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Map Datum
Maps and charts consist of lines, like meridians and parallels, which
represent the earth on a flat surface. Each chart and map is based on
a particular model of the earth's surface which consists of an ellipsoid
and the distance of that ellipsoid from the center of the earth. In
addition, the chart is made using a particular projection of the surface
of the earth onto paper.
The GP system is based on the World Geodetic urvey of 1984
(WG 84); therefore, best accuracies from the NAV 1000 PLU are
obtained using the WG 84 datum. Altitude presented on the
display is height above mean sea level based on the height above
the geoid, an equipotential model of the earth's surface based on
gravity. This model, like a model of the magnetic variation around the
globe, is contained within the unit.
The NAV 1000 PLU provides geodetic coordinates (LAT/LON) and
a map projection referred to as a transverse mercator. To work with
maps and charts around the world, the NAV 1000 PLU provides 11
geodetic map datums and one user defined datum.
Magellan uses the abridged Molodenskiy Datum Transformation
algorithms for these calculations. The constants used in the
equations are based on averaged information obtained by
geodocists and is generally closer than five meters to most local
control points.
The Map Datum option lets you select the datum that corresponds to
the chart you are using. In the legend of your chart, you will find
reference to the datum upon which the chart is based. We provide
only eleven pre-programmed transformations. But we provide a list
of constants you may program yourself. Just remember that a given
position can be accurately placed on a chart only when you have the
unit's datum setting matched to that of the chart.
A-9
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