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Author Topic: real north versus magnetic north  (Read 8565 times)
K3GC
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Posts: 118




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« Reply #15 on: February 29, 2012, 06:37:51 PM »

My Iphone gives me true or magnetic Smiley
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WB6BYU
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Posts: 13334




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« Reply #16 on: February 29, 2012, 07:08:10 PM »

But of course noon local time may not correspond with the civil time in your timezone.

A faster version of the stick method is to mark the tip of the shadow, wait a while (an
hour is probably long enough if you started with a tall enough stick) and mark the tip
of the shadow.  Those two points should form a line East/West.  Then draw a line
perpendicular to it to show North.  At least it was close enough for Boy Scouts.
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AK7V
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« Reply #17 on: March 01, 2012, 08:06:49 AM »

Many compasses have an adjustment that can be done to compensate for the offset.  I set my compass up for my QTH (determining offset in degrees from a website), and as long as the needle lays within the outline (follow your compass instructions), the compass itself points true north.
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KA5N
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« Reply #18 on: March 01, 2012, 08:41:17 AM »

If you have a plat survey of your property, the nice surveyors have indicated true North
with an arrow. 
You don't even have to go outside.
Allen
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KC9TNH
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Posts: 304




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« Reply #19 on: March 02, 2012, 04:27:51 AM »

I think the procedure is to wait until exactly noon local time and then the stick's shadow will point true north - provided you are in the northern hemisphere.
I'll be home havin' coffee in the morning while you're waiting for both of Mickey's hands to be straight up.  Grin

1. Place a stick into the ground at a spot where it will cast a shadow. Mark the shadow's tip with a stone, twig, or other means. This first shadow mark is always west--everywhere on earth. (picture it, draw it out, whatever you have to do)

2. Wait 20-30 minutes until the shadow tip moves a few inches. Mark the shadow tip's new position in the same way as the first.

3. Place your left foot on the first shadow tip. Place your right foot on the second shadow tip. You are now facing north. This is true everywhere on earth.

* There is a JAG officer (attorney) version of this with 74 PowerPoint slides and lots of case law but so-called "primitive" people have been taught & found able to grasp this method for a long time.  If you have further questions please post them & I'll rope in my grand-daughter to explain it...

An interesting thing occurred once on a simple day-flight up to Phillips for a great breakfast at a little off-airport diner. There are areas north of here that will make your compass spin-around like a circus carousel.
Lots of ore in the ground.
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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
K0JEG
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Posts: 669




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« Reply #20 on: March 02, 2012, 01:19:40 PM »

I think the procedure is to wait until exactly noon local time and then the stick's shadow will point true north - provided you are in the northern hemisphere.


At least until UTC stops tracking the Earth's rotation...

http://en.wikipedia.org/wiki/Leap_second#Proposal_to_abolish_leap_seconds
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K6AER
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« Reply #21 on: March 02, 2012, 02:05:35 PM »

Just hit HERE on your GPS and walk 180 degrees south. Turn around and look at the spot where you hit HERE. That is true north.
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W8JX
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« Reply #22 on: March 02, 2012, 02:09:39 PM »

Many compasses have an adjustment that can be done to compensate for the offset.  I set my compass up for my QTH (determining offset in degrees from a website), and as long as the needle lays within the outline (follow your compass instructions), the compass itself points true north.

Actually those adjustment are not really for that.  They are meant for tweaking it when it is installed in a car, truck or airplane to compensate for disruption in magnetic field cause by vehicle. It is designed to be calibrate to magnetic north and you use the deviation in your are when you navigate by it. If you tried to tweak it to work properly in your area it would become very unpredictable in another area for accuracy. Best and easiest way to be sure locally is with a hand held GPS that can plot position and directions based on true north.
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VE9AAE
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Posts: 31




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« Reply #23 on: March 05, 2012, 07:55:11 PM »

"Actually those adjustment are not really for that.  They are meant for tweaking it when it is installed in a car, truck or airplane to compensate for disruption in magnetic field cause by vehicle. It is designed to be calibrate to magnetic north and you use the deviation in your are when you navigate by it. If you tried to tweak it to work properly in your area it would become very unpredictable in another area for accuracy."

Actually, if it is a standard handheld compass (say a Silva or Suunto) then that is exactly what the adjustment is for.  The needle will always point to magnetic north, if you are using a map then you would adjust the declination based upon the annual shift for your area and the age of the map (generally calculated for grid north as you would be using the grid lines when setting bearings, etc).

Unless you have a compass designed for use with a vehicle, it is advised that one move several meters away from the. Veh or your bearings will be off.

As far as 'tweaking' it for your area, that is what you should do, and it will be quite accurate unless you have gone a long distance (at least across the state/province), but making those adjustments are just part of normal navigation SOPs.

GPS is a wonderful tool (especially in the arctic, where your compass is not that useful and a local Innuit guide comes in handy) but the basics will always be there when all else fails............pun intended!
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W8JX
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« Reply #24 on: March 05, 2012, 08:20:32 PM »

Actually, if it is a standard handheld compass (say a Silva or Suunto) then that is exactly what the adjustment is for.  The needle will always point to magnetic north, if you are using a map then you would adjust the declination based upon the annual shift for your area and the age of the map (generally calculated for grid north as you would be using the grid lines when setting bearings, etc).

If you are hard core using a map and compass you will calibrate compass to MAGNETIC north and use map that shows deviation for area you are in. To do otherwise would require constant recalibration.   Many many year ago we used to check and calibrate the accuracy of the compasses in large military aircraft. All bases had a place called  "compass rose" for these calibrations and they were calibrated to magnetic north and pilots/navigators used the correction for area they were in at time. You do it this way because you have a constant, magnetic north that you reference and use map correction. When you try to tweak compass you loose your constant reference of what you are sure of.  


GPS is a wonderful tool (especially in the arctic, where your compass is not that useful and a local Innuit guide comes in handy) but the basics will always be there when all else fails............pun intended!

While I still carry a compass in field/woods in pack, I have not really used one for over 15 years now.  My main handheld GPS also has a backup electronic mag compass. In the woods and terrain I travel a GPS gives me a repeatable accuracy not possible with a compass.  
« Last Edit: March 05, 2012, 08:22:45 PM by W8JX » Logged

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WB6BYU
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Posts: 13334




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« Reply #25 on: March 05, 2012, 08:31:05 PM »

Quote from: W8JX

If you are hard core using a map and compass you will calibrate compass to MAGNETIC north and use map that shows deviation for area you are in. To do otherwise would require constant recalibration.   Many many year ago we used to check and calibrate the accuracy of the compasses in large military aircraft. All bases had a place called  "compass rose" for these calibrations and they were calibrated to magnetic north and pilots/navigators used the correction for area they were in at time. You do it this way because you have a constant, magnetic north that you reference and use map correction. When you try to tweak compass you loose your constant reference of what you are sure of.  



I regularly use Orienteering maps that are aligned to magnetic North.  The problem is that
Magnetic north keeps moving relative to the local terrain.  Easier to adjust the declination on
a compass than to reprint all your maps every few years.

If you are in an airplane, then you might need to readjust your declination periodically because
you are covering long distances.  But for a hiker on foot, adjusting it every few years, or when
they travel to another part of the country, is adequate.  (That's why the Orienteering maps are
magnetic - so they don't have a problem with everyone remembering the declination.) 

The GPS is better in poor visibility because it will lead you to a point, rather than in a direction.
But for standing in one place and taking bearings to plot on a map, I find the compass is better.
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VE9AAE
Member

Posts: 31




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« Reply #26 on: March 05, 2012, 09:00:53 PM »

W8JX,
Valid points for an aircraft I'm sure, but it's probably a safe bet the majority following the thread are dealing with land navigation/orientation.
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K0IZ
Member

Posts: 737




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« Reply #27 on: March 06, 2012, 06:17:25 AM »

I used the stick and solar noon method to align my antennas and solar panels.   Very precise.
1.  Select a straight stick or pole, about 3 ft long.
2.  Pound stick into ground a few inches.  Align stick to be perfectly vertical using a level.
3.  Enter your longtitude in solar noon calculator to get time adjustment for solar noon vs time zone noon.

http://www.solar-noon.com

4.  Get time zone noon for your month and day from calculator.  At my place in Colorado, for example, time zone noon (for solar noon) will vary from about 11:45 to about 12:15, depending on date.  If you are near an edge of a time zone the difference will be greater.
5.  At the prescribed time zone noon, mark the shadow from the stick.  That is exactly true north.
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WB6BYU
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Posts: 13334




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« Reply #28 on: March 06, 2012, 07:18:12 AM »

Why bother with a stick?   You've got the tower right there, and it should be plumb.
Just mark the tip of the shadow at local solar noon and aim the boom of the antenna
at your mark.
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W8JX
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Posts: 6038




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« Reply #29 on: March 06, 2012, 07:50:02 AM »

The GPS is better in poor visibility because it will lead you to a point, rather than in a direction.
But for standing in one place and taking bearings to plot on a map, I find the compass is better.

If you use a better hand held GPS with a built in electronic compass it will act just like a compass standing still if you want. My GPS has Topo Maps in it too.
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