--- In OziUsers-L@yahoogroups.com
, "The Stewart Family"
> Thanks to Dave, JG and Ron for your replies. I think you have
clarified things well. However, your answers have now raised another
question for me.
> My GPS Download/Upload is set to WGS84 (as it is) and the NMEA
output string is also WGS84 and I have my preferred settings for
projection as UTM. Now, if I am using moving Map mode and the map
loaded was calibrated upon the Datum AGD66 are the eastings and
northings shown as AGD66 or WGS84; as stated before my location on
the map is correct, but I am curious as to what datum are the
eastings and northings being displayed in.
> Another example would be If Noel below had his map loaded into OZI
would he have needed to change his GPS from WGS84 to AGD66 to see the
correct eastings and northings for the Paper based Map which was
> I cannot test this at the moment because my maps for my current
location are GDA94 which is compatable with WGS84.
Ok, let's try this again. If you set your Garmin GPS to AGD66, then
two things happen. First off, coordinates displayed on the GPS
screen, are relative to AGD66 datum. Secondly, position coordinates
sent to OziExplorer via NMEA protocol, are relative to AGD66 datum.
If you do that, then you must also change the setting in Ozi, for
which datum to expect in coordinates received via NMEA. Ozi will
automatically translate coordinates received in whatever datum, back
to WGS84, no matter what. WGS84 is the common denominator, so to
speak. Everything passes through it, no matter the source or
destination. Now, there are at least 2 more datums to deal with
here. The Ozi display datum setting determines, relative to which
datum, Ozi will display coordinates. So, if you have Ozi display
datum set to AGD66, then Ozi will translate any coordinates to AGD66
for display. As for the map calibration datum, in order to display
the map image, Ozi will automatically translate the calibration from
whatever non-WGS84 datum, to WGS84. Still, this has no effect on any
other datum setting. Ozi is only keeping track of the pixel
coordinate to lat/lon coordinates in WGS84 datum. If you move the
mouse pointer around the screen, Ozi translates the pixel coordinates
to WGS84 lat/lon and then to whatever datum you've selected for
display. Ozi will also convert the final lat/lon coordinates to
whatever grid you may have selected such as UTM. It should be
obvious that the grid coordinates are displayed in the same datum as
the lat/lon coordinates.
Let's see if we can sum up some rules here:
1. Calibrate map images using whateve datum you want, so long as the
coordinates that you enter for the calibration points, are relative
to that datum. There is no interaction with any other datum
setting. You may use a map calibrated in any datum, with any other
datum setting for NMEA, display, etc.
2. Set your Garmin GPS to WGS84, unless you simply must have non-
WGS84 coordinates displayed on the local GPS, because you're also
using a paper map, with non-WGS84 coordinates marked. If you do
that, then change the Ozi setting for NMEA datum, to match what
you've set on the GPS. Here again, there is no interaction with the
Ozi setting for display datum, nor does it matter what datum was used
to calibrate the map image. It's completely independent.
3. Set the Ozi display datum to whatever your heart desires. If you
must have displayed coordinates in the same datum as a paper map,
then choose the datum of the paper map, for the Ozi display datum.
If there are marked gridlines on the map image, some people prefer to
have Ozi display gridlines overlaid with those of the image. In such
case, it may be convenient to have the Ozi display datum set to the
same as the datum for the map image's gridlines. This may or may not
be the same as the datum used to calibrate the map image. So, still,
it's independent. There is no interaction with any other datum
It's all a bit complicated by Garmin's bad design choice.
Furthermore, as I mentioned previously, if you configure your Garmin
GPS for Garmin interface mode instead of NMEA mode, then position
coordinates reported via PVT protocol, are using WGS84 datum.