This is something I ran across in a video recently, and was a new one for me in regards to using a compass for navigation.

The presenter said that there could be a deviation between actual magnetic north and what a compass says it is. That hadn’t ever occurred to me, but it seems possible if the manufacturer didn’t accurately align the compass magnet(s) and the compass card (or even needle). In that case, what is indicated as magnetic north by the compass would be off by some amount.

The video therefore suggested checking one’s compass to see if there was any error between actual magnetic north and what the compass indicated.
(From this point on, keep in mind that when using map or other actual geographic features, there may be a difference between true north and magnetic north, i.e., magnetic declination, that must be factored into the readings.)

The example in the video was sighting along a true north/south road, and if, after compensating for the magnetic declination for one’s location, the compass reading is off from north/south, then the device has some error that will affect all other readings.

My examples:
Among others, I have two high quality compasses, a Francis Barker M-73 made in the UK and (sometimes) issued to British military forces, and a Finnish Suunto. Both have magnetic compass “cards” that rotate to magnetic north rather than separate magnetized needles with compass dials that can be rotated manually. The M-73 card is calibrated in 1 degree increments, and the Suunto in 0.5° increments. The fine divisions are visible because both compasses have magnification viewers.

The way I checked them today was to take azimuth readings from two points readily identifiable on Google Earth: the intersection of two roads and the sharp peak of a mountain 8+ miles away. I recorded the two readings, 203° for the M-73, and 199.5° for the Suunto. Although the Suunto has finer calibration markings, sight aiming is less precise because all it has is a short line on the unit that’s pointed at the sighting target. The M-73 has an aiming notch and wire similar to common lensatic compasses, and is therefore more precise.

The next step was to determine the current magnetic declination for the location, and that was with a magnetic field calculator on the NOAA site (see link below). The declination was shown as 8.29 East degrees and the calculator added that figure to my two readings to get true north values. The converted azimuth to the peak for the M-73 was 211.3° and for the Suunto 207.8°.

After the field work, I went to Google Earth to determine its bearing from the road intersection to the mountain peak: 210.5°. The differences between the Google bearing and what I got with my compasses were 211.3 – 210.5 = 0.8° for the M-73, and 210.5 – 207.8 = 2.7° for the Suunto. So, based on this one experiment the magnetic north determination of the M-73 is very close, and less than the calibration of the compass card. The Suunto’s difference was a bit greater, but that could have been due to the crude sighting method it’s necessary to use with it.

All this is obviously of interest only if we have very precise instruments and need precise direction determinations. It’s not something to worry about if your compass can be swallowed with the intention of recovering it later at the prisoner of war camp.

I am curious if this is something anyone else was aware of, and whether you have made any attempt to check your own compasses.

NOAA link:
https://www.ngdc.noaa.gov/geom...agcalc.shtml#bearing

When I used to go offshore sailing, we would write the appropriate deviation [declination] on the charts we were using as it varies, depending on where you are on the earth. It also changes over time, although so slowly it can be disregarded for all usual purposes.

We would also take it into account for land navigation, though IIRC it wasn't so critical over short distances. Again, there were booklets (and now, websites) that will give your appropriate measurement wherever you happen to be.

Maps normally note the deviation and you have to take that into account with your calculations.

In my teens and twenties I did a fair amount of solo backpacking in the SW part of Colorado -- the wilderness areas within the Rio Grande forest. I hiked a good portion of the Continental Divide Trail in southern Colorado. I used mid-line compasses from REI (ones with mirrors, to better sight distant objects) and USGS topo maps -- the system works. I lost the Continental Divide Trail a number of times on my hikes, as back then it wasn't marked very well in places. Map & compass work always got me back on course, albeit with moments of frustration that I lost the goat-path-like trail in places.

USGS maps list an inclination angle (true north vs. magnetic north) on each map. If there were errors in my compass or the maps, they were less than my user-induced errors.

Just so it’s clear, the video I referenced above is about checking to see if a compass actually points to magnetic north, and not about declination per se.

And of course, magnetic declination shifts over time. According to NOAA, at my location it currently shifts 0.10° west per year. I have maps dating back decades and that 0.1° adds up.

I learned compass work in the military and have used it here the Yoop. But without long lines of sight to move from one sighting to the next I find traditional compass to be a PITA. And use of a compass at night takes some doing. I use GPS (Garmin) now exclusively. And I will printout Topo maps and research my trips with Google Earth before I wander off into the bushes. All that said, I take a good quality compass with me on every trip. In the Yoop you deal less with deviation than in other parts of the country.

A couple of youse guys are using the term "deviation" incorrectly. The correct term, in the context that you used it, is "variation."

"Variation" is the difference between true north and magnetic north. This depends on your position. I'm not sure about marine charts, but for sure, aeronautical navigation charts show isogonic lines, that depict the variation. The line that has 0° variation (the locus of all positions where true north and magnetic north are the same) is called the agonic line. It does drift somewhat, it currently runs roughly from a bit west of Chicago, down through the New Orleans area.

"Deviation" is the error caused by physical factors unique to the specific compass, such as metal or electrical components in an aircraft installation, or in the case of a handheld compass it might be the enclosure. There is frequently a way to adjust the compass, to minimize the deviation. Aboard ship, there's a pair of metal spheres whose position can be adjusted -- these are known as the navigator's balls.

Every airplane that I have flown has a correction card adjacent to the magnetic compass, that shows for each 30° heading (30°, 60°, 90°, etc.) what that particular compass will indicate, taking the deviation into account.

The mnemonic that we taught for distinguishing between variation and deviation was: "True Virgins Make Dull Company." Using the first letter of each word, you can remember that True course +/- Variation = Magnetic course. Apply deviation correction to magnetic course, to arrive at the compass heading.

https://www.magnetic-declination.com/

Good site for those who want to know what their current location's declination is.

There are only a few locations on Earth where a magnetic compass points exactly to the True (geographic) North. The direction in which the compass needle points is known as Magnetic North, and the angle between Magnetic North and the True North direction is called magnetic declination.
Magnetic declination varies both from place to place, and with the passage of time.

quote:

Originally posted by bald1:

There are only a few locations on Earth where a magnetic compass points exactly to the True (geographic) North. The direction in which the compass needle points is known as Magnetic North, and the angle between Magnetic North and the True North direction is called magnetic declination.
Magnetic declination varies both from place to place, and with the passage of time.

Looks like "declination" is synonymous with "variation."

In all my years of flying, I always heard it referred to as "variation" -- I never heard "declination" used.

quote:

Originally posted by V-Tail:
In all my years of flying, I always heard it referred to as "variation" -- I never heard "declination" used.

Must be a flying thing.
Instructions with my compasses all refer to the difference between magnetic and true north as declination.

Currently I have a knock off of the honking Brunton Professional Transit. As well as an old late '90s Army issue lensatic tritium Cammenga 3H series (21-26460-02e) compass. And for small pocket jobs, I have a Recta Swiss Army Type DP6, which is the same as Suunto MB-6, and a classic Silva Type 27.

And most all YouTube videos on the subject refer to this as declination
https://www.youtube.com/result...magnetic+declination

And the NOAA site I linked above also calls it “declination,” as do things like Army map reading field manuals.

^^^^^^^^^^^^^^

Indeed

And to your query, no I've never checked to determine if my compass "arrows" in fact pointed accurately to magnetic north. Interesting notion you've raised though.

That said, further compounding any calibration such as you undertook is what is known as local attraction. Local attraction is a result of materials such as iron or nickel deposits located near the earth’s surface at a specific spot which will impact the compass. High voltage power lines, car bodies, and flashlight batteries will impact the compass too.

Within the Army we use mils or degrees. Air Force uses feet instead of meters.

FAGS (field artillery guys) use direction and deflection.

My iPhone has a built-in compass app. Analog plus digital display. I can tell it to use magnetic north or true north. It’s aware of my current position (GPS), and the magnetic declination at my position.

Must be aviation term vs. ground term. I also have always known it as magnetic variation.

Airwingers vs ground pounders

It’s variation on a chart and declination on a map.

Also, since magnetic north moves, how old are your compasses?

Large, rocky formations can and will affect a compass, especially if there is iron present.

Stone mountain here, just off the end of PDK has a note about it on the sectional.

I taught Map and Land Navigation at the Field Artillery School, at Ft. Sill, Oklahoma (1971 -1973). Magnetic declination is the angle between magnetic north and true north. It can be either east or west depending on where you are located. U. S. military maps have (had) a declination diagram in the bottom margin showing the angles between magnetic, true, and grid (UTM) north. This allowed you to set the declination on your compass (we used the M-2 in mils) for your particular location.

But wait, there's more! Magnetic north moves over time. So beneath the diagram was a note to adjust the declination angle a certain amount each year from the map's publication date. Even that was not exactly precise because magnetic north's movement, although not rapid, is somewhat random.

quote:

Originally posted by trapper189:
Also, since magnetic north moves, how old are your compasses?

I don't think that would matter. The maps/charts change, for the movement, but the needle still points to the magnetic pole. There's a lot more difference using the same compass on the East coast vs West than old vs new maps.

quote:

Originally posted by bald1:
That said, further compounding any calibration such as you undertook is what is known as local attraction.

Yes, that’s always something to consider as well.

When I took my readings yesterday I deliberately moved away from the steel signpost nearby and checked the power lines. It’s also why I ditched the magazine holders with their magnets that I once had on a “battle belt.”

As with any test, experiment, examination, it’s necessary to repeat it under different conditions to confirm the results. That’s why small sample sizes such as a single five-shot shooting group are essentially meaningless. I may check my compasses in different locations if the spirit moves me, and possibly add another one or two.

The magnetic pole does of course move, but as stated, a properly constructed compass will always point to its current location. That’s why it’s necessary to know the proper declination for one’s own location such as provided by the NOAA site. An interesting fact that I ran across is that the pole’s movement speed has increased markedly in recent times. Some people believe we’re due for a magnetic pole reversal that has occurred regularly throughout the planet’s history, and when that happens, it would have … interesting results.