic Declination
This is the angular difference between true North and magnetic North. This value differs depend-
ing on the location of the survey site. You can determine the magnetic declination for a survey
site from several sources: magnetic declination maps provided by the U.S. Geological Survey,
commercial software, or Internet sites (see
listing for details).
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