Angle of declination

[Umsaskis]

This Christmas I got a new AMC White Mountain Guide, and I was comparing the maps with my very much older guide, as well as an even older (20-years old) guide. I was surprised to see that, in the two older guides, the angle of declination listed on the maps was 16.5 to 17 degrees. However, on the new maps, the angle of declination is 15-15.5 degrees. I know the magnetic north pole moves around a little bit over time, but does that affect our angle of declination from year to year? Or were the older maps incorrect?

 

[TJsName]

Oh yeah, they are on thr move. The earth's magnetic field is actually getting ready to flip. Cool.stuff!

 

[Cat in January]

What do you mean getting ready?

 

[HH1]

As a good reference and to calculate your exact mag. declination, go here:
http://www.ngdc.noaa.gov/geomag/declination.shtml

 

[TJsName]

What do you mean getting ready?

Well, as much as the science can predict this far. It can take a thousand years to do it, so I would still generally trust your compass.


http://www.physics.org/facts/frog-magnetic-field.asp

 

[Stan]

Oh yeah, they are on thr move. The earth's magnetic field is actually getting ready to flip. Cool.stuff!

This is an excellent example of relative motion ... the magnetic field is remaining the same, it's just that we're all flipping out!

As for declination ... sometimes referred to as "variation" in the nautical world ... many maps (called charts in the nautical world) state the annual change and direction of the change. As a practical matter for hikers using a compass, it can be ignored since your reading need not have that degree of precision and accuracy within a few degrees is generally sufficient as you would rely equally on other things to establish your position and the direction you need to take to get to your destination (e.g. topography, contours, altitude, geographic features ...)

On the other hand, if you are triangulating a few mountain tops to work out a position then the accuracy becomes important because the difference of a few degrees over several miles can lead to significant error. I've never triangulated while hiking and I don't know anyone that has but I have laid out direction to major peaks along a bushwhack route (while in the comfort of my home on a flat surface with parallel rules and a compass rose and no wind blowing) in order to check my track along the way.

 

[John H Swanson]

The movement on the magnetic North Pole has a different effect depending upon your position. So the declination change might be one or two degrees as you note. But in places in northern canada you might need the declination for the current year. I once saw a map showing the historic position of the np quite interesting. It might be available on one of the links

 

[peakbagger]

For most folks if they not surveyors, they don't notice the difference in the Whites year to year. The absolute easiest way to check if you have a map is stop at a known point on a map and then shoot to another known point (like the top of a mountain). The difference is the actual declination. If you are really picky, surveyors sometimes maintain sighting courses they use to check their transits. I think there is one in Fryeburg and one in Portland but I have no idea how to find them. Generally most folks are lucky with a baseplate compass to stay within 10 degrees. A sighting compass http://www.princetonwatches.com/sho...=SS002110011&gclid=CMeu2r6ahbwCFa4-MgodH1AA0A used with care can keep you plus or minus 1 degree. I use one on for my AT boundary work.

 

[Tom Rankin]

A quick calculation shows that the error per degree per mile is about 92 feet. So, if I was traveling 3 miles to a distant point, and I was 5 degrees off, I would miss the mark by about a quarter of a mile! So, being accurate is important. And as others have said, correct often, based on clues you get along the way.

 

[peakbagger]

We used to take boy scouts out with a sighting compass in winter on a do a four or five mile closed course. With a bearing compass they usually were within 50 feet on approximately 1 mile traverses. We were going point to point so measuring distance was not a factor.

I have done some bushwhacks over the years where we were in dense spruce fir along a long flat ridgeline with few distinguishing characteristics where we had to estimate distance and then turn an angle to another bearing, in that case any compass error was overwhelmed by the error in distance. In those situations there is a lot to be said for a GPS. That is pretty rare as generally there are plenty of topo features that can be used for an end point of a compass run so estimated distance error isn't an issue. Where it is handy is when the estimated distance in the field far exceeds the expected distance from the map, which means you are most likely lost or on the wrong compass bearing.

 

[Nessmuk]

A quick calculation shows that the error per degree per mile is about 92 feet. So, if I was traveling 3 miles to a distant point, and I was 5 degrees off, I would miss the mark by about a quarter of a mile! So, being accurate is important. And as others have said, correct often, based on clues you get along the way.

The trigonometric "small angle approximation"... An easy rule of thumb is to figure approximately one unit of cross track error for every 60 units of track traveled. Here is a graphic that use to explain what happens if you ignore declination, or have a small compass error. (click to enlarge)

 

[sardog1]

I've never triangulated while hiking and I don't know anyone that has but I have laid out direction to major peaks along a bushwhack route (while in the comfort of my home on a flat surface with parallel rules and a compass rose and no wind blowing) in order to check my track along the way.

You need to get out of the New England forest more often. It's a fairly useful technique out West and in Alaska.

 

[RoySwkr]

A couple of notes here:
Yes the declination does change with time but not very fast around here, and it is different but not by much in different parts of the Whites. If you also go to Baxter Park or the Adk the difference is worth noting.

Many maps are not quite oriented to true North (DP can no doubt explain why with typical map projections only one parallel line on a map can point exactly to N) and the same is true of the UTM grid. On typical USGS maps of the Whites, grid N is different from true N by about a degree.

I have done "triangulation" but more commonly "unangulation", that is if I'm following a featureless ridge I shoot to a known point off to the side to see how far along the ridge I've progressed - because I know (I hope what ridgeline I'm on, only one compass bearing is needed and any error in using declination will put me at the wrong place. But a lot of my compass work uses direct bearings, i.e. from somewhere I see the car parked in a log yard and set the bearing on the compass to head for it. I don't need to know the declination or even the numerical value of the direction I travel, but just follow the compass arrow.

 

[Nessmuk]

You need to get out of the New England forest more often. It's a fairly useful technique out West and in Alaska.

In the Adirondacks, especially outside of any broad vistas offered from well exposed higher elevations, it is can be rare to get well placed identifiable distant terrain suitable for triangulation. If you are in such a place, most likely you know where you are anyway. What is useful in that case it the ability to use the reverse method... you can identify those visible distant peaks with map and azimuths to them from your known current location.

Elsewhere, on the other hand, you don't really need the classic equally spaced 3 distant objects for triangulating lines of position, or 2 objects at the optimum 90 degrees for resection of lines of position to your location. One azimuth measurement taken off a single known distant land feature will do if you know that you are standing on a known linear feature, such as the edge of a stream, river, lake, trail, or long ridge line. The compass azimuth is one line of position, crossed with the line feature you are standing on as the second crossed LOP. Most often when bushwhacking this is all you will get, but it is enough to learn your location along that feature. Combine that knowledge with observing the immediate surrounding terrain, correlate it with the map, and you have your pinpoint location identified.

 

[Nessmuk]

But a lot of my compass work uses direct bearings, i.e. from somewhere I see the car parked in a log yard and set the bearing on the compass to head for it. I don't need to know the declination or even the numerical value of the direction I travel, but just follow the compass arrow.

True enough. Any way you do it, be cautious of simply following that compass arrow. I used to say that after a map or direct terrain measurement with the compass, I don't even need to know what the azimuth is, just follow the arrow. But now I always make mental note of what azimuth my compass is set to. Then just occasionally glance at it to be sure the dial has not accidentally been turned. I learned that the hard way one time during a particularly difficult bushwhack through dense young spruce.

After putting compass to map and dialing in the azimuth, I never paid attention to what the actual azimuth numerical angle was. After some time of crawling through the blowdown and dense spruce, observations with the terrain I was in were not making sense with what I expected to have happened by now, even at my slow speed. I stopped to remeasure my planned course from my last known location. Sure enough, my compass dial had gotten turned by more than 30 degrees by the vegetation, and I had been following that course in error for who knows how long. I made a best estimate of where I could be, and headed for probable known terrain to get reorientated. That points out what I tell my students... you are allowed one navigation mistake at a time. Catch it early and it is easily correctable. Make more than one mistake and in nearly-featureless terrain your problems can multiply exponentially.

 

[DougPaul]

After putting compass to map and dialing in the azimuth, I never paid attention to what the actual azimuth numerical angle was. After some time of crawling through the blowdown and dense spruce, observations with the terrain I was in were not making sense with what I expected to have happened by now, even at my slow speed. I stopped to remeasure my planned course from my last known location. Sure enough, my compass dial had gotten turned by more than 30 degrees by the vegetation, and I had been following that course in error for who knows how long. I made a best estimate of where I could be, and headed for probable known terrain to get reorientated. That points out what I tell my students... you are allowed one navigation mistake at a time. Catch it early and it is easily correctable. Make more than one mistake and in nearly-featureless terrain your problems can multiply exponentially.

This is one of the reasons that I carry a map, a compass, and a GPS. While I can navigate using map and compass alone (and did so for many years), the GPS adds redundancy and after an error, can give you an accurate new heading as long as you have a good target waypoint and a reasonable current location fix. (For instance, when conserving batteries, I have turned my GPS on long enough to get a bearing, turned it off, and then followed the course indicated by the bearing using my compass. Had I wandered from that course for any reason, I could have trivially obtained a new bearing and course.)

Definition note:
* Bearing: a heading pointer toward a location. If you move toward either side of a direct line toward the location, the bearing will change to compensate.
* Course: The course from points A to B is the straight line between them. It does not change as you move around and you can be to one side or the other of it while traveling between the points.


BTW, there is a nice map of the wanderings of the magnetic north pole here: http://en.wikipedia.org/wiki/North_Magnetic_Pole and http://en.wikipedia.org/wiki/File:Magnetic_North_Pole_Positions.svg It has wandered ~1000 miles over the last century.

Doug

 

[sardog1]

One azimuth measurement taken off a single known distant land feature will do if you know that you are standing on a known linear feature, such as the edge of a stream, river, lake, trail, or long ridge line.

Handy if you're on your butt in a seat in the Yukon Flats (and you've kept track of the meanders ...). Not so handy if you're standing on the shore of one of the three million lakes in the Great Land.

 

[Nessmuk]

This is one of the reasons that I carry a map, a compass, and a GPS. While I can navigate using map and compass alone (and did so for many years), the GPS adds redundancy and after an error, can give you an accurate new heading as long as you have a good target waypoint and a reasonable current location fix. (For instance, when conserving batteries, I have turned my GPS on long enough to get a bearing, turned it off, and then followed the course indicated by the bearing using my compass. Had I wandered from that course for any reason, I could have trivially obtained a new bearing and course.)

Right, but don't forget about the terrain itself adding redundancy. That's what told me something was wrong in my case. At least in this part of the world, "featureless" densely vegetated terrain does not stay completely static very long before subtle changes to slope and other changes give away clues to where you are. If you add the knowledge of expectation of what you are about to encounter, or should encounter as you travel, that is a very powerful navigation aid. Add in other clues such as the sun when it is visible, and even of this day's known weather and wind direction, or observation of pine tree growth pattern caused by prevailing wind direction, all these and many more are valuable natural aids to navigation. People have long paid attention to these navigation clues along with correlation to map and compass. I fear that the art of doing so is becoming lost with too many folks (though certainly not all) relying way too much on the singular focus of using GPS as a must-have crutch, rather than as an additional aid with other skills in their navigation toolbox.

Definition note:
* Bearing: a heading pointer toward a location. If you move toward either side of a direct line toward the location, the bearing will change to compensate.
* Course: The course from points A to B is the straight line between them. It does not change as you move around and you can be to one side or the other of it while traveling between the points.

Other confusing terms with these that are often incorrectly used interchangeably:
* Heading: The actual direction you are "heading" or moving toward. May or may not coincide with bearing or course.
* Azimuth: Simply the measured angular direction, a numerical value, to some thing or place. Does not imply a bearing or course or heading until you plan or begin to move in one of those directions.

 

[Nessmuk]

Handy if you're on your butt in a seat in the Yukon Flats (and you've kept track of the meanders ...). Not so handy if you're standing on the shore of one of the three million lakes in the Great Land.

Another one of my sayings to students is: "you have to know where you are before you can know where you are". Take that to any level you want... if I were to be blindfold dropped into some unknown wilderness and I had absolutely no clue which lake shore I was standing on, it would be rather difficult to determine my location from an azimuth of a non-unique shaped hill in the distance. Tell me that I have hiked in a known direction of some miles to encounter a lake on the map of known shape and size, then I can hypothesize which of those hills I see is which on the map, and act on that until where I think I am makes sense with where I have been and what I can now see. Or... if what I see is as unique as Mount Rushmore or some other absolutely unique bluff, then the rest is easy.

The point is, as it was when I was a navigator for the Air Force using very limited instruments, you begin with a dead reckoning position with as much accuracy as you can get (meaning - pay strict attention to direction, speed, and time from where you know you have been to where you are going). Then work from that assumed position to refine your exact location with whatever aids and visuals you can muster.

As an aside, I did rely very heavily on the GPS as the handy tool that it is when in the Yukon Flats, but that was during competitive races. Losing the GPS would likely mean losing the race, but not losing my way. I also had an accurate map of the permanent land and islands (not all of them are so permanent) that I constantly visually correlated with the map. In a few cases with planned shortcuts on the river, the land had been eroded so much in the two year gap since I was previously there, that the GPS would have taken me to into wrong channels and over newly formed shallow shoals. Eyes always open and brain engaged. And... absolutely keep track of each and every meander I paddle by.

 

[sardog1]

Another one of my sayings to students is: "you have to know where you are before you can know where you are". Take that to any level you want... if I were to be blindfold dropped into some unknown wilderness and I had absolutely no clue which lake shore I was standing on, it would be rather difficult to determine my location from an azimuth of a non-unique shaped hill in the distance.

When I taught navigation, I often started the first lesson with an imaginary flight to Alaska. I told the students I'd give them a parachute and equipment and supplies to keep them alive for a couple weeks. They'd be blindfolded on the trip up there. One by one, I'd remove the blindfold and push them out of the plane over Alaska, guaranteeing a safe landing.

First question: "What do you do with the compass I give you?" Some would want to go south, some north, some east. In each instance, I'd point out that depending on where I dropped them, they'd be long dead before they came across any sign of human activity, including any road. The right answer? Throw the compass away– it's just extra weight that serves no purpose in this scenario if you don't have a map or least some prior knowledge of the terrain.