[Cdt-l] etrax 30 Question

Larry Swearingen larry.swearingen at frontier.com
Wed Jan 22 18:50:49 CST 2014

Hello jerry,
Thank you for that explanation.  It was very helpful.

Larry Hoodad Swearingen

From: bcss at bresnan.net 
Sent: Wednesday, January 22, 2014 4:54 PM
To: 'Larry Swearingen' ; 'Tjamrog08' 
Cc: cdt-l at backcountry.net 
Subject: RE: [Cdt-l] etrax 30 Question

If by “BLM Maps” you are referring to USGS topo maps you are correct, they don’t match.  The topo maps are all projected in North American Datum of 1927 (NAD27) and the BC waypoints are in World Geodetic System of 1984 (WGS84).  For most purposes, NAD83 (North American Datum of 1983) is almost identical to WGS84.  (NAD 83 is the USA’s implementation of WGS84) If you look at the lower left corner of most of these maps it will tell you the difference in position between NAD83 and NAD27. The wording is something like “To place on the predicted North American Datum 1983 move the projection lines 4 meters north and 51 meters east as shown by the dashed corner ticks.”   


The important thing is to match the coordinates on the map you are using with the settings in the GPS.  To make the Bear Creek waypoints match the NAD 27 maps in your Etrex  go to “setup”, “position format”, “Map Datum” and change the setting to NAD 27 CONUS.  Set the Position Format to UTM UPS.  Then the gps will work with the UTM grid on the older maps. (Remember to switch it back for the BC maps.)  


The reason the BC maps are in WGS84 is because it is the easier for users.  The native language of GPS receivers is WGS84, and they come set up in that datum when they are purchased.  The Mapbook maps are gridded in WGS84 to facilitate this.  



best wishes,


Jerry Brown

mailto:bcss at bresnan.net



From: cdt-l-bounces at backcountry.net [mailto:cdt-l-bounces at backcountry.net] On Behalf Of Larry Swearingen
Sent: Wednesday, January 22, 2014 1:47 PM
To: Tjamrog08
Cc: cdt-l at backcountry.net
Subject: Re: [Cdt-l] etrax 30 Question


I have been doing that.  When I do that I need to setup the gps for whatever coordinate

system the map uses so I’ve been practicing for that using my BLM maps and also the 

Bear Creek maps.  I’ve been looking up the Coordinates to a known Bear Creek WayPoint

after changing the Coordinate System to that used on the BLM Maps and locating that point

on the BLM Map.  BLM map coordinate system is not the same as those used on BC Maps.

However the gps will convert the way point coordinates to whatever you set it up for.


I agree though.  The middle of the Gila Wilderness is not the place to try and figure this out

for the first time.  :>)  I do hope to not get so far lost that I’m off a BLM Map.  That would be Lost !





From: Tjamrog08 

Sent: Wednesday, January 22, 2014 1:07 PM

To: Larry Swearingen 

Cc: Frank Gilliland ; cdt-l at backcountry.net 

Subject: Re: [Cdt-l] etrax 30 Question


You got it, HooDad.   

Not covered in GPS manuals is being sure to align the settings on your GPS with whatever physical maps coordinate format it has along the margins. You often need to do a little decimal estimation, as the points along the margins are marked in only three or four places.   

I was using  Jerry Brown's maps, and it took a bit of for me to be able to (1) establish a GPS reading where I as at and then (2) relate that to the physical map.  I'd practice this before I went.  

It saved my ass twice when I was so far off the CDT that I was off of the narrow corridor on the map.  What you do is hit Your exact "lost"  location on the GPS, then orient the map magnetically, and then see just how far off and in what direction you'd have to travel ( usually bushwhack) back to get to the CDT.  

You must know how to do this, and you might not be calm enough to figure it out the process  if you start to panic wandering around out there.

Yes, practice.  

I would also practice at home navigating to a single waypoint.  


Uncle Tom 


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