Magellan GPS PROMARK X, User Manual

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SETUP
ProMARK X
3-13
measurements are based on the height of the position above (or below) the
ellipsoid being used by the currently selected datum, or the HAE. When you
select HAE, the unit always displays the height of the position above the
ellipsoid of the currently selected datum.
The other model is the geoid. The geoid is the surface over which water
would flow if free to adjust to the effects of gravity and the earth’s rotation.
This is often referred to as the Mean Sea Level (MSL).
Since the geoid is not a mathematically smooth surface and does not always
correspond to the ellipsoid, the unit adds the Geoidal Height (GH, or the
difference between the geoid and the ellipsoid) to the HAE to calculate the
height of a position relative to the geoid. This is the Height Above the Geoid
(HAG).
We said above that the geoid and mean sea level are the same; this means
that HAG and height above MSL are the same. In the ProMARK X they differ
only in the value used for GH.
The ProMARK X contains a global geoidal model; when you select MSL, the
unit calculates the altitude of a location using a standard value for GH that
is accurate to within a few meters. This is usually accurate enough for most
applications even though the difference between the unit’s GH and true GH
changes rapidly over several miles.
If known, you may use the actual GH of your area. This will increase the
accuracy of the position fixes calculated by the unit, especially when
collecting data for differential positioning over long baselines. Select HAG
to enter your own value for GH.
NOTE
If you are not sure which reference to use, select ALT to display
elevation above mean sea level.
Access SETUP 10. Press the RIGHT ARROW to select ALT, HAG, or HAE.
If you select HAG, you must enter a value for Geoidal Height. Press
ENTER . If a value is displayed, press CLEAR to erase it. Key in a new value
and press the RIGHT ARROW to select positive or negative. Press ENTER
to save the new value.