Datum question

[Orso]

Let's say I plot a course on my pc using the National Geographic "TOPO!" program, and then I want to transfer those waypoints to my Etrex (yellow model) using the pc cable. ###The TOPO! program uses WGS84 as the Datum, which is also what my GPS is currently set on. ###However, the topographic map I will be using in the field, the ones you buy at map stores uses a Nad27 datum. ###And I know the datum on each hardcopy map may be different, but for this conversation lets all agree that the actual map uses a Nad27 datum.

The question is this...As I am following my GPS, and it is leading me to each of my waypoints, what will be the difference from where I actually am, versus where I plot my self on the map? ###The GPS and the map using two seperate datums. ###

Can I download the waypoints from the TOPO program(WGS84) to my GPS(WGS84), and then change the setting on the GPS to the NAD27 datum, and then fully expect to be physically on the earth where I plot my self on the map using the coordinates from the GPS? ###Gosh!! I hope I am explaining myself clearly.

Please throw any quesions back at me if you don't understand what I am asking. ###I am planning to do some solo hunting in the LPNF for the A32 deer hunt later this year, and I just want to feel comfortable with all the aspects of the GPS.

By the way I will be taking my buddies GPS on my trips with me also, that way I will have a back-up, being on my own and all.

[Tinhorn]

Hi Brave Orso,

I always assumed the GPS converted all the saved coordinates between datums automatically


I used a program I found on the internet to convert these coordinates, maybe try it and see if they jive on the GPS

Nad27:
39:46:34.4
107:17:30.3

UTM for Nad27:
303725
4405224 ### zone 13

WGS83:
39:46:34.4
107:17:32.6

UTM for WGS83:
303675
4405433 ###zone 13

Tinhorn

[Topclamdigger]

The GPS will do converstions automatically ( Map Datum <---> Mapdatum), when you change the Map Datum setting in the Setup. Yes, the Waypoint will be in the same position(map/ground).Caution: when entering waypoints into GPS manually, be sure the Map Datum setting of the GPS is set to the Map Datum used in the coordinates that are entered.

[Gun Docc]

Howdy Orso ,

i also use the National Geographic "TOPO!" program and it is a good mapping program

what i do to match my paper USGS maps is to set it up in the National Geographic "TOPO!" program to use the same datum as the paper maps , Nad27 and then also set your GPS to this same datum and they all are working from the same datums and your plotted courses or waypoints will be exactly the same

setting the mapping software to use the Nad27 datum is easily done:

when you have the program running click the preferences tab and you will see a box open up with several options , highlight coordinate display and another box will show some more options and one of these is the Nad27 datum so click it to show a checkmark beside the option and your set

good luck on your outdoor ventures

[Orso]

Thanks for the input you guys. ###Some good points. ###I think the idea of changing the Topo program to the datum of the paper map is the smart way to go. ###Have everything, Topo program, GPS, and paper map, all on the same page. No punn intended.

Tinhorn, I will check out the coordinate and let you know.