declination correction on compass

[adimo]

I see a lot of talk about adjusting declination on the compass and I am not really sure why is it so important. Instead, why not do this:

Know your local declination. Put the map on the ground, put the compass along the map grid and bring the needle to the N marking. Turn the map E or W to the local declination and keep it fixed. Now find your destination and read the bearing from the compass. Sight that bearing and follow it in the field.

Why not orient the map instead of messing with the compass?

 

[leghog]

Setting the declination on a compass is just faster. Set the declination once and be done with it. Otherwise you must add or subtract the declination every time when converting between grid and magnetic azimuths.

Brunton makes compasses with tool-less declination adjustment. Some are VERY inexpensive. Their USA-made TruArc3 has tool-less declination and a global needle and can be had for about 10 bucks.

 

[gadgetgeek]

adimo, its a good skill to have, but a lot of people don't trust their math as much, so they prefer a set and forget method. And if you are using the map instead of doing the math, there is a greater risk of adding in small errors. But as long as a method works, go for it.

 

[Bob W]

I really don't mind the small screwdriver tool for adjusting declination on the Suunto M3 Leader compasses. The action is more smooth and sealed than the tool-less models I've tested.

"Set and Forget" beats the he11 out of fiddling with the map every single time. Most times I take a direction reading I'm not even looking at the map.

For example, if I'm walking "North to the road," checking the map is unnecessary. Conversely, if I'm going "around the west side of this ridge into the canyon below," I use the map frequently to study topo lines while the compass stays in the pocket.

As often than either of those scenarios, I'm not using the map or compass regularly. I don't care where I'm at.

And for the record, I'm not one of those people that needs to physically orient the map on the ground, lol. I've seen people do that. Believe it or not, I've also seen people turn the road map every time the car goes around a corner. Turn two corners and the map's now upside down! Seriously, I've seen people do that.

In New Mexico you'd have to drive six hours for the declination to change only 2 degrees. Up north the declinations are closer together so you might have need to change it more often.

But most people aren't going to change the declination that often. If I live at 13d declination and go for a hike where it's 11, 12, 14, or 15... why bother messing with it? Unless you're in a competition or using the compass for survey work, a few degrees difference isn't going to matter.

 

[leghog]

"Set and Forget" beats the he11 out of fiddling with the map every single time. Most times I take a direction reading I'm not even looking at the map.

Whether you set the declination on the compass or simple add/subtract, there is no more "fiddling with the map every single time" one way or the other. As an aside, afield, you'll likely refer to the map MUCH more often than ever referring to the compass.

 

[leghog]

If I live at 13d declination and go for a hike where it's 11, 12, 14, or 15... why bother messing with it? Unless you're in a competition or using the compass for survey work, a few degrees difference isn't going to matter.

Depends on the distances travelled. A few degrees can make a huge difference, even when deliberately aiming off.

 

[pinnah]

The other approach is to draw declination lines on your map, but that's a hassle. I use an adjustable compass and won't go back.

 

[Maven]

If you have a compass with adjustable declination or if you mark the bezel ring with your declination theres no "messing with the compass" Unless you travel far enough from your "home" isogonic line youll always be accurate, youll never forget to orient the map or do the math to compensate for declination and be off by however many degrees your declination is.

So in my area Im looking at 12.25degW. Forget to adjust declination with the map on just one bearing and your off by more than a half mile (2,800feet!) in just a single 2.5mile leg of your travels.

 

[Maven]

Depends on the distances travelled. A few degrees can make a huge difference, even when deliberately aiming off.

1 degree over one mile is 92ft(rounded)



[Distance traveled X sin(degree error/2) ] x 2 = distance off.

 

[RussMo]

Were I go 4-wheeling/hunting in Alaska it's as much as +19 Deg, 21'. You better know where your going cause you will be WAY off on a 25-50 mile back country hunt

 

[leghog]

Depends on the distances travelled. A few degrees can make a huge difference, even when deliberately aiming off.

1 degree over one mile is 92ft(rounded)



[Distance traveled X sin(degree error/2) ] x 2 = distance off.

One degree aint squat. A few degrees can be, particularly if additive to the 1-3 degrees of compass resolution. Can really be a problem when deliberately aiming off.

 

[Maven]

One degree aint squat. A few degrees can be, particularly if additive to the 1-3 degrees of compass resolution. Can really be a problem when deliberately aiming off.

Yeah, one degree over a mile is 92 ft. Not a real big deal for general travel by map. 90 feet off and you could visually find your actual destination .But forget to adjust for declination just once in a high error area and you're way off. Don't adjust on one 1/2 mile long bearing in my area and you're off by well over 550 ft. Just in a single leg. 5 miles and you're more than a mile off from where you wanna be.

 

[leghog]

Yeah, one degree over a mile is 92 ft. Not a real big deal for general travel by map. 90 feet off and you could visually find your actual destination .But forget to adjust for declination just once in a high error area and you're way off. Don't adjust on one 1/2 mile long bearing in my area and you're off by well over 550 ft. Just in a single leg. 5 miles and you're more than a mile off from where you wanna be.

Declination here is 10 degrees West.

But even setting the declination, if your compass is 2 degree resolution, and you aim off 3 degrees, and you are off just a degree that 92 feet may now be 600 feet. And you won't know if it's 600 feet to your left or your right which negates the whole purpose of aiming off to begin with. Sure you'll find what you're looking for, but you'll also lose time doing it which again negates why you aimed off to begin with.

 

[John A. Larsen]

Declination here is about 22 degrees easterly, big problem if you do not adjust for it. I use a Suunto compass that you can set the declination as when tired you may not remember to adjust for it. As far as trying to hit a specific spot dead on, I would rather do an "Off Set" where I know that I am right or left of the target, works best if there is some definite land or man made feature, that when you get there, you know to turn right or left and walk to the target. Did that a lot, remember looking for a trail hut on the Appalachian Trail about 0300 one winter night at about -20 degrees, did not want to have to wander up and down the trail, looking for the hut. Off Set to the North (moving East to West) hit the trail, turned South and found the hut in about 5 minutes. John

 

[leghog]

Declination here is about 22 degrees easterly, big problem if you do not adjust for it. I use a Suunto compass that you can set the declination as when tired you may not remember to adjust for it. As far as trying to hit a specific spot dead on, I would rather do an "Off Set" where I know that I am right or left of the target, works best if there is some definite land or man made feature, that when you get there, you know to turn right or left and walk to the target. Did that a lot, remember looking for a trail hut on the Appalachian Trail about 0300 one winter night at about -20 degrees, did not want to have to wander up and down the trail, looking for the hut. Off Set to the North (moving East to West) hit the trail, turned South and found the hut in about 5 minutes. John

"Off Set' and aiming off are the same.