Followers 0

# Deviation, Declination, Dip, Magnetic & True North?

## Recommended Posts

Ok, my terms are messed up, and searching past threads didn't get me what I wanted. A while back the topic of true vs. magnetic north came up, and several ways to measure it were presented. One was particularly clever. How can I measure the difference with the maximum accuracy? Can it be done to 0.1 degrees or better, even though my compass is a cheap little 2" job? (I know I can't use that accuracy, but just wondered what's possible)

Ok, my terms are messed up, and searching past threads didn't get me what I wanted. A while back the topic of true vs. magnetic north came up, and several ways to measure it were presented. One was particularly clever. How can I measure the difference with the maximum accuracy? Can it be done to 0.1 degrees or better, even though my compass is a cheap little 2" job? (I know I can't use that accuracy, but just wondered what's possible)

I usually print out Topozone maps of the local area I'm exploring. These are nice since since they give thne current declination, rather than the one printed on the map (which tend to be out of date).

Here in the NE area we use the acronym AMC (Add to Map for Compass). It works here since magnetic north is west of true north. And the AMC happens to be a regional hiking and advocacy group, so it's easy to remember.

As for accuracy - what's the point? I'm lucky to read a degree from my compass. If I add 17 degrees (a typical local declination value) I still can't read better than a degree. I also doubt that the values of declination can be easily found to be accurate to .1 degree (since it constantly changes). You would practically have to get a value for each day!

The main thing to visualize is the little pair of arrows on the bottom right of a map. Remember which side of true north the magnetic arrow is. If you can't visualize east and west, heck, just think right or left.

Edited by Papa-Bear-NYC

Photobuff -

Here is the NOAA site that provides the declination amount.

If you still want to measure it yourself, one way is to use a PID that has an azimuth station. Get someone to hold a vertical target at the AZ mark and sight to it from the main mark with your compass. The AZ mark's bearing in the NGS datasheet is based on True North.

Clicking on Topozone from a geocaching benchmark page, like Papa-Bear-NYC said, seems to be the easiest thing to do.

I have one of these Suunto KB14s and it's excellent for benchmark hunting. It is graduated to ½ degree and has both backsight and foresight numbers on the scale.

Try using the LARS rule. LARS equals Left Add, Right Subtract. Starting from the type of azimuth you currently have, move towards the type you want, then apply the correction based on the direction of the movement.

Here is a nice Worldwide view I have used when traveling international, just so I have a rough idea ahead of time (assuming I have hiking / geocaching plans).

Here is a Good Info website, but with too many pop-ups! Wikipedia also has a good section.

Very, very simply:

If you have a decent map, but no benchmark handy, and know here you are on the map:

1) Pick a distant target (mountain top, etc.).

2) Determine your true bearing to the object from the map (draw a line on the map from your position to the object).

3) Measure (by eye) the magnetic bearing to the object with your compass.

4) Subtract 3) from 2) You may get a negative number. This is your Magnetic Declination.

5) If it is a positive number, it is west declination, and should be added to a magnetic course to get a true course. "East is Least but West is Best"

This will be rough, obviously, but it works.

Also, don't forget that any GPS receiver that has an electronic compass built-in (and maybe all GPSr's??) has the worldwide magnetic declination table built-in. On my Magellan Meridian Platinum it is simple to set the unit down (away from metal!) pointing to magnetic north, then without moving it, change it to display true heading. That might be pretty darn accurate, at least well within a degree, anywhere in the world (except northern Canada!).

Well I never use a compass.

I have one in case of emergency.

Before GPS I used TOPO Maps.

I was taught from time knows when to always be oriented in all situations.

Knowing your directions at all times.

But I always know where North is.

Now the variations,deviations,declinations...

Each has a way of coming to a closer relative position.

By Astronomic Observations.

ASTRONOMIC OBSERVATIONS

By Magnetic Observations.

MAGNETIC OBSERVATIONS

By Declination Charts.

ISOGONIC CHART

DECLINATION

Hey all, thanks- I'm working my way through the links. Been kinda scarce lately as my wife managed to fall and break her arm, so I'm in caregiver mode. One of lifes unplanned events! Afraid it's also put a crimp on benchmarking for the time being.

Anyone remember these??

Can Dead Men Vote Twice At Elections

True Virgins Make Dull Companions At Weddings

D= Deviation (compass error)

M= Magnetic

V= Variation

T= True

W= Westerly

If I'm using a paper map I'll put a line on it that represents Magnetic North

Then orient the map with a mag compass & ferget the rest of the math.

It's quick & simple & it'll get ya within a degree or two.

For *most* practical applications that's close enough.

If ya really want to split hairs get a compass that reads in "Mils"

6400 Mils = 360 degrees or 17.77777 mils per degree

Are we confused yet.....

JW

E=Easterly

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.