Marking declination lines on a map

Discussion in 'Other Skills' started by Bob_Spr, May 20, 2019.

  1. Bob_Spr

    Bob_Spr Supporter Supporter

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    I recently watched John McCann's video demystifying declination. He suggested marking your declination on your map and indexing from that instead of the true north lines. He said to verify the declination graphic on your map is correct. I checked mine, which is supposed to be 5 degrees but it is only 2. If I want to mark my map correctly, where do I put my protractor to mark my 5 degrees? The bottom right corner?


    John' video
     
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  2. riokid87

    riokid87 Scout Banned

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    I found it simple enough to just add or subtract the G-M angle. I don't think drawing new grids on the map would work for me. Too much chance for error.
    I didn't watch the 19 minute video. Maybe you could summarize the advantage of drawing new grids?
     
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  3. halo2

    halo2 Curmudgeon in Training Supporter

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    My understanding, but you should verify since I'm week on M&C, is to index off of the map grid lines; that is, grid lines are MN so set your declination angle then use the orienting lines superimposed over the grid lines and draw from the baseplate straight edge.

    That said, declination also changes over time so depending on when your map was made, its listed declination could be off the current declination for that area.

    Have you considered a compass that's adjustable for declination?
     
    Last edited: May 20, 2019
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  4. slysir

    slysir Supporter Supporter

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    Most maps already have the protractor declination at the bottom right. I agree with @halo2 to not put the lines on the map because declination changes. Just set your compass. If your compass isn't adjustable "East is least. West is best".

    -John
     
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  5. Bob_Spr

    Bob_Spr Supporter Supporter

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    The idea is to draw declination on your map so you do not have to remember to add or subtract when you plot a course. By doing do, you do not have to change the declination or remember to change the declination on your compass (if you can change it). Also, if you use the marked map and orient it with the drawn lines, it's easier to match? the terrain. Example, I am on a hill and am looking north according to my compass. The next place I want to go is a my 4:00 o'clock. I line up on it and see it is 111 degrees from MN. I can easily orient my map with the drawn lines, shoot an azimuth of 111 and plot my course, without math. Or if I make notes while traveling, I can easily transfer the notes to the map based on the drawn lines instead of having add/sub declination when transcribing my notes.

    My primary compass is a MC2 global, which has adjustable declination. But the other compass I use a lot is a military lenstatic - no adjustments. I was thinking of using maps with the drawn in lines so I could use either one interchangeable with the map and not have to remember to add/sub.

    Unfortunately, what the protractor on the map says and what it measures with other protractors is different.
     
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  6. riokid87

    riokid87 Scout Banned

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    So true North is the North pole where the earth rotates on its axis. On military maps it is depicted with a star because the North star ,Polaris, is very close to true north. Considering the distance to Polaris vs the relatively small distance from the s to n pole, the slight difference between the n pole and Polaris is inconsequential for navigating such relatively short distances.
    Mag north is direction mag compass points and can change
    Grid north is the grid lines on your map. I think the diff has something to do with the earth being a sphere and our maps being flat.
    Anyway that's how I remember it from my land nav classes in the army back before GPS and Google maps.
     
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  7. riokid87

    riokid87 Scout Banned

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    To
    To orient a map to mag North, we just set the map on something flat, but not a metal vehicle hood, lined up the edge of the compass on one of the longitude lines, then rotate the map and compass until the compass needle pointed to o degrees on its face.
     
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  8. leghog

    leghog Guide

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    I just adjust the declination on my compass. This is set for 10 degrees West, the local declination where I stomp around.

    [​IMG]
     
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  9. JOttum

    JOttum Nights Watch Supporter Bushcraft Friend

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    If the declination isn't drawn accurately on your map it's not a big deal as long as YOU know how many degrees to add or subtract when going fro. Map to compass.

    For example: you've found that the your area is 5 degrees East of Magnetic North. Just adjust your compass and calculations as such. Doesn't matter if the little angle printed on the map is accurate.

    I hope I haven't confused you more.
     
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  10. slysir

    slysir Supporter Supporter

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    I realize you are trying to make things as easy as possible. Only you can decide which method works best for you. But it's so simple to take a bearing off the map with true north, then if you're 5 degrees east then turn the bezel 5 degrees counter clockwise. If you're west turn the bezel clockwise. You're now good to go!!

    -John
     
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  11. JOttum

    JOttum Nights Watch Supporter Bushcraft Friend

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    you said that way better than I did!
     
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  12. DuctTape

    DuctTape Scout

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    I have always used magnetic north on my maps. We would draw a few lines with pencil. Why use true N grid lines and adjust for declination when one can simply orient to magnetic north lines?
     
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  13. snapper

    snapper Guide

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    I'm with Duct Tape on this one. I've always adjusted my maps to take magnetic north into consideration. Also, while no expert, in post #4 above John stated to remember that "East is least and West is best" but that doesn't work everywhere in the USA. You need to be on the "correct" side of the magnetic north line for that to work. In many parts of the county it's the exact opposite; which is why putting magnetic lines on your map will work for whatever side of the line you're on.

    That's all for now. Take care and until next time...be well.

    snapper
     
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  14. slysir

    slysir Supporter Supporter

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    Explain. You are either dead nuts on true north or you are east or west of it.

    -John
     
    Last edited: May 23, 2019
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  15. leghog

    leghog Guide

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    If you are going to use the declination diagram on the map, check the age of the map data. The depicted declination could be off by quite a bit. Better is to check the declination at NOAA before heading off.

    https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml?useFullSite=true

    I'm fairly certain he was speaking of easterly declination and westerly declination.

    I always remember EMGA (Easterly declination, Magnetic to Grid, Add).

    If EMGA, then:
    EGMS (S being Subtract)
    WMGS
    WGMA

    EMGA. It's been burned into my memory for more than four decades.
     
    Last edited: May 23, 2019
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  16. snapper

    snapper Guide

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    John - Check out what leghog has said above. He did a better job of explaining it than I did. I'm also going to try and find a link to a map that came with my old Silva compass that shows what I'm talking about.

    That's all for now. Take care and until next time...be well.

    snapper
     
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  17. slysir

    slysir Supporter Supporter

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    There may have been some misunderstanding on what the old phrase "east is least, west is best" means. It's a saying to help remember whether to add or subtract to adjust for declination.

    Here's where it gets confusing. If you are east of true north, you adjust for west declination. Meaning you add your declination to the bearing. West of true north you subtract.

    East declination you subtract=least
    West declination you add=best

    But as I stated in an earlier post, all you have to do if you don't have an adjustable compass is just rotate the bezel.

    @leghog post #8 is a good visual. He shows that his declination is 10 degrees west. So his orienteering needle is set 10 degrees plus zero=best.

    I truly hope that people just trying to figure this out aren't getting more confused!! :rolleyes:

    -John
     
    Last edited: May 27, 2019
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  18. Glenn Rowe

    Glenn Rowe Supporter Supporter

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    Okay, I've done this a lot. On any USGS map the True North line is always accurate but the Magnetic North line may not be. Check and verify the current declination before doing this.

    1. Carefully extend the True North line out far enough that it can be used with the biggest protractor that will fit on the map. Bigger ones are easier to read and use, and therefore more accurate.

    2. Using the True North line as Zero (obviously), locate the correct degree of Declination on the protractor. Mark it on the map with a dot.

    3. Carefully draw a line back from the dot to the base of the True North line (the spot where the Magnetic North line first branches off).

    4. Verify with your compass and you're done.

    Henceforth, at least until Magnetic North moves again, you can use the Declination Line(s) to orient the map to Magnetic North, and use the compass without having to consider/adjust for the declination.

    Back in the day, for maps we used a lot, we'd use a parallel ruler to locate and draw declination lines in several key places on the map.
    https://www.draftingsteals.com/2123...hXJWdIG56ke1FC8Kjz_Olc5Od2kVQMtgaAuIaEALw_wcB
     
    Last edited: Jun 17, 2019
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  19. Yknpdlr

    Yknpdlr Scout

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    I have taught basic and advanced land navigation for more than 30 years, including to Homeland Security, Law Enforcement, EMS, SAR, and licensed wilderness guides. I have always used the draw magnetic north lines on my maps, especially for my personal backcountery travels.

    It is true that the declination has changed over time, and the MN pole seems to be in runaway mode in recent years. However, as checked recently, the declination in my area has changed by only a degree to a degree and a half over all the years I have been using the method, so even my early drawn maps are still quite useable. I have over 300 USGS maps in my collection, very many have MN lines on them and I have no intention of throwing them away.

    By the way, if using this method do not simply extend the short MN line as drawn on the map in the declination diagram. The USGS will tell you that the angle as drawn is only representative of the actual angle, especially for small angles. Extending that short MN line will almost certainly result in error. You must use a grade-school protractor and a long straight edge placed on the edge of the map (the TN line) to be accurate enough. Draw the first line along the protractor set to the declination angle and you can fairly accurately shift the straight edge over by eye to create a set of parallel MN lines across the entire region of interest or the entire map. Works great, try it. Don't forget that your compass is really a protractor and can be used as such in a pinch. But a real protractor will give you a longer and more accurate starting reference line.

    By the way, I find those sayings such as : "east is least, west is best", "LARS", "EMGA", etc. only lead to confusion. Students easily get confused because going from compass to map is the opposite application of going from map to compass. From which way do you add, when do you subtract? It is easy to figure it out from first principles depending on which reference angle you begin with, and which reference angle you want to end up with. I simply use my two arms to tell me which angle is larger, which is smaller. I turn myself into the declination diagram. For example, in my area, MN is west of TN. I stand with my right arm pointing straight up, my left points slight left. I imagine a landscape object in the first quadrant to my right, say around 45 degrees. The method works regardless of the actual 360 degree angle I am converting. If I am going from a MN (compass reference) angle to the TN (map reference), it is obvious that it is a larger angle from my left arm than it is from my right arm. That will tell me whether to add or subtract, depending on which reference angle I am converting from to the other.

    In more advanced versions of teaching, I offer 4 methods of going from map to compass and vice versa. Some methods may sound complex, but a little practice goes a long way to understanding what is happening. Absolute understanding of reference angles and the declination diagram is essential. Going through a few examples should clear things up in any of the given methods.

    1) Orient the map to earth, as riokid87 suggests above. Initially do this by visual orientation of the map with visible landscape features, which alone may be enough to navigate by the method of "terrain association" without the need to apply the compass to the map. This is the only method in which you use the magnetic needle when the compass is on the map, hence does not work near magnetic materials, such as on a car hood, or a table top with metal supports or screws. All azimuths will be relative to MN on map and with compass alone on earth. No need to add or subtract anything. All the other methods use the compass only as a protractor with map reference lines when the compass is on the map. This method is inconvenient without a fixed level flat unmoving surface (difficult to do in a moving canoe). Must be free of nearby magnetic materials.

    2) Draw magnetic north lines on the map, as discussed. No need to worry about nearby magnetic materials. "Ignore the magnetic needle when compass touches map". All azimuths, as measured either on map or on earth with compass alone will be relative to MN. No need to add or subtract anything. Map need not be oriented to earth, works in any random orientation. Nearby metal has no effect (ignore the needle when compass is on the map).

    3) Use the grid north lines already drawn on the map. In my area, GN happens to be offset from TN by a small fraction of a degree, so I can assume it is actually TN. In mid-latitudes the difference can be as much as 3 degrees near the edge of a grid zone, so that the "grid declination" can easily be accounted for if necessary. Reference on the map is relative to TN. After measuring the angle on the map, or when going from earth reference (MN) to map reference (TN), add or subtract to change the compass bezel azimuth as required (use the arm method). No need to worry about nearby magnetic materials. "Ignore the magnetic needle when compass touches map". Map need not be oriented to earth, works in any random orientation. Nearby metal has no effect (ignore the needle when compass is on the map).

    4) Use a compass with a declination adjustment. If the orientating arrow is correctly rotated within the bezel, the compass itself will resemble the declination diagram. All angles measured will be referenced to TN both on map and on earth with the compass alone. No need to add or subtract anything. But you have to remember to re-adjust the compass if you travel very far to a new area of declination. No need to worry about nearby magnetic materials. "Ignore the magnetic needle when compass touches map". No need to add or subtract anything. Map need not be oriented to earth, works in any random map orientation. Nearby metal has no effect (ignore the needle when compass is on the map).

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
    Last edited: Jun 27, 2019
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  20. Yknpdlr

    Yknpdlr Scout

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    This is still very confusing. What does "you are east of true north" mean? If I am in an area with west declination (generally east of the Mississippi River (approximate location) zero agonic line) and use my compass on a randomly oriented map as a protractor to measure an azimuth relative to TN, then I must add, not subtract the declination to the compass bezel when I pick the compass off the map and use it in the field.

    However, oppositely, if I use my compass to measure an azimuth to a landscape feature in the field, then I must subtract declination from the measured MN azimuth when plotting the azimuth on the map as a protractor to get to a TN reference.

    No declination adjustment is necessary in either direction in the cases of having MN lines drawn on the map (method #2 in my post above), or if the map has been precisely aligned with earth (method #1 in my post above). In both of those cases all measured azimuth angles will be with respect to MN and can be used directly without conversion.
     
    Last edited: Jun 30, 2019
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  21. Nathan H

    Nathan H Tracker

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    In the British army we used the saying "Mag to grid get rid"

    Meaning when taking a compass bearing we would subtract the correct declination (declination angle add the yearly angle since the map was produced until today's date

    and Grid to mag add meaning a bearing from the map to out compass we would add the correct declination.

    I have to point out I'm in the UK and therefore slightly east of magnetic north.

    Im not 100% sure if this would work in the USA as some of you will be west of magnetic north
     
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  22. leghog

    leghog Guide

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    Regardless of where in the world , just remember E-M-G-A. Which means Easterly declinate, convert Magnetic to Grid by Adding declination angle.

    Therefore, if EMGA, then:

    EGMS (S is subtract declination angle)
    WGMA
    WMGS

    Just remember EMGA.
     
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  23. Yknpdlr

    Yknpdlr Scout

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    Why not just look at and understand fundamentals of what the declination diagram is actually telling you, because if you can't remember the silly sayings, you risk getting the translation wrong by double the declination angle.

    The compass is simply no more than a protractor, people. A protractor measures angles relative to a specific defined baseline starting line. We happen to have two possible reference baselines to transfer from one to the other (sometimes, only when necessary). If you know which baseline you start from and which baseline you are going to with a different base reference angle (which is not always necessary, depending on which map with compass method is used), it is obvious when and what to add or subtract from what. Forget the silly sayings, which only add a confusing and potentially disastrous step. Why bother trying to remember an unnecessary intermediate step when you can better focus on basic information. I say again when making the translation: starting from observing the declination diagram,"which angle from the one baseline to the other is larger, which is smaller" , this will make the add or subtract decision for you.
     
    Last edited: Jul 11, 2019
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