GPS Setup – Map Datum
By Blake Miller
“Datum is a mathematical model of the Earth which approximates the shape of the Earth, and enables calculations such as position and area to be carried out in a consistent and accurate manner. Lines of latitude and longitude on a map or chart are referenced to a specific map datum. Every chart (or map) has a map datum reference. The map datum is usually listed in the title block of the chart.[1]”
Datum is a term that GPS users should be aware of. It is a set-up function and option found on all receivers.
Figure SEQ Figure \* ARABIC 1 The units page where you can change map datum.
In essence, map datum is a mathematical model used to take information from a spherical earth and adjust that information for placement on a flat map. Of course we can get much more technical but in reality, that is about all the hiker/hunter needs to know.
It is the application of datum that is important to hikers.
Today, most US Geologic Survey (USGS) topographic (topo) maps are based on a map datum called the North American Datum of 1927; or NAD 27. This information is found at the bottom left of a topo.
Figure SEQ Figure \* ARABIC 2 The bottom left of a topo map showing the Map Datum.
The issue for GPS users is that receivers are set at the factory to another datum known as WGS 84.
WGS 84 and NAD 27 are not the same. 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 units will take 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.
Let’s be clear about this. If I enter the same coordinate (e.g., latitude & longitude, UTM) data in a GPS but use a different datum I will go to a different location.
If I am going to plot coordinates (from the GPS) on to a topographic map or take information from a topographic I recommend using NAD 27.
In
the topo to the left, the map’s datum is set to NAD 27. To use the
coordinates provided I need to set my GPS receiver to match the map’s
datum.
To change my receiver’s (Garmin 60CS) datum settings I will do the following:
- Select and press the menu button twice. The following screen will come into view.
- Now select “setup”.
- Select “units.”
- Rocker down to “Map Datum” and then press the enter button. A drop down menu of datum options will be displayed.
- Select either WGS 84 or NAD27 CONUS. CONUS is the abbreviation for Continental United States.
Finally, there are four points to consider about map datum.
First, the GPS receiver has datum information for many countries around the world. If I were to travel to South Africa shifting to the correct datum would be the right thing to do.
Second, GPS software is flexible. For example, waypoint coordinates (e.g., Latitude and Longitude) saved while using WGS 84 will change and convert automatically if NAD 27 is later selected later.
Third, anytime I provide coordinates to a location I always ensure my friend receiver’s datum settings match mine, or I shift my setting s to match his.
Fourth, when using mapping software on a PC or Mac (e.g., such as Terrain Navigator, or National Geographic’s TOPO) for trip planning ensure the program is set to the correct datum or shift the GPS’s datum to match the software. This is important when down loading GPS coordinates or routes to the GPS receiver.
Setting the correct map datum to match a topographic map is a great step for accurate navigation with a GPS receiver. It’s quick and easy.