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Thread: Got caught by the change in declination

  1. #1
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    Got caught by the change in declination

    I had read that Magnetic pole is moving impacting declination but I really hadn't let is impact backcountry navigation until yesterday. I was following my very hard to follow AT boundary section and kept ending up off the line. The monument tablets are usually about 900 yards apart unless the terrain is steep. where they will chop up the line into shorter pieces and set an intermediate monument. I had the points plotted in my GPS but usually use my compass to stay on line. I was on a long straight bearing slabbing a steep downhill slope so its quite tough to stay on line to look for blazes. Whenever I looked at my course plot I would be on straight line to the east of my intended route on a fairly straight line. I would then drop down slope and would usually pick up a blaze and then would take another bearing.

    My AT boundary maps call out true bearings and when I had corrected the bearing to magnetic I had my compass correction set to 18 degrees which used to be accurate for the area around Andover Maine. I checked the current declination today and its closer to 15. Therefore all my bearings were 3 degrees east of where the actual boundary ran. On 900 yards the error is 236 feet which is enough to get way off a faded intermittently blazed line.

  2. #2
    Senior Member TCD's Avatar
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    Good reminder!

    But you have to check that a little more often...based on the NOAA map, it looks like that change from 18 to 15 took about 40 years to happen...

    https://maps.ngdc.noaa.gov/viewers/h...l_declination/

  3. #3
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    Hmm...my map adventures map of the Whites from 2014 has a declination of 17

  4. #4
    Senior Member TCD's Avatar
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    Changes all the time. I like paper maps, but they don't keep up with those kinds of changes very well. The NOAA site is probably the definitive resource.

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    Senior Member ChrisB's Avatar
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    Quote Originally Posted by peakbagger View Post
    ... all my bearings were 3 degrees east of where the actual boundary ran. On 900 yards the error is 236 feet which is enough to get way off a faded intermittently blazed line.
    Peakbagger, on a good day my bearings aren't close to 3 deg on! Ten? Maybe!

    I'll tweak declination when the poles switch again.
    Nobody told me there'd be days like these
    Strange days indeed -- most peculiar, mama
    .

  6. #6
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    One of challenges of following boundary lines is there are 3" aluminum disks glued to ledge or set on top of short pipe in the ground every 900 yards. Hard to find even with perfect bearings and no hobblebush.

  7. #7
    Senior Member Nessmuk's Avatar
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    Be as careful as you will, holding a line without witness points to within 2-3 degrees on a consumer grade compass is a battle not to have your track look more like a random walk. Have you ever XC skied across a lake or wide open field and looked back at what you thought was skiing in a perfectly straight line? The number of zigs and zags is amazing.

    When traveling point to point, most experienced navigators will rely mainly on terrain association with map as the primary method, and use the compass as a general guide to get started and as occasional reference along the way.
    "She's all my fancy painted her, she's lovely, she is light. She waltzes on the waves by day and rests with me at night." - Nessmuk, Forest and Stream, July 21, 1880 [of the Wood Drake Canoe built for him by Rushton]

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    Quote Originally Posted by Nessmuk View Post
    When traveling point to point, most experienced navigators will rely mainly on terrain association with map as the primary method, and use the compass as a general guide to get started and as occasional reference along the way.
    I understand, and have some limited experience, using terrain association with map as the primary method to bushwhack to a big targets like slides, ponds, waterfalls, back to a trail, etc. But the OP is trying to locate a series of points with very small markers.

    Isn't terrain association too gross a method for such small targets? You could be 10 feet away and not see the marker.
    Last edited by Tom_Murphy; 09-16-2019 at 09:13 AM. Reason: I can't spell.

  9. #9
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    Running boundary in the Maine woods is a challenge at best. The general concept of the AT relocation effort in Maine 30 years ago was to get the trail up on the ridgeline and out of productive timberland. Prior to the relocation very little of the trail was "owned in fee" it was mostly over privately owned predominately timber lands. The western Maine AT was mostly on active logging roads running parallel to the ridgeline and due to fire regulations tended to go from sporting camp to sporting camp as cooking fires were banned on private land without the permission of the landowner or if a Maine guide was present. Maine private lands are under the right pass doctrine where the public is presumed to be allowed to pass over private land unless it is specifically posted and that worked for the AT up until the seventies. After a fairly famous lawsuit over township common lots which the timber companies lost, they owed the state lots of money. The state was in the mode where the fairly new land conservation movement was starting to rise up and the solution was trade money owed for land. The timber companies didn't want a strip of AT running over their prime timberland so they agreed to sell land to pay down the debt with the understanding that the AT would be relocated up onto the ridgeline. This led to a string of Maine public reserve lands along the new AT route and then the National Park Service stepped in and bought corridor to connect the state owned lots. The general concept was to protect a swath of land on top of the ridges for the AT. The lines were laid out on paper with little regard for the actual topography. If the topo is steep they will put intermediate tablets usually at 300 yards but I have learned that is good indication that I will be crawling up a steep slope hanging from trees or having to leave the line and then reacquiring it at the top of a cliff.

    In my AT boundary section the boundary line runs roughly on the edges of the summit "plateaus" so the boundary is slabbing steep slopes and occasional cliffs. Keeping a straight line point to point means holding tight bearings using faded painted blazes to get back on line. I have a sighting compass I normally use but it doesn't have built in declination so I used my plate compass that does. I agree land navigation is far easier shooting for terrain features but running lines is what I agreed to do. The tablets are supposed to have witness trees but in some areas it was regenerating woodlands left over from the last timber cut so they trees were small and didn't survive and in that case finding a most likely buried tablet is darn close to impossible without resurvey.

    I actually was on mission to find a missing tablet and lugged my surveyors metal detector up the mountain and then bushwhacked through the woods with an 18" pole sticking up out of my pack which made for tough going. I spent a couple of hours of searching and it remains missing. Luckily its a point on line instead of a corner. Not the first time I have determined that the original survey crew selected by the NPS on lowest bid took shortcuts. Considering the terrain and it being pre GPS, its still an impressive piece of boundary work. The claim is the AT in Maine has a longer boundary than Yellowstone National Park.
    Last edited by peakbagger; 09-16-2019 at 10:33 AM.

  10. #10
    Senior Member Nessmuk's Avatar
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    Quote Originally Posted by Tom_Murphy View Post
    I understand, and have some limited experience, using terrain association with map as the primary method to bushwhack to a big targets like slides, ponds, waterfalls, back to a trail, etc. But the OP is trying to locate a series of points with very small markers.

    Isn't terrain association too gross a method for such small targets? You could be 10 feet away and not see the marker.
    "point" taken, and true enough. It is the nature of the beast. Without using professional surveyor's methods and very expensive equipment (both on establishing the original track and on the re-finding of markers) it is a difficult task. It would not be unusual for some people to be 2 feet (or less) away and never see a marker on a tree they pass by. On a bushwhack, an error of 10 feet rarely matters at all unless you are close to a dangerous drop.
    Last edited by Nessmuk; 09-17-2019 at 09:41 AM.
    "She's all my fancy painted her, she's lovely, she is light. She waltzes on the waves by day and rests with me at night." - Nessmuk, Forest and Stream, July 21, 1880 [of the Wood Drake Canoe built for him by Rushton]

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