Thales Navigation Mobile Mapper - Manual (Page 112)

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However, MobileMapper uses only the WGS84 datum to
project data onto the map screen, i.e. to position features spa-
tially on a map. One way to visualize this is if you were to
select NAD27 or EUR79 as your datum in either Mo-
bileMapper Office or the receiver and moved the map cursor
over a feature, you would display coordinate numbers (lat/
lon or northing/easting) that were calculated using this da-
tum. If you switched to the WGS84 datum, the coordinates
displayed on the MobileMapper receiver's Position Screen
or in MobileMapper Office suite's coordinate display win-
dow would change - but the position of the feature on the
map screen would NOT change.
Why is this? Calculating coordinates doesn't take much com-
puting power but using powerful map projection algorithms
does. Moving features around on the small map extents used
by workers in the field does not warrant any decrease in re-
ceiver map display performance. It would take an unneces-
sary increase in time to display a map that would hardly
change in appearance. To re-project positions on a Mo-
bileMapper Office map screen using a variety of datums
would not be a problem for the PC and its larger screen.
However, this is a GIS function that is outside the purpose of
MobileMapper Office, which is designed as a GPS data
communication, display and validating tool and not as a GIS
in itself. Because Thales Navigation does not know what al-
gorithms your GIS uses for map projections, you should al-
ways transform MobileMapper positions using the same GIS
you use to manage your existing maps. This is the best way
to assure conformance of MobileMapper positions to your
GIS maps and databases.