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Azmuth And Distance


Buggy5151
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And I assume the person means from True north.

I'd assume the person meant magnetic. If I expected someone to take a bearing using a compass, I'd use magnetic since not everyone knows about or has a compass that will adjust to local declination.

 

Of course in my area, the declination is less than 1°, so I don't worry either way.

 

Jamie

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I would hope that the cache owner meant True North, but I would not assume it. The owner should list which he used. I have done caches where the owner did not state, and one where he did not know there was a difference, even when it is significant (13d E declination.)

When you find out which is used, you can do a projection in your GPSr, or use your compass in the field. If your compass does not have a declination adjustment, put a mark at the declination and line the needle over that mark and read true on the Az ring scale.

To determine the declination at your location, use your GPSr. Some GPSrs show you the local declination when you set the North Reference to Magnetic (like some Garmins.) For others, look at the two bearings from your location to a waypoint with the reference set to True and then Magnetic. Subtract the two bearings and you will get the declination. If the True bearing is larger, the declination is East.

 

Have fun.

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Good answers from all. The problem seems to be semantics. Both bearing and azimuth seem to mean the same thing, but are different.

 

1) Bearing: This is an angular direction measured from one position to another using reference lines. Generally used to indicate a magnetic or radiometric (in aviation) reference line.

 

2) Azimuth: The horizontal angular distance from a reference direction, usually the northern point of the horizon, to the point where a vertical circle through a celestial body intersects the horizon, expressed in degrees. Generally measured from position of Polaris (North Star) on the horizon in the Northern Hemisphere (aka True North). We'll let the Aussies explain the Southern Cross in another post. :(

 

Ok, this is where things get confusing. I'm really trying to keep this simple. Bearing is commonly used to refer to a magnetic heading (direction), while azimuth is commonly used to refer to a true north heading (direction).

 

Topo maps are oriented to true north, so if you draw a line between two points, and schlepp a compas on it, you've got an azimuth. Unfortunately our planet's magnetic fields don't align perfectly to Polaris, and wiggle all over the place in invisible isogonic lines. We have to correct from a map by applying this isogonic difference (referred to in another post as the "deviation"). Isogonic lines either have an easterly or westerly correction (someone pointed out topo maps list this at the bottom). For easterly, going from true to magnetic direction, you add the deviation. Subtract for westerly.

 

Alas! Unless you've got a really fancy (and expensive!) GPSr and set it up to correct for all of this magnetic deviation, your GPSr doesn't care about all of this, and utilizes True North (which would be... ta-da, an azimuth!). On the flip side, contrary to what I saw mention in another post, there isn't a floating magnetic compass out there that can correct to true north (I've only seen one very, very, very expensive electronic compass that does this, at least in my lifetime).

 

I'm betting the cache creator is a stickler for such finer points of definitions (doesn't interchange bearing and azimuth as most folks out there do), and means azimuth (what your GPS knows). Set your GPS in compass mode and follow the numerical azimuth given at the cache redirect.

 

Or, email the cache creator and ask them what in the daylights they were talking about. :lol:

 

Whew. I'm sure I just confused things even more. I'll take my newbie "found 8 caches" self and go hide under a rock again (though I've been orienteering for 20+ years).

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And I assume the person means from True north.

I'd assume the person meant magnetic. If I expected someone to take a bearing using a compass, I'd use magnetic since not everyone knows about or has a compass that will adjust to local declination.

Normally I’d agree with you. But technically, when not specified, true north is to be assumed. Because this chap used the term azimuth I make the assumption he has some knowledge in this area, and thus is more likely to know that true north is to be assumed.

 

Of course this isn’t necessarily so, but in the absence of more information it’s the guess I would make given his use of the more uncommon term.

Edited by Thot
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I would hope that the cache owner meant True North, but I would not assume it.

Wouldn't you have to assume something to continue with the cache?

 

Of course if you want to wait for a reply you could write the owner and ask.

 

Usually I won't have read the details that carefully until I'm on the hunt. If I don't make an assumption I'd have to give up, go home, declare a DNF, email the owner and wait for a reply.

 

My choice is to make my best guess and give it a try. If I don't find it, then try the other possibility. As it turns out the declination is small near me so it may not matter.

Edited by Thot
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Thot:

You are correct, I would assume True north first. For one cache, this is what I did. Then I had to do the bearing again using Magnetic North to find the cache. I think it is in better form to specify when writing the cache description, which is what I do and did for 3 of my placed multi-caches which involve bearings, projection and triangulation.

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An easier way to find your area's declination, look at any topo map. It's noted at the bottom left of the sheet. You don't have to buy one, just look at one in the store.

There is no need to even go that far to get it. When I switch my GPSr to read magnetic, it gives me the declination. I can switch to magnetic, get the declination, and then switch it back to true where it is normally. Quick, easy, and accurate. What could be better and easier.

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When I was a little tyke, growing up in the midwest, I spoke the same kind of English as everyone else in town. Of course I was shocked to find out that folks from other parts talked "funny" or just plain wrong. Eventually, I learned that there wasn't just one right way.

 

Likewise with the word "bearing". An orienteer and a boy scout seem to have learned that a bearing is always relative to magnetic North. But to be unambiguous, you must specify your reference when giving a bearing. The reference can be anything. For example, traditionally bearings used in a vehicle such as a ship or a aircraft are given relative to the heading of the vehicle. Yes indeed, when a pilot reports traffic at two o'clock (a bearing), it is in reference to the pointy end of the plane, not true North, and not Magnetic North.

 

Unless you've got a really fancy (and expensive!) GPSr
My GPSR is the cheapest one I could find at the time (A Merigreen) and it can be easily set up to read either magnetic or true.
there isn't a floating magnetic compass out there that can correct to true north
I'm not sure what you mean by "correct to true north" but my Silva Ranger 15T has a little screw where you dial in the magneric variation and then N is always true North.
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Magnetic declination

 

From this picture you can have over a 20 degree differance in a bearing.

 

You need to know if they took a mag bearing or a true north bearing.

 

In the northeast, the compass needle points more than 20 degrees west of true north, while in the far west, it errs by as much as 21 degrees east. Since one degree of compass error equals 92 feet per mile of ground error, you must take this difference account when you navigate.

 

;)

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Ok,This is what the Cache says to do ! I'm sorry if I didn't give all the facts.

This should help.Thanks!

 

(This is an offset cache. Collect information at the first location that indicates where the cache is hiding. Directions to the cache is given by azmuth and distance. )

 

(The cache is located DISTANCE from the cache on a heading of 335 degrees true. ) :o

 

Thanks for all the help.

Edited by Buggy5151
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Except maps give the declination including any magnetic anomelies in the area. Which is one reason declination lines aren't nice smooth, even lines when shown on a map.

 

USGS quad 24,000 topo maps are quite old in many cawses. The declination given on the map are often way off. Your GPS is a better indicator as mentioned above as it gets its info from recent data.

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Bug,

 

As you can see, the cache owner was kind enough to supply the word "true," which tells you which "north" to use. You then can use your GPSr, set to true, or a compass if you know how to adjust from magnetic to true (which the posts above describe).

 

In my neck of the woods, the magnetic declination isn't far from "true" and if DISTANCE isn't too far, any variation wouldn't matter much. If you are someplace with a 20 degree difference, and DISTANCE is large, you would likely have to try both without the tip from the cache placer.

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