I recently purchased a Garmin Montana. I want to use it to find ngs and other control with coordinates. My question is, what datum is used by handheld Gps and Google earth? I was thinking it was wgs 84? Anybody have any idea? Thanks.
Does Not Matter
Whatever datum, a hand held only gets you so close. If you cannot find it from a few feet away, you best give up surveying.
I do recall that I seemed to be too far away from the NGS ties, when I realised my gps was set to NAD 27 not 83. Now that makes a big difference.
Paul in PA
Agree with Paul, either NAD83 or WGS84 will work for what you want to do. I think GE is WGS84 if it matters. GE has a lot of inaccuarcy even over small areas. I plotted 38 points this week in GE from an aerial job I controlled earrlier this year, GE had upadted the photos since the targets were placed, even across this city there was quite a bit of error in GE, I imported NAD83 points, some were very close, others not so close, so point is for the application you are looking at the datum + the handheld accuracy is only a tool to get close to start looking.
SHG
Google Earth uses WGS84. And if you actually want to use Google Earth on a hand held device, you probably want to get an Android smartphone... which all shockingly have built in GPS and plenty of free or pay apps from the Play Store that will do what you want possibly better than Google Earth.
All GPS work in WGS84 and do conversions to other datums selected from their menu.
The older ones did a null conversion from WGS to NAD83, in keeping with that old military document that said they were the same.
I'd be curious if your new model does also. To test this, set the display to the lat/lon format with the finest resolution, probably DDD.ddddd and set one of the datums. Enter several waypoints and record their coordinates on paper. Switch to the other datum and examine to see if any of the waypoints changed coordinates slightly. The resolution of that format is comparable to the difference between WGS and NAD83, so even if they do attempt a real conversion some points may not change, but some out of a bunch of points would be expected to. I've done this on two older Garmin handhelds and never found a change.
To prove the procedure is a good test, try NAD27 and see that the numbers do change.
I think that the WAAS corrections are based on ITRF00 reference frame and therefor at least some of the handhelds which are receiving WAAS corrections are displaying ITRF corrected coordinates. I am pretty sure my garmins do.
In my area this is between 1 and 2 meters total shift from NAD83xx, but may be more in other parts of the country.
So that difference is in the fringe of the noise of the accuracy of most such devices. However some of my experiences with long term averages seem indicate that the ITRF values may be what you are getting.
Of course not all of the photos in google earth are perfectly registered or corrected either.
- jlw
Google Earth uses SPHERICAL WGS84 ...
... not ellipsoidal WGS84. That makes a difference, especially since with satellite photos they cover such a large area that only the nadir (plumbline point) has any semblance of having an accurate position. The further you get from the nadir of any photo, the further the imagery deviates from its true "geodetic" position.