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Old 09-20-2013, 09:56 PM   #1
HH1
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Backpacker - True or False

On page 43 of the October edition of Backpacker magazine, navigation specialist Molly Absolon dishes out advice to folks who "Don't Know How To Deal With Declination".

Referencing a couple of diagrams, her text concludes with an example:

"Say you're in Washington's Cascades and your map tells you declinaton is 17 degrees east. If your desired heading is 25 degrees, add 17 to follow a correct heading of 42 degrees to your destination".

Is Molly being truthful or pulling our leg?
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Old 09-20-2013, 11:49 PM   #2
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I'm not familiar with the article or the current declination for that area, but that sounds about right judging from on-line declination maps.
I guess it may depend on what year the map was printed...and if I understand your question correctly.

I'm sure a few others here might be able to help also...

Last edited by Justin; 09-21-2013 at 12:13 AM..
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Old 09-21-2013, 12:00 AM   #3
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I should have provided more detail, Justin. The article is about how to adjust your map-derived heading for your compass, depending on whether you are in area of east declination (ie. west of the Mississippi) or west declination (ie. east of the Mississippi), then getting into the add/subtract declination game.

The 17 degree east declination for the Cascades I think is fine.
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Old 09-21-2013, 03:28 AM   #4
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Quote:
Originally Posted by HH1 View Post
On page 43 of the October edition of Backpacker magazine, navigation specialist Molly Absolon dishes out advice to folks who "Don't Know How To Deal With Declination".

Referencing a couple of diagrams, her text concludes with an example:

"Say you're in Washington's Cascades and your map tells you declinaton is 17 degrees east. If your desired heading is 25 degrees, add 17 to follow a correct heading of 42 degrees to your destination".

Is Molly being truthful or pulling our leg?
I just looked up that article online. What she says is horribly confusing and inconsistent. The declination diagram she shows is correct for her location in the western U.S. But the compass she shows doesn’t make any sense with it. The compass she shows does not match the declination diagram… in the Dec diagram magnetic north is correctly shown to the right (east) of true north. But in her compass it is shown as the opposite.

Molly's declination and compass diagrams are at best inconsistent and confusing, and at worst wrong (reproduced here from Backpacking magazine). How confusing is this?:


She doesn’t specify how she references the “desired heading” of 25 degrees. Is it relative to true or to magnetic? If relative to magnetic north, then you don't need to consider declination at all on the compass. If relative to true north, then she is still wrong anyway.

So let’s start over. Always look at the declination diagram. If your map doesn’t have one, draw it in for reference. Now take an easy example for illustration. Let’s say you are in Seattle where the declination is 17 degrees east, and you want to actually walk in the direction of true north (000=360 TN). If you did that, where would your compass needle point? It would point off to your right (east), by 17 degrees.

If you “box the needle” to take you in the direction of true north, your compass would look just like the declination diagram. Compare the declination diagram to my version of the compass image shown below. The baseplate direction of travel arrow points to TN, the needle points to MN, and the rotatable dial with the orienting arrow is boxed to be aligned underneath the needle:


You hold the compass to walk toward the direction of travel arrow on the baseplate, while keeping the needle boxed. Note that this is 000-17=347 degrees as dialed in to your compass will take you in the direction of true north. You in fact dial your compass to desired True heading minus Declination (in this Seattle location case, 347 degrees). But if you simply look at the dec diagram and make your compass match it, then you don't really have to worry about doing the math in the wrong direction.

To verify this another way, say now you want to walk in the direction of magnetic north (000 on your compass). As you can see from the declination diagram, that would be 17 degrees relative to true north on your map. But magnetic north is 000 on your compass. What is that relative to true north? By simply looking at the declination diagram, MN is obviously 17 degrees to the right of true north. So to go from mag to true, add 17 to mag (in this case of the Seattle location). Once again, looking at the declination diagram provides the correct math for you as shown in my declination and compass image below:


Molly is unclear what she means by desired heading. Assuming she means as measured on the map, then it would be a true north reference heading of 25 degrees. Use the dec diagram and draw where the line would be at 25 degrees relative to TN. Is it to the right or to the left of the 17 degree MN line? To the right, of course. By how much? By 25-17=8. As you can see from the example I just gave, to walk in that direction you would set your compass to 25-17=8 degrees. 25 degrees true is 8 degrees to the right of magnetic north. If I interpret what she means by 25 degrees desired heading correctly, then Molly has made a classic 2x declination error of 34 degrees.


Note, for locations east of (roughly) the Mississippi, the declination diagram is the reverse, MN is to the left (west) of TN - the opposite of what it is in Seattle. The same principles apply, just draw out the diagram.
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Last edited by Wldrns; 10-19-2013 at 09:12 AM..
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Old 09-21-2013, 06:43 PM   #5
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Methinks the hardest part of compass instruction is trying to keep it simple. I try to teach but have a hard time keeping it simple. I am down to "the needle consistently points in one direction, all the number and verbiage relates to that fact." If you want to walk North follow the needle and if you want to walk south head in the opposite direction. When you want to get more precise take a course from Wldrns who understands it better than most and is willing to teach at any level. Like anything else it takes practice, practice, practice.
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Old 09-21-2013, 08:36 PM   #6
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Quote:
Originally Posted by HH1 View Post
On page 43 of the October edition of Backpacker magazine, navigation specialist Molly Absolon dishes out advice to folks who "Don't Know How To Deal With Declination".

Referencing a couple of diagrams, her text concludes with an example:

"Say you're in Washington's Cascades and your map tells you declinaton is 17 degrees east. If your desired heading is 25 degrees, add 17 to follow a correct heading of 42 degrees to your destination".

Is Molly being truthful or pulling our leg?
I can see the story now... "After repeated attempts to contact navigation specialist Molly Absolon to provide clarification went unanswered, co-workers realized she was several days late returning from a back country outing."
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Old 09-22-2013, 05:02 PM   #7
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This is one reason why it is good to draw magnetic north lines on your map when using an orienteering compass...it eliminates the adding/subtracting confusion.
If the declination is 17 East, draw a few lines on your map with a pen and your compass that are 17 from the left or right margin lines on the side of the map (which is True North).
Then all you have to do is align the edge of the base plate of the compass with your known starting point and your destination/way-point on your map, and then turn the compass dial until the red arrow at the bottom of the dial is parallel with your drawn magnetic north lines.
Then hold the compass out flat in front of you, and turn your body until the needle is within the red arrow at the bottom of the dial, then look up and follow the direction of travel arrow at the tip of the compass, and off you go...

If the declination is 14 west like it currently is for most of the Adirondacks, then draw lines that are 346 (360-14) from the side margins of the map.

Last edited by Justin; 09-22-2013 at 06:30 PM..
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Old 09-22-2013, 05:23 PM   #8
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This is one reason why it is good to draw magnetic north lines on your map when using an orienteering compass...it eliminates the adding/subtracting confusion.
If the declination is 17 East, draw a few lines on your map with a pen and your compass that are 17 from the left or right margin lines on the side of the map (which is True North).
Then all you have to do is align the edge of the base plate of the compass on your map with your known starting point and your destination/way-point, and then turn the compass dial until the red arrow at the bottom of the dial is parallel with your drawn magnetic north lines.
Then hold the compass out flat in front of you, and turn your body until the needle is within the red arrow at the bottom of the dial, then look up and follow the direction of travel arrow at the tip of the compass, and off you go...

If the declination is 14 west like it currently is for most of the Adirondacks,
Then draw lines that are 346 (360-14) from the side margins of the map.
Exactly right Justin. Although there are several other methods you can use, I do like this the best. I teach this as my favorite, but I don't always require my students to draw the MN lines, and I might not always have time to prepare new maps myself when I go out out a quick SAR mission in a new area. But I do draw the MN north lines on most of the maps I use most often.

These diagrams depict pretty much everything you need to know about using this particular technique of drawing MN north lines on the map and using the compass with the map:
Attached Images
File Type: jpg pg1.jpg (134.7 KB, 168 views)
File Type: jpg pg2.jpg (93.4 KB, 167 views)
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Old 09-22-2013, 06:47 PM   #9
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I think you need to write for Backpacker.
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Old 10-19-2013, 08:37 AM   #10
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Nicely stated, Wldrns, very clear and concise.

Now if we could just get the earth's magnetic field realigned to follow lines of longitude all this confusion would go away. Maybe we should talk to the Canadians, who are closer to the pole. I'm sure Neil could help on that one.

Back to being serious, I have a map of the world that is drawn with South facing up. It's a good reminder of the conventions we make up as helpful guides and their correspondence to the physical world. For anyone interested in the development of maps and the problem of navigation, read Dava Sobel's book Longitude.
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Old 10-19-2013, 09:58 AM   #11
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I was taught "West is best; East is least." In other words, you add west declination or SUBTRACT east declination to get your desired heading....

Take it easy,
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Old 10-19-2013, 12:06 PM   #12
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I was taught "West is best; East is least." In other words, you add west declination or SUBTRACT east declination to get your desired heading....

Take it easy,
desmobob
Rules of thumb generally only work when one understands all the assumptions behind them, and takes heed of those assumptions when using the rule. For example, are you starting with a true bearing and converting to a "desired heading" that is magnetic, or are you starting with a magnetic bearing and converting to a "desired heading" that is true.
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Old 10-19-2013, 03:48 PM   #13
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Rules of thumb generally only work when one understands all the assumptions behind them, and takes heed of those assumptions when using the rule. For example, are you starting with a true bearing and converting to a "desired heading" that is magnetic, or are you starting with a magnetic bearing and converting to a "desired heading" that is true.
Exactly. Many can recite the "east is least..." thingy, but in my classes few will tell you what it means, and they are just as likely apply it wrong anyway. That's why I strongly focus on the declination diagram as a physical pictorial that you can draw and keep in your head. It is easy to relate the diagram to the the parts of the compass. Understand that and you are less likely to forget what is behind the process and how to apply it to navigation. Explain it clearly and most people get it fairly easily.

After that, I find the most confusing hurdle that people learning have trouble with is knowing when to rotate the azimuth dial, and when to turn the compass baseplate as a whole. It is an indication of incomplete understanding of the process, so this part needs careful explanation. When they don't know what to do, they most likely will grab the dial and rotate it to....(?)... their eyes glaze over and scratch their head in a state of confusion. I tell them that when you turn the dial to some directional azimuth (either on the land or on the map), the compass makes an angle measurement of direction and stores that number in its "memory". What you do next depends on if you want to make use of that direction stored in memory (rotate baseplate only), or erase the previous measurement and save a new different direction in memory (rotate azimuth ring). Do a few examples of each and it should make sense.
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Old 10-19-2013, 04:19 PM   #14
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I don't do the add or subtract thing because you can't do math when you're hypothermic. I prefer to just rotate the bezel by a set number of lines. Direction dependent upon the desired effect. True to magnetic or magnetic to true. And using the magnetic/true diagram as a guide and knowing the mag n pole location
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Old 10-19-2013, 05:27 PM   #15
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How about the compasses where the declination for your location is preset on the compass? What are the pitfalls to that?
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Old 10-19-2013, 05:43 PM   #16
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How about the compasses where the declination for your location is preset on the compass? What are the pitfalls to that?
Nothing wrong with that method. But that adjustment might not be an option on inexpensive (but otherwise quality) compasses that many people have. Of course you have to remember to change the adjustment when you travel to another area with a different declination. And I have seen people make the adjustment in the wrong direction on the compass, giving them a 2x declination error. Once again, referring to the declination diagram compared to the compass would come to the rescue.

With the declination adjustment on the compass you don't need to draw magnetic north lines on your map, but you would need true north lines instead. If the map has UTM grid lines, you could in most cases use the north/south grid line as a TN reference, but just know that it can be as much as 3 degrees off from TN in the lower 48 states (usually much less). If you are primarily using terrain observation skills rather than blindly following a compass heading, a 3 degree error may not be significant. DEC Rangers and SAR generally use this method and all directions are relative to TN (but not necessarily with every ranger, so always check to be sure).
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Old 10-19-2013, 06:08 PM   #17
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How about the compasses where the declination for your location is preset on the compass? What are the pitfalls to that?
A good idea if you are always hiking in the same locale. Else a royals pis. For me hikin in the cats one weekend and ME north woods the next would mean many adjustments

The magnetic grossing of the map approach
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Old 10-20-2013, 10:54 AM   #18
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How about the compasses where the declination for your location is preset on the compass? What are the pitfalls to that?
I'm almost always bushwhacking in the same area. My compass has been corrected for 14*W declination for so long, when I go somewhere out of the area to hike, there's a real possibility I'll forget to change it. I try to maintain the the routine of looking at the declination adjustment before putting the compass lanyard around my neck, every single time I put it on.

Take it easy,
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