GPS Set-Up: Why It Makes A Difference

In my land navigation classes, I emphasize that hiking groups should all be on the “same page” regarding the set-up options of their GPS receivers. There are several essential steps that ensure accurate position reporting, rendezvousing, and returning to camp safely.

While planning a journey or at the trail head, taking the time to adjust settings among hiking partners is critical. Before departing, validating map datum and coordinate format should be a priority. First, match the map’s datum. A topographic map identifies datum in the map key. Once the datum is identified ensure that all GPS receivers are set to match the correct datum. See the illustration below.

Map datum is a mathematical model of the Earth used by map makers. Datum allows for the accurate transfer of geographic data from a spherical earth to a flat map. In the United States, there are three common map datums found on topographic maps. These are WGS 84, North American Datum 1927 (NAD27) and NAD83. Select the datum that is used on the map. Receivers are set at the factory to WGS 84.

Note that WGS84 and NAD27 (North American Datum 1927) are not the same.

Let’s be clear about this. If I enter the same coordinate data (e.g., Latitude & Longitude or UTM) in a GPS but use a different datum I will go to a different location.

To illustrate this point, if I have two identical receivers, each with waypoints set to the same latitude and longitude yet set to different datum I will find that the GPS receivers will guide me to different locations. In Central Oregon, the difference is approximately 100 meters east and about 40 meters north. Even though the coordinate information has been entered correctly, the datum difference will cause an approximate 100 yard error.

Second, establish what coordinate system will be used by the Hiking partners. Generally, coordinate system is referred to as position format in most receivers. There are many options. The most common is Latitude and Longitude and there are three distinct formats. These are:

1. Degrees, minutes, & seconds (read as dd mm ss.s in the receiver,)
2. Degrees, minutes.minutes (dd mm.m), and,
3. Degrees.degrees (dd.d)

Degree, minutes.minutes is the common default setting selected by the manufacturer.

Here again, match the map. Most topographic maps are printed using Degrees, minutes, & seconds and there is a deference.

Universal Transverse Mercator (UTM) in another coordinate system and is an excellent option. Many maps will have both coordinate systems. UTM is much simpler to use than Latitude and Longitude; it is more intuitive.

Finally, there are other considerations about position format and map datum. When overseas be aware that many countries have their own unique position format and map datum.

GPS software is flexible. For example, coordinates data (e.g., Latitude and Longitude) saved while using WGS 84 will change and convert automatically if NAD 27 is later selected later.

Lastly, anytime I provide coordinates to a location I always ensure my friend receiver’s datum settings match mine, or I shift my settings to match his. Only then will I enter the information.

Setting the correct map datum and position format to match a topographic map is a necessary step for accurate navigation with a GPS receiver. It’s quick and easy.