GPS Navigation – Declination

Having a GPS navigation system is a great addition to one’s outdoor gear.

It can open up realms of adventure previously open only to the hardiest souls, or give the devoted couch potato a fun outdoor diversion close to their home, as in geocaching.

There are a few important details to understand about GPS navigation, and mobile navigation in general.

One of these is the subject of “declination”.

Quite simply put, declination is the difference between “true north”- the actual axis of rotation of the globe, and “magnetic north”- the topmost point of emanation of the magnetic field surrounding the globe, to which compass needles point.

One of the characteristics which make declination a confusing subject is that the magnetic north is constantly changing. It has moved approximately 900 miles closer to the true North Pole over the last 100 years.

Declination is expressed as an angle. Looking at a compass with the needle pointing north, the declination to true north will be a few degrees either east or west of the needle, depending on your location on earth. There is a line where the two of them are directly in line (the “Agonic Line”), which runs down the midwest of Canada and the United States, and there is no declination.

Virtually all topographic maps show the declination for that particular region. However if the map is more than 10 years old, the declination may have changed enough to seriously throw off your attempts to navigate accurately if you use the declination shown on the map.

A portable GPS navigation unit in the field is not enough. You should always carry a good compass, preferably one that you can adjust the declination of. Along with this, it’s a very good idea to have a topographic map of the area you’ll be travelling in.

A good compass will give you the ability to navigate without having your GPS navigation system turned on all the time, wasting battery power. Also you’ll develop your skills using a compass to navigate, so you aren’t wholly dependent on your GPS unit, should it decide to go dead, break, get itself lost from you, etc.

As well, having a topo map will help you to locate yourself relative to physical landmarks around you, judge distances, enter waypoints you’d like to go to, etc.

Your GPS unit can be set to display headings and bearings relative to either true north or magnetic north.

Where this becomes very important is to make sure that your compass, and your GPS, are both pointing in the same direction.

You can buy an inexpensive compass that simply points to magnetic north, and have your GPS unit reference to magnetic north as well. This is fine and will serve your purpose.

Virtually all topographic maps are aligned with true north. If you decide to use a map in the field, you will need a compass with the correct declination set, and your GPS portable navigation device set to true north, so all three are pointing in the same direction.

You can find the correct declination for any area in the United States by going to the website for the National Oceanic & Atmospheric Administration (NOAA) and entering in the zip code for the area you’re interested in. It will give you the correct declination for that area. Outside the USA, enter in the latitudet and longitude for the area you want to know about.

Go to: www.ngdc.noaa.gov/geomagmodels/Declination.jsp

So get out there, stay safe, use your common sense, and have fun!

Read more about GPS mobile navigation systems, and GPS navigation in general, at http://Gpst.ForHiking.net.