Arizona Topo Datum

[muskybankr]

What is it?

[jbensman]

It depends on what you set it to in MapSource.  Preferences, then Position.  It will tell you datum the map is currently uisng and allow you to change it. 

[jbensman]

I should clairify.  That is what will be displayed in MapSource. 

For your GPS it is the same thing.  In the setup you chose what datum to use

[Seldom]

Since my OR300 only shows WGS84 and something called Zanderj as datums.  And cgpsmapper uses WGS84 by default, why would anyone want to use a datum other than WGS84?

Also, what's the difference between a datum and a spheroid?  My OR lists WGS84 separately as both a datum and a spheroid.  I thought the datum described the spheroid, and as such was the same thing.

[jbensman]

Maybe someone can explain it better, but I would describe the datum as where the lat/lon (or UTM) is measured from.  But the thing to understand is it makes a difference.  If you mark a waypoint in the NAD83 datum and then change the datum to NAD27, the latitude and longitude will be different.  So to accurately describe a point on the earth, you have to give the lat and long AND the datum you are using.  I must say I am really suprised your GPS would not at least have NAD83 (conus) and NAD27 which most USGS topo maps use.

[Indrid Cold]

http://forums.esri.com/Thread.asp?c=93&f=984&t=98998

[Seldom]

I must say I am really suprised your GPS would not at least have NAD83 (conus) and NAD27 which most USGS topo maps use.

Well, my 60CSX lists more datums than I can count, but my OR only lists WGS84 and the Zanderj, which uses an "International" spheroid.  Indrid's link did a pretty good job of describing the difference between a datum and a spheroid. It seems that every datum has its own specific spheroid, but a spheroid can be used by more than one datum.

I use GM to generate my Garmin maps.  I frequently need to use different projections and datums for data I get from outside sources to add to a map, but the output is always WGS84, so I don't understand the need to reproject or change datums in MapSource or on the GPSr.

[eaparks]

The Oregons have the same datums for choices as the 60Csx does.  3 names of Datums in the 60CSx have been changed slightly on the Oregons but all of the ones in the 60CSx are available on the Oregons.  Same number, same alphabetical order even with "None" and "User" being the last 2.

[Seldom]

Correct.  My problem was that I only saw the ones below WGS84.  I'm still trying to figure out why the others are needed in the GPSr, though.

[Boyd]

I get confused by all this as well. But I think one use would be to make the GPS display match a paper map. See: http://www8.garmin.com/manuals/UsingaGarminGPSwithPaperLandMaps_Manual.pdf

[jbensman]

Boyd is right about a paper map as lat/long and UTM is marked on them in either NAD83 or NAD27.  I remember the preGPSmap days when all you got from your GPS was lat long.  You would set your GPS to UTM, and use the UTM grid and a clear plastic card making off meters to plot your postion on the map or to get the position of a spot on a map to enter into your GPS. 

The other reason why you may need to change is someone else may be using another datum.  Its like meters vs yard.  If your distance is in yards and someone else is using meters, one of you much convert.  N38 16.562 W87 25.984 in NAD27 is an entirely differnet place than N38 16.562 W87 25.984 in NAD83.  It should be within 100 feet, but it is not the same point. 

[Seldom]

Thanks, guys.  I downloaded Boyd's link.  Guess I'll have to dig out some paper maps.

[-Oz-]

everything was converted to WGS84 for getting combined to the map.

[Dickfoster]

That's why most GPS units have the choices they do although WGS84 is what the GPS constellation uses and is universal globally no matter where you are.

Without so many old paper maps floating around there would be no need to change datums at all as WGS is already corrected for the all the lumps and bumps of the earth.
At least as they were up to 1984 anyway.
All the previous datums on which the old paper maps based were best fit spheroids that could only approximate the true shape of the earth in any given region.  

For example the central reference point for NAD was completely arbitrary and chosen due to the limitations of what methods were available for conducting geodetic surveys of the day and that long before satellites came along. It was hard work back then too.
 
BTW there is no such thing as an accurate static reference or datum that can be good forever as the shape and gravity model are undergoing constant change. There will always be a need for updates therefore there will always some disagreement in absolute positions that are based on different datums. In other words, get use to the idea of close enough.  

A former and original member of the old, now gone Defence Mapping Agency (DMA). It's The National Geospatial Intelligence Agency or some such today. People who obsess over this kind of stuff. Talk about yoiur nit pickers but they are.

I get confused by all this as well. But I think one use would be to make the GPS display match a paper map. See: http://www8.garmin.com/manuals/UsingaGarminGPSwithPaperLandMaps_Manual.pdf