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Old 09-11-2012, 01:30 PM   #21
John H Swanson
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My flight instructor years ago told me something that has proven true over and over again and I have never forgotten... "All navigators make mistakes. The difference between a new navigator and and an experienced nav is how quickly the mistaken is caught and corrected..." Best done before the pilot even notices.
How true. Even following a trail, it is amazing how quickly I catch my diversion off of the trail. I often will stop in my tracks just two steps off the trail. I snap back from the zone I was in - as if the alarm is flashing. It's all subconscious...as if I feel it. Then I look around, take a few steps back and get back on the trail.
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Old 10-26-2012, 12:37 PM   #22
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Of course before you can draw the magnetic north lines you need to get the declination from somewhere. And yes, it does change over time. The declination printed on the bottom of some of my paper topos is 4 degrees different than the current declination for that area.

Gmap4 uses software and data supplied by NOAA to show you the current declination for the center of the map. Just click Menu ==> Declination on/off. The current declination appears in the lower left corner and updates if you change the center of the map.

Gmap4 default map: http://www.mappingsupport.com/p/gmap4.php

Gmap4 homepage: http://www.mappingsupport.com/p/gmap4.html

Joseph, the Gmap4 guy
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Old 10-26-2012, 01:51 PM   #23
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Of course before you can draw the magnetic north lines you need to get the declination from somewhere. And yes, it does change over time. The declination printed on the bottom of some of my paper topos is 4 degrees different than the current declination for that area.
Wow, you must be using some very old USGS maps, or maps that are much closer to to the magnetic poles than even those surveyed back in the 1940's here in the Adirondack region. I have to go all the way back to the 1901 era maps before I see any difference as much as 3 degrees around here. Farther north and west of us (closer to the magnetic pole), the change over time is indeed much more dramatic. Although historically useful for several reasons, I don't think many people actively navigate around here from those older maps.

There are several web sites where you can calculate declination and drift over time. NOAA has a new official site.
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Old 10-26-2012, 08:13 PM   #24
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I just messed around on that NOAA site...you can also get the actual field strengths for a particular location.
I am not familiar with the units, though.

nT?? Would that be nano Tesla?
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Old 10-26-2012, 08:32 PM   #25
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I just messed around on that NOAA site...you can also get the actual field strengths for a particular location.
I am not familiar with the units, though.

nT?? Would that be nano Tesla?
Correct you are. There's a short description relative to earth magnetic field intensity here.
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Old 10-27-2012, 09:34 AM   #26
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Correct you are. There's a short description relative to earth magnetic field intensity here.
Thanks, Wldrns...I work with many NdFeB magnets in the design of high speed PMDC motors, but with much, much higher flux densities. I also have a calibrated gauss meter (for measuring field strengths in motors and mag bearings) but it is not responsive to such low values.
Funny how I live with this every day, and never realized the absolute values of the Earth's magnetic field.

Very informative, thanks again.
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Old 10-27-2012, 10:16 AM   #27
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I also have a calibrated gauss meter (for measuring field strengths in motors and mag bearings) but it is not responsive to such low values.
But even though it can't measure absolute values, your compass is in fact responsive to such low values.
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