NMEA and datums?

Javalin

Hello all,

Anybody out there know how to work out from a standard Bluetooth GPS unit, what Datum its using for LAT/LONG calculations?· Most current ones are WGS84 but there are several similar datums.

The unit in question outputs the usual RMC, GSA, GGA, GSV, ZTG messages - i've looked but cannot see any references to the datum in these messages.

It must be possible, as TomTom can do it correctly, but from just reading the LAT/LONG its obviously not WGS84......

I am not aware of the unit taking input for proprietary messages....· (i.e. some will output something like $GPDTM (gps Datum) messages)

Thanks,

James

Post Edited (Javalin) : 5/19/2008 1:51:44 PM GMT

Phil Pilgrim (PhiPi)

If you've got another GPS unit whose datum you can select, just run through the list until the Lat/Lon agrees with that of the mystery unit. This may not be determinative, though, if you're at a location where two or more datums yield the same coordinates.

-Phil

Javalin

Hi Phil,

Yes I could - but i'd like to find a method (i.e. a parameter in an NMEA message / etc) to be able to detect in code reading the unit (any unit) that its not using WGS84..... Must be possible like I said as TomTom read this unit correctly....

Additionally - I know its not WGS84 (not mentioned clearly in the original post) as the data is overlaid in a KML file onto Google Earth - which does use WGS84. Its always nearly consistently out..... odd.

Solution may be a new(er) unit but seems a shame...

James

Javalin

Update (for those that are interested).

It turns out that Google Earth takes decimal degree's in a KML file, and displays dd mm ss. The gps via NMEA outputs ddmm.ssss where mm.ssss are minutes and seconds (in 60'th's - i.e. not decimal).

Hence the mismatch between the google earth displays and the gps data....

Solved!

James