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Backpacker Magazine – June 2012

Manually Adjust Declination

Change declination to navigate more accurately over long distances.

by: Molly Absolon

by Supercorn
by Supercorn

The difference between true north (how your map is oriented) and magnetic north (where your compass points) is called magnetic declination. In order accurately trek to your destination along a bearing, you have to adjust and correct for it.

The line of zero declination, where true north and magnetic north intersect, runs from the southern tip of Florida to northern Lake Superior, and near that line, declination won’t much affect navigation. The further east or west you move away from the line of zero declination, and the higher your latitude, the more declination will impact wilderness travel.

In Southern California, for example, declination is about 14°E, in Upstate New York it’s about 13°W, and in Alaska, it may exceed 35°E; if it’s not accounted for, that’s enough to equate to more than a mile of error over five to ten miles of off-trail travel.

If you have a compass with adjustable declination, it's easy to automatically correct for it. Just set the compass to the declination in your most common hiking area. It’s hard to follow a bearing within closer than 5 degrees in the wilderness, so unless you’re driving hundreds of miles to hike, you won’t have to reset your compass’s declination. Set your GPS to display True North bearings so they work together seamlessly. You can determine the declination of your area by entering your zip code into the calculator from NOAA's National Geophysical Data Center. 

To manually adjust for declination, you have to do the arithmetic of adding and subtracting the declination to and from your bearings as you navigate in the field. You can shoot bearings, identify landmarks, then correct the magnetic bearing by adding to (if your declination is east) or subtracting from (if your declination is west) the declination before translating it to a map.

Accurately translate a map-based bearing to a compass heading by doing the opposite: subtract the declination from the map’s true-north reading (if your declination is east) or add to the map-derived bearing (if your declination is west).

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READERS COMMENTS

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ND HITT
May 09, 2014

Once again, another Backpacker article that is pointless. Here, I summed it up this verbose garbage with one link: http://www.ngdc.noaa.gov/geomag-web/#declination

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bakpacker
Dec 29, 2013

It's strange that this article is so sloppy. The points raised in earlier comments could have been fixed easily, and it would have helped to add NOAA's website URL.

Star Star Star
vjb1976
Dec 22, 2013

The magnetic declination map is misleading because the declination actually changes over time. Another method for orienting your map is to use magnetic north without any correction on your compass. To do this, go to the NOAA website as the article suggests and find the current value of the magnetic declination for the area of concern. Then take a protractor and pencil and draw a magnetic north line on your map. Then use this line and your compass without declination compensation to orient the map. This method is used in the sport of orienteering except that the maps are drawn with magnetic north straight up rather than true north. Also, don't rely on the magnetic north data on USGS maps. Most of those are out of date. Just look up the current value on the NOAA website.

Star Star Star
vjb1976
Dec 22, 2013

The magnetic declination map is misleading because the declination actually changes over time. Another method for orienting your map is to use magnetic north without any correction on your compass. To do this, go to the NOAA website as the article suggests and find the current value of the magnetic declination for the area of concern. Then take a protractor and pencil and draw a magnetic north line on your map. Then use this line and your compass without declination compensation to orient the map. This method is used in the sport of orienteering except that the maps are drawn with magnetic north straight up rather than true north. Also, don't rely on the magnetic north data on USGS maps. Most of those are out of date. Just look up the current value on the NOAA website.

Star Star Star
vjb1976
Dec 22, 2013

The magnetic declination map is misleading because the declination actually changes over time. Another method for orienting your map is to use magnetic north without any correction on your compass. To do this, go to the NOAA website as the article suggests and find the current value of the magnetic declination for the area of concern. Then take a protractor and pencil and draw a magnetic north line on your map. Then use this line and your compass without declination compensation to orient the map. This method is used in the sport of orienteering except that the maps are drawn with magnetic north straight up rather than true north. Also, don't rely on the magnetic north data on USGS maps. Most of those are out of date. Just look up the current value on the NOAA website.

Star
robert morgan
Dec 20, 2013

Right, I am looking at it in December 2013 and see the same apparent discrepancy. Why do they say Florida when it is nowhere near Florida. Perhaps map is not "true"

Riff
Jun 14, 2012

Why does the article say that the 0-Degree line goes through Florida, yet the picture shows nothing of the sort, it goes right through New Orleans?

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