Thales Navigation Mobile Mapper - Manual (Page 111)

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Which system/datum you use can have a very profound re-
sult. For example, the difference between NAD27 and
NAD83 is minimal in Bloomington, Indiana, about 42
meters in Albany, New York, about 70 meters in Las Vegas,
Nevada and as much as 100 meters in parts of California.
Why are there such differences? The NAD83 is a closer and
more refined mathematical description of the shape of the
Earth in North America. For this reason, the WGS84 datum
uses elements of the NAD83 datum to calculate coordinates
in North America.
Similar discrepancies are seen around the world when calcu-
lating positions using different datums. To make matters
even more complicated, different organizations, govern-
ments and corporations use different algorithms to calculate
positions for their own maps. This is not normally a problem
as long as an organization's maps are in agreement with
themselves. Because Thales Navigation cannot know what
algorithm you wish to use to make MobileMapper data con-
form to the datum you are using, we export our positions in
world standard WGS84 coordinates. This allows your GIS to
make the transformations that are consistent with your map
collection.
For the purposes of collecting or updating map data, both
MobileMapper Office and MobileMapper receiver software
will display the coordinates of features imported from your
GIS maps in whatever datum you wish. This means that if
you move the map cursor (in either MobileMapper Office or
the receiver software) over a feature on a map screen, the co-
ordinate numbers (lat/lon or northing/easting) will be dis-
played in the coordinate system/datum of your choice. This
allows you to compare the coordinates of features on a Mo-
bileMapper map with coordinates you might have in a data-
base.