Gps Quality Indicators - Magellan MobileMapper - GPS Software User Manual

GPS quality indicators

Tracking 3 satellites
will allow you to calcu-
late a 2D position (lat/
lon) using the last alti-
tude recorded by the
receiver. You need to
track at least 4 satel-
lites to get a 3D posi-
tion (lat/lon/altitude).
To achieve the speci-
fied accuracies for
MobileMapper Pro, you
will need to track 5 or
more satellites.
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MobileMapper Pro will warn you if the PDOP, and therefore the
accuracy, of your positions has dropped below the level you se-
lected during Setup (see page 65, the PDOP alarm option). If
you see this warning you may press the ESC button and con-
tinue logging.
However, if you are recording line or area features, you might
want to return to the field when the GPS satellites are in a bet-
ter configuration overhead (the constellation changes slowly
but constantly).
And if you are recording a point feature, you should strongly
consider moving to another location with better satellite re-
ception and recording an offset (distance and bearing) to the
feature.
GPS Quality Indicators
At the bottom of each logging screen, you can see two num-
bers that give you a good indication of how accurate your po-
sitions should be.
The first is the number of satellites that the receiver is track-
ing.
The second number is the Positional Dilution of Precision
(PDOP), which is an estimate of accuracy that the receiver
constantly calculates using the geometry of satellites in the
sky. The more satellites that are being tracked and the more
evenly they are distributed around the sky, the better the ac-
curacy. PDOP values less than 4 or 5 are good. If the PDOP
value is over 10, you should consider moving to an area with
a clearer view of the sky and recording features with offsets.