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Declination - which do I trust?

Guest scooterj

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Guest scooterj

I have a compass with adjustable magnetic declination. The chart that comes with my compass says that the magnetic declination where I live is 4 degrees east. However, my GPSr (Garmin eTrex Legend) says it is 2 degrees east.


This weekend I'll be doing some back-woods cache hunting in an area where according to the chart that came with my compass the magnetic declination is 2 degrees east. However, the topo map I have of the area says the declination is 0 degrees.


It sounds to me like my chart is off by 2 degrees, but I'd like to know for sure. Is there a simple way to determine what the actual declination is in a particular location? I know that being off by 2 degrees won't make a whole lot of difference (if even any) for my cache hunt, but it'd be nice to know what the accurate information is. icon_smile.gif


[This message has been edited by scooterj (edited 31 August 2001).]

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Guest bob_renner

The declination changes over time. Many maps were generated 10-20 years ago. The declination may have changed since then. The declination readout in the GPSR is supposed to know about the changes and should give you a more accurate readout.


You could set the GPSR to magnetic, leave the compass declination set to 0, and use the mangetic bearings from the GPSR.



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Guest Geoffrey

I had a magellan and a garmin GPS. I noticed here in detroit that the magellans were pretty good, but the garmins are way off. My 3plus and Vista show a declination of 7 in detroit michigan, whereas it should be 3.

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I agree with bob_renner that you should set your GPS to display magnetic bearings and leave your compass on magnetic as well. It makes things much easier.

Here on the west coast, magnetic variation is much greater than in Kansas; nautical charts here will say something like "VAR 19 15'E (1996) Annual decrease 5' " So you can figure out that in 2001, magnetic variation in this location is about 18 degrees, 50 minutes East.


Your GPS contains tables that provide a variation figure based on a model of the planet, and they should be reasonably accurate. Of course, local magnetic influences are very common, and are often not taken into account in either maps or GPS tables.


As a practical matter, you should obtain a magnetic bearing to your destination from your GPS, navigate toward the waypoint with your compass, but obtain updated bearings from your GPS from time to time to keep yourself headed in the right direction.

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Guest scooterj

Well, I completed the hike yesterday and using what the GPS said the variance was I found that the compass got us right where we needed to go each time, so it looks like the GPS won out over my chart.

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Guest CharlieP

Originally posted by scooterj:

Well, I completed the hike yesterday and using what the GPS said the variance was I found that the compass got us right where we needed to go each time, so it looks like the GPS won out over my chart.


I will have to go with the chart. A declination program I use shows for 39N 95W (eastern Kansas) a declination of 3d 46 min east as of Jan 2002. The pole is moving west and this figure will decrease about 7 minutes per year. I don't understand why the GPS has such a large error in the declination.




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Guest xploregon

Originally posted by jndery:

What compass are you using to get bearings that precise?


I have a Normark MC-280 [Finland] with 2 degree increments. The bearing line is 1/2 degree thick.

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