Discussion: GPS settings

Hi all,

I am new to orienteering and recently bought myself a Garmin etrex GPS but unfortunately was unable to understand and set the GPS to use with my map. I tried to set it using MGRS, but i just can't figure out why the reading is different from my topo map.

May i know how i should go about setting these to be able to use it together with a topo map;

1) Position Format
2) Map Datum

Any advise is much appreciated. Thanks.

The actual GPS readings are in Lat/Lon, so you can do the conversion either in the unit, or in the software that handles the map overlay. Your profile doesn't say where you are from, but the best datum for North America (and the one shown along the edge of USGS topo maps) is NAD27. WGS84 is the default for many GPS devices because it's the best fit globally. The eTrex menu lets you change the UTM datum, but I don't remember which option it is off the top of my head.

Position Format
As for setting it to match up map.
The etrex can either be set to display the following two formats
DD MM'SS'' Degrees minutes and seconds
DD MM.mm' Degrees min and decimal min
For a topo map you will choose best to fir the relevent scale and grid to make it easier to interpret

Another consideration is what kind of map you're using. Look at the map for longitude/latitude markings. Some maps have several systems. If the map has grids for degrees and minutes, you might use DD.MM.mm, because it would (in this case) be more difficult to use seconds.

When I'm using GPS with an orienteering map, or some other map of a small area, I prefer using UTM. UTM essentially divides the earth's surface like orange slices divide the surface of a peeled orange. Within each 6 degree slice (zone), the UTM coordinates are rectangular grids (easting and northing instead of x and y). The coolest part of UTM is that the numbers represent meters. I usually demonstrate it to people by taking a big step N, S, E, or W, and show them how the coordinate changed by 1. (This works best with a very clear view of the sky.) As long as your map doesn't sit where two slices touch, you can do easy math to determine distances between points.

UTM wiki

The problem with that wiki is that most of the maps in it are Mercator. That means all the north lines are parallel, and you can't get a good feeling for the orange slices. (There is a US UTM image that shows them.) Here is a map of Canada that better shows the UTM orange slices.