Garmin and ATS77 Datum (Eastern Canada):

Posted by:
Mitch Lane Windsor, NS, Canada mitchdawn.lane@ns.sympatico.ca

        Hey all, I'm hoping I can help some folks out here who may be
in the same situation I'm in. I'm the Survey Supervisor for a
construction company here in Nova Scotia, Canada. I've recently bought
several Garmin GPS 12Maps and have found them to be one of the most
useful tools we own for test pit locations, simple mapping, and most
importantly control monument locations. The problem I had however was
the datum our monument values are in - there was no way to precisely
convert them for use with the Garmins.
        After buying the Mapsource CD however, the situation changed
dramatically for the better. In the CD software, it's possible to
enter values for user datum and grid that are more refined than what
can be entered directly in the Garmin. Because of this - and after
some experimenting with the figures - I can now enter points with
their ATS77 values (our local Datum) in the CD software, upload them
into the 12Map, and confidently find those points in the field with
pinpoint accuracy. It also works downloading as well, I just download
waypoints into the software and select the user datum and grid from
preferences and have ATS77 valued points. I'm sure this same principle
will work for other local datums as well - use the Mapsource software
for user datums rather than the Garmin - but I have no familiarity
with other grids other than our own, I recommend purchasing a CD and
do some experimenting.
        Anyway, I'll give the figures here so anyone with Mapsource
who needs ATS77 value conversions can do so with confidence. Good luck
and happy GPS-ing!

User Defined Grid:

        Longitude origin = W64.5000 (zone 5)
                          W61.5000 (zone 4)
        Scale Factor = 0.9999
        False Easting = 5499996 (zone 5)
                            4499996 (zone 4)
        False Northing = 0

User Defined Datum:

        Delta x = 0
        Delta y = 0
        Delta z = -4.5
        Delta Semimajor Axis = 2
        Delta Flatness = 0.00022