map datum dilemmas

[sugahjohn]

I don't know if either of these questions have been covered fully in another thread concerning GPS and GPS units, but here goes:

1.) Are there any drawbacks in composing a route made up of waypoints from maps utilizing differing map datum so long as you import the waypoints with the correct map datum setting on the GPS unit. More specifically, as a training exercise, I've been trying to put together a route to Owl's Head using primarily Nat'l Geo Topo (NAD 83) and MapSource US Topo 24k (WGS 84). From what I understand, these two datums are very similar, so there should be no problem in using them both on the same route. My question stems from the fact that--aside from the fact I can be a bit anal retentive--as I was trying to get the exact lat/long for the Lincoln Woods Visitors Center (which, so far I've been largely unable to isolate) I came across online (topozone.com) a map that's datum was NAD 27. Now, given DougPaul's Crawford Notch example from another thread, it seems that NAD 27 and NAD 83/WGS 84 are significantly dissimilar. That being the case, if I import (or input) a NAD 27-based waypoint correctly into my GPS, will it "fall into line" with the NAD 83/WGS 84-based waypoints already in the unit when I set up a route?

2.) Does anyone have any experience using lithium batteries in the Garmin 60CS? (It seems that the only options for Battery Type in System Setup are Alkaline and NiMH.)

Thanks!

 

[MichaelJ]

If you change the datum in your software, then type the point in, then change the datum back, it will end up in the right place. By the same token, if you change the datum in the GPS, then type the point into it, then change the datum back, it will end up in the right place.

(in fact, changing the datum back is technically unnecessary for this task, you should just do it so as to keep your overall settings how they were when you started)

 

[sugahjohn]

If you change the datum in your software, then type the point in, then change the datum back, it will end up in the right place. By the same token, if you change the datum in the GPS, then type the point into it, then change the datum back, it will end up in the right place.

(in fact, changing the datum back is technically unnecessary for this task, you should just do it so as to keep your overall settings how they were when you started)

LOL! I'm soooo confused, saith Vinnie Barbarino.

 

[DougPaul]

1.) That being the case, if I import (or input) a NAD 27-based waypoint correctly into my GPS, will it "fall into line" with the NAD 83/WGS 84-based waypoints already in the unit when I set up a route?

Depends. There are datum standards for transmitting locations between software and a GPS (use WGS-84, IIRC), but not everyone follows them... Some use WGS-84 as per spec, some use the display datum of the GPS/software.

Safest thing to do it to test it by:
1. Set the datum to what you want on both the GPS and software.
2. take a waypoint on your digital map and ditgitally transfer it to your GPS.
3. Set the datums to WGS-84 on both the GPS and software and check to see if it reads out the same waypoint lat/lon.
4. Try this for all 4 combinations of datum settings if you really want to be sure. (You could also test both upload and download if you wish.)

Garmin GPSes have both Garmin protocol mode and NMEA protocol mode. It is always possible that one mode is implemented correctly and the other incorrectly... (Other brand GPSes likely have proprietary protocol modes too.)

FWIW, I believe that Garmin GPSes with Mapsource software do the right thing. I have heard reports of problems with other combinations, but I don't remember which ones.

2.) Does anyone have any experience using lithium batteries in the Garmin 60CS? (It seems that the only options for Battery Type in System Setup are Alkaline and NiMH.)

AA photo lithiums are fine. The only thing that setting does is calibrate the battery status meter (which is of very limited use for lithium and NiMH cells due to their rectangular discharge curves).

When fresh, alkalines and lithiums are both 1.5V and NiMH 1.25V. Thus a fresh NiMH will read partially charged on an alkaline setting. Use the alkaline setting for lithiums. (I just leave it on alkaline and mentally factor in which kind of cell I am using.) No matter what kind of cell you are using, the GPS shuts down when the voltage gets down to about 1V/cell.

Doug

 

[MichaelJ]

LOL! I'm soooo confused, saith Vinnie Barbarino.

Once any single given point is in the software on your computer, its datum no longer matters. The software will handle any and all conversions, either to display it onscreen (in the software's datum) or to transmit it to the GPS (make sure the software and the GPS agree on the datum ... any datum).

So if you know the coordinates of a point in NAD83, set Nat'l Geo Topo to NAD83 and create the point. Is the next point in NAD27? Set Topo to NAD27 and type the point in. They'll all appear on the map in the correct location.

Now, set your GPS to WGS84, and the Topo software to WGS84. Export the points in the GPS. Done!

 

[sugahjohn]

Thanks for getting back to me and providing clarification, MichaelJ. Thanks, as well, to you DougPaul for your explanation. I play around with it and see what happens. I take it, though, that if things ultimately seem right on the Garmin software on my computer, they should seem right on the gps, assuming that I've downloaded the waypoints/routes properly. Thanks!

 

[DougPaul]

In theory, all (most?) GPSes use WGS-84 internally and do any required conversions during input/output. This would also be a good way to write software, but of course, the author is free to do whatever he wants.

MichaelJ:
Coordinates are never meaningful without the datum. I expect that most software has an internal default datum and once a set of corrdinates is converted to the default all further internal use assumes the default until the coordinates are output, which requires an appropriate datum conversion. Just because the user need not be aware of the internal default does not mean that it isn't there.

If the software and GPS follow the spec properly, electronic transfers of data between them will always use WGS-84, no matter what the human interface datum is. Thus, setting the datum before transferring data should not be necessary. However, given that some of the hardware/software does not meet spec, your suggestion of setting both the software and the GPS to WGS-84 is as good as any. If you determine that your software and hardware meet spec, then the whole problem goes away and you don't need any special settings to transfer data.

Doug

 

[CaptCaper]

Back parking spaces are N44 03.797 W71 35.306 in 84. If this helps.

Also the Lithium's will work but you won't have much of a warning via battery status icon as the batteries run out. There known for that. Running great then crash, empty. Take extras I always do.

2300 or so rechargeable and Lit's as back up's in single digit temps.

Jim

 

[MichaelJ]

Coordinates are never meaningful without the datum.

Of course. I was only implying that once it's in the software, the software will take care of it internally. Only at the moment of being input or exported out does a user-specified datum become relevant again (from the user's perspective) for stored points.

At least in good software.

 

[kltilton]

You can use this handy link to convert from NAD27 coordinates to state plane coordinates to NAD83 coordinates if you are interested. It is part of the NGS geodetic toolkit(http://www.ngs.noaa.gov/TOOLS/ ).

http://www.ngs.noaa.gov/TOOLS/spc.shtml