true north compass deviation?

[forestgnome]

True north is 17 1/2 degrees to the right(east) of magnetic north. Does this hold true everywhere on Earth? I was thinking about what a compass might do on the equator or on a pole, now my brain hurts Thanks

 

[NH_Mtn_Hiker]

No, it can vary significantly depending where you are on the planet.

For example:
Magnetic Deviation across North America
Another:
Another Deviation map

Note: These are both dated 1990 so they are probably off a degree or two.

 

[SAR-EMT40]

Like NH_Mtn_Hiker said. Not only is it different all over the planet it also changes at any given location slightly from year to year. It is always listed on the USGS 24K topo maps what the declination is at the center of the map for the year that the map was made. I don't have a map in front of me but I believe it also lists what the change per year is also.

Keith

 

[NH_Mtn_Hiker]

Like NH_Mtn_Hiker said. Not only is it different all over the planet it also changes at any given location slightly from year to year. It is always listed on the USGS maps what the declination is at the center of the map for the year that the map was made. I don't have a map in front of me but I believe it also lists what the change per year is also.

Keith

It actually varies a little from day to day as does the amount it varies. The magnetic poles are in constant motion. The magnetic pole in the northern hemisphere (incorrectly called the north magnetic pole) moves as much as 15 miles a year generally towards Siberia.

 

[SAR-EMT40]

It actually varies a little from day to day

Yes, but the map only lists the yearly change.

Keith

 

[diehard]

Estimated Value of Magnetic Declination

Here's a web site which might help you determine the correct value of magnetic declination. You can plug in, both your location and the date

http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp

Jim cahoon

 

[moonrock]

Short answer: Magnetic north pole is located in Northern Canada. So declination is westward from the east coast, and eastward from the Rockies & west coast. Declination also increases, the farther north you go.

Until it "flips", when all bets are off for a few years/decades. As it is now, anomalies in near-equatorial parts of the world are strengthening. But whether it'll happen this century is very debatable.

Moonrock

 

[NH_Mtn_Hiker]

moonrock wrote:
...Magnetic north pole is located in Northern Canada...

I knew someone would bite.

In magnetism, opposites attract and likes repel...right. Therefore, the south magnetic pole is actually the one that's in the northern hemisphere, otherwise the north needle on your compass would point the other way, wouldn't it.

It's true, you read it here on the internet.


Edited for clarity.

 

[Remix]

Dont forget that there could be significant local deviations due to ore deposits. Some member, I believe, wrote about issues on the NY side of the Champlain valley. I believe areas of significant distortion are noted on the maps.

 

[Mark]

Hopefully, this isn't too dumb of a question, but why don't cartographers draft maps with vertical lines that point to magnetic north instead of geographic north? Then I wouldn't have to remember whether I need to add or subtract the deviation printed on the map. (I'm sure when I'm lost and hypothermic, I will get this wrong.)

 

[MichaelJ]

The flip isn't instantaneous, and while it is flipping, the earth's magnetic field, which protects us from all sorts of stellar badness, will be greatly weakened. Depending on who you read, either we will need to stay inside for a while, or all life on earth will be wiped out by electromagnetic storms boiling off the atmosphere, when this happens.

It's interesting reading - Google around, keeping a mind's eye on who you're reading for a source.

 

[Papa Bear]

Like NH_Mtn_Hiker said. Not only is it different all over the planet it also changes at any given location slightly from year to year. It is always listed on the USGS 24K topo maps what the declination is at the center of the map for the year that the map was made. I don't have a map in front of me but I believe it also lists what the change per year is also.

Keith

Note that although Topozone duplicates maps of varying age, they publish current magnetic declination for that location. If you use the "print this map" option you will find it at the bottom right of the page.

 

[Tom Rankin]

Hopefully, this isn't too dumb of a question, but why don't cartographers draft maps with vertical lines that point to magnetic north instead of geographic north? Then I wouldn't have to remember whether I need to add or subtract the deviation printed on the map. (I'm sure when I'm lost and hypothermic, I will get this wrong.)

It certainly could be done that way. (But remember as it was said already, it's a moving target). Over time, the convention has been to align North with the spin of the Earth's axis. Celestial navigation is based on this orientation. This also has the benefit of making the Sun rise in the East and set in the West (at least at the equator).

 

[HikerBob]

Then I wouldn't have to remember whether I need to add or subtract the deviation printed on the map.

Most likely (hopefully) your compass has an adjustment for declination so it's simply a matter of setting it to the current value for the location you are hiking and you can forget about it.

Bob

 

[Stan]

Hopefully, this isn't too dumb of a question, but why don't cartographers draft maps with vertical lines that point to magnetic north instead of geographic north? Then I wouldn't have to remember whether I need to add or subtract the deviation printed on the map. (I'm sure when I'm lost and hypothermic, I will get this wrong.)

One reason is that the magnetic pole drifts, hence the long term changes in declination (called variation on nautical maps which also have a compass rose showing both true and magnetic north). True north pole is a constant, notwithstanding our slight wobble. Maps would have to be reoriented periodically, good for sales perhaps but more confusing in the long run.

Another reason is that maps are used by more than just hikers with magnetic compasses. Instruments such as GPS and gyroscopes don't depend on the earth's magnetism for direction.

There are mnemonics to help remember the adjustment. Mohamed suggested a good one on a similar recent thread over at AMC ... AMC= "Add Map Compass". This only applies where there is westerly deviation and means that after you lay out your true course you add the declination.

Another mnenomic is "east is least (subtract), west is best (add)", referring to the fact you add westerly declination and subtract easterly declination.

It is the opposite going from a compass reading to a plot on the map.

You're right, Mark, a situation of distress is not the time to fuss with this stuff. That is why it is good practice to plot and lable your courses, out and back, on your map in the comfort of your kitchen for bushwhacks and for escape routes.

P.S. Though sometimes used to describe declination (also known as variation on nautical maps), "deviation" in compass terminology actually refers to the distortion of the reading caused by metal, say in a ship, your car or your belt buckle. Those familiar with nautical navigation think of the sequence Compass, Deviation, Magnetic, Variation, True ... which is remembered as "Can Dead Men Vote Twice". Now, here's the quiz, is there a sailor out there who'll tell us the opposite memory aid for True, Variation, Magnetic, Deviation, Compass?

 

[solo_hiker]

another mnemonic

It is the opposite going from a compass reading to a plot on the map.

Before I got a set-and-forget Silva, I could never remember what was what with declination. So I made up this mnemonic for myself:

"Maps Tell Almost Everything"

which stands for "Mag To True Add East". This is all you need, since it's the opposite for west declination, and you just reverse this relationship for true to magnetic, ie, True To Mag Subtract East. So from the one mnemonic, all four rules can be deduced.

Interestingly, a few years ago I tried to get the editor of Backpacker to put this in their 'know-how' section. But he said no since it only applied to east declination, and you'd need another rule for west. Hmmmmm......ok. :-0

Hope it helps. Ciao Solo

 

[Peaks]

Orienteering

Hopefully, this isn't too dumb of a question, but why don't cartographers draft maps with vertical lines that point to magnetic north instead of geographic north? Then I wouldn't have to remember whether I need to add or subtract the deviation printed on the map. (I'm sure when I'm lost and hypothermic, I will get this wrong.)

Try orienteering. These maps are drawn to magnetic north.

 

[linehant]

>>...the magnetic pole drifts, hence the long term changes in declination <<

Indeed... there's an interesting chart in Sierra Club's Land Navigation Handbook showing declination for London England between 1600 and 1980:

Year Declination (deg)

1600 8 E
1650 1 E
1700 7 W
1750 18 W
1800 24 W
1850 22 W
1900 16 W
1950 8 W
1980 6 W

If someone had set the declination adjustment on their Silva compass in 1600, fallen asleep for 200 years, and then forgot to reset the adjustment before setting off on a hike in 1800, they probably did not end up exactly where they'd intended.

 

[dave.m]

If someone had set the declination adjustment on their Silva compass in 1600, fallen asleep for 200 years, and then forgot to reset the adjustment before setting off on a hike in 1800, they probably did not end up exactly where they'd intended.

Same would be true for the old, defunct Tates compasses, which were known to be wildly inaccurate. I'm sure you've heard the old proverb...

"He who has a Tate's....






Bwwaaahahahahahaha

 

[KayakDan]

On a somewhat related tack,why don't land maps use a "compass rose" as used on nautical charts? I navigate by magnetic north when kayaking(sight of land-legs >5nm)and it's dead simple. Why not do the same on land?