Accuracy over distance...

[Jay H]

Hey folks, just curious as to what would be considered the approximate error one can expect when trying to take a visual bearing over a certain distance via a compass and map.

Standing on the summit of Windham High Peak in the northern Catskills, I'm looking at a bunch of peaks that I know are in the Berkshires or the Taconics. From the map, I know that the Berkshires are just about level, latitude wise, with the blackhead range in the Catskills, which is just south of Windham. I took a visual bearing with my compass to what we thought is Greylock and got 59deg. This is not a lensatic compass, a standard flat Brunton compass so even that will be off for sure. But I went home and was playing with MapTech's Terrain Navigator and plotted a straight line distance from Windham HP summit to Greylock summit. I got a distance (straight line) of 54.5 miles and a TRUE bearing of 66deg.. with the -14.5deg declination, this should be 51.5deg magnetic bearing from Windham to Greylock. So I'm off about 8.5degs over 54.5 miles. Is this to be expected given the approximation of my location, the distance, and how steady of a visual bearing I can take with my compass?

Interestingly enough, there is a restaurant NW of the catskills by Windham which claims it has a 5 state view:

NY (duh!)
VT
CT
MA
NH

I easily believe the top 4, but not sure about the NH part, I'd have to see that with an expert to believe that... Or perhaps, somebody in NH with a large red flare...

Jay

 

[Tom Rankin]

Not sure about the compass, but I hope it would be accurate to about 5 degrees. Any more, and I could guess from the Sun's position!

As for the 5 state view, maybe try Google Earth? The Mohonk tower claims 6, BTW,

NY / NJ / PA / CT / VT / MA.

 

[sleeping bear]

I read somewhere that it was reasonable to assume that a bearing could be up to 6 degrees off.

I've set up several canoe orienteering courses where students paddle to a certain spot and take a bearing accross the lake. Inevitabley they all complain that my bearings are off no matter how many times I check and back-check them. Of course, with canoes there is a lot more room for error as drifting is a common problem. However, their corrected bearings are usually within 5 or 6 degrees of mine.

 

[Nessmuk]

Hey folks, just curious as to what would be considered the approximate error one can expect when trying to take a visual bearing over a certain distance via a compass and map.

8 degrees is too much. Distance has nothing to do with it. With your compass you should ideally be able to be accurate to 2 degrees, I'll give you 3, or 4 at most depending on a variety of error causing conditions, not 8. I wouldn't be satisfied with 8 unless I could find a very good reason. Does it remain the same if you move a few tens of yards or more? - it should for the distance of the target. I verified both your location/target coordinates and expected compass bearing using another map source and calculation. Something is wrong when you get 8 degrees of error.

Of course make sure you are not holding the compass close to any obvious bits of ferrous metal, a car, a belt buckle, wrist watch or other jewelry, concrete (which has rebar). I once had a girl in a training class who kept getting the wrong bearing every time, yet when I used her compass it agreed with mine. I finally discovered she was wadding up the lanyard in her hand under the compass. For some reason she had put a metal split ring on the lanyard, which screwed up her reading.

it can be very difficult to positively identify a distant peak. Quite often a closer, though significantly lower high ground can be easily confused with the intended distant target. Since you are a long distance away the perspective will be quite flat. GoogleEarth shows Mt Greylock disappearing into the surrounding terrain when viewed at a shallow angle from the SW. Assuming you are holding the compass correctly and not near anything to throw off the magnetic field, then I'd guess that's what is happening.

 

[Jay H]

Of course, I know that Greylock has the war memorial on top or as I call it, the Giant Pawn, but no binoculars with me. I took a bearing to the northernmost peak, I don't think the visibility would allow me to see into VT. It could possibly be the small peaks around the tricorner marker (MA/NY/CT) or maybe even a smaller peak like Catamount or so. It was a fairly prominent peak in a bunch of smaller peaks. I can go back to TN and play with the bearing some more to see what line of sight I am seeing at 59deg mag. bearing. TN is good for that, I even have the 3D view that I can play with.

Jay

 

[DougPaul]

With a good sighting compass (eg Silva Ranger) you should be able to get within 2 deg, 1 if you are really careful.

Stand at one known object and sight on another a mile or so away and compare to a topo to see how well you can do.

Doug

 

[John H Swanson]

Despite being pretty good at navigation, I've stopped trying to estimate position using triabulation. My efforts to locate my position have been very inaccurate - and my terrain association can give far better position ....usually to about +/- 50m ....sometimes better....such as when I'm standing on a benchmark. I can only conclude it is from the accuracy of the bearings. I can not give you a bearing error, but 8 percent would not surprise me. To this point, I'd like to see someone repetitively demonstrate their ability to determine a bearing within 2 degrees of actual.

Bear mind that I use a base plate compass and not a mirror sighting compass or better technology.

 

[NH_Mtn_Hiker]

I took a visual bearing with my compass to what we thought is Greylock and got 59deg.

it can be very difficult to positively identify a distant peak. Quite often a closer, though significantly lower high ground can be easily confused with the intended distant target.

Berlin Mtn.(2,818 Ft.) in the Taconic's is just west of Mt. Greylock and is 58.8 deg. true and 51 miles from Windham High Peak.


Just a thought. ....as said earlier, an 8 deg. error is a lot.

 

[sardog1]

Despite being pretty good at navigation, I've stopped trying to estimate position using triabulation. My efforts to locate my position have been very inaccurate - and my terrain association can give far better position ....usually to about +/- 50m ....sometimes better....such as when I'm standing on a benchmark. I can only conclude it is from the accuracy of the bearings. I can not give you a bearing error, but 8 percent would not surprise me. To this point, I'd like to see someone repetitively demonstrate their ability to determine a bearing within 2 degrees of actual.

Bear mind that I use a base plate compass and not a mirror sighting compass or better technology.

Come along on a night bushwhack sometime. I expect and achieve 2 degree accuracy with my Silva Ranger whenever using the mirror in the daytime. I expect and achieve 3 degrees if held at waist or chest level. Four degrees would make me very cranky with myself.

The key when not using a mirror is to concentrate first on aligning the compass direction of travel arrow square to your shoulders. Then lift your chin straight up and down to look at the distant object and the compass alternately. When starting off on a bearing, resist the temptation for your eye to be caught by whatever happens to stand out in your field of view -- your chin should pick out the object on the bearing line, not your eye. And watch out for iron or steel on you that you're unaware of -- belt buckle, bra wire (yes, Virginia, it happens to some of you), ice axe, rifle barrel, etc.

 

[Nessmuk]

Despite being pretty good at navigation, I've stopped trying to estimate position using triabulation. My efforts to locate my position have been very inaccurate - and my terrain association can give far better position ....usually to about +/- 50m ....sometimes better....such as when I'm standing on a benchmark. I can only conclude it is from the accuracy of the bearings. I can not give you a bearing error, but 8 percent would not surprise me. To this point, I'd like to see someone repetitively demonstrate their ability to determine a bearing within 2 degrees of actual.

Bear mind that I use a base plate compass and not a mirror sighting compass or better technology.

I completely agree with you that terrain association is a primary navigation method. Accurate navigation is all about putting together dozens of clues all around you, that constantly change... all to your benifit. I've always said that it would be a rare situation indeed that I would need +/- 10 meter accuracy claimed by a GPS. If I need that much accuracy then it would be obvious from the edge of the cliff or shoreline I am standing on. Otherwise, map study and the terrain tells me where I am with sufficient accuracy.

But I disagree that triangulation isn't as accurate as you'd like it to be. Of course rarely do you use only one method alone such as triangulation (more properly called resection in this mode). Both terrain association and the compass work together as complimentary nav aids, along with several other techniques all at the same time. And you don't always have pure triangulation availble here in the woodsy sight limited northeast anytime you want it. I must say though, that 2 degree accuracy is not out of the question. Though it may be slightly easier to sight with, you don't need a mirrored (Ranger style) compass. Your rectangular baseplate with rotating liquid filled bezel should work just fine.

I've been teaching backcountry map and compass navigation for more than 15 years and learning on my own a lot longer than that. I expect my students to get within 2 degrees in real backcountry triangulation exercises where the target is well defined, though at the outside 4 degrees may be acceptable. They get better as they develop experience holding the compass at chest level and sighting by facing the target squarely.

 

[HH1]

Just To Add To Your Woes, Jay...

...your readings are off by a lot more than 8.5 degrees.

Mass and the rest of the northeast is in a region where magnetic declination is west; as such magnetic bearings are LARGER than true north bearings (use the mnemonic 'west is best' to remember that).

Therefore, in converting a true bearing to a mag bearing, you ADD magnetic declination; SUBTRACT it in converting from mag to true.

So, the magnetic bearing of your Terrain Navigator's calculated 66 True bearing is 66 + 14.5 = 80.5 degrees, not 51.5 degrees.

That means your compass reading was out by 80.5 - 59 = 21.5 degrees.

To check out your compass, borrow a friend's and take a sighting on some distant object - they should read within a few degrees of each other. Sometimes compass needles go funny.

Also, if convenient, take a bearing on the North Star; it should read somewhere between 12 and 17 degrees mag.

All in all, my guess is that you were sighting on something other than Greylock.

Cheers!

 

[Nessmuk]

...your readings are off by a lot more than 8.5 degrees.
...So, the magnetic bearing of your Terrain Navigator's calculated 66 True bearing is 66 + 14.5 = 80.5 degrees, not 51.5 degrees.

That means your compass reading was out by 80.5 - 59 = 21.5 degrees.

Geeze... the simplest of mistakes... I'm embarrassed that I overlooked checking that detail in the initial analysis. The discussion emphasis shifted to acceptable compass error rather than focusing on the things that can really kill you.

 

[Jay H]

OOPs, thanks for the oversight... Must of been some kind of ice-cream induced brain freeze. CADET (Compass to True Add East or perhaps "Compass ADd East to True") is the mnemonic to remember the mag->true conversion, but I guess I must of relapsed.

Anyway, for sure then it isn't Greylock.

Jay

 

[Nessmuk]

OOPs, thanks for the oversight... Must of been some kind of ice-cream induced brain freeze. CADET (Compass to True Add East or perhaps "Compass ADd East to True") is the mnemonic to remember the mag->true conversion, but I guess I must of relapsed.

Anyway, for sure then it isn't Greylock.

Jay

I hate remembering mnemonics because mnemonics is so hard to spell. Anyway, it is easier to picture what is happening fysically than remembering which way to add/subtract by using a phunny word.

Picture in your mind the globe with the magnetic north pole in northwest Canada. That's where your compass needle points. For those of us located here in the east, that puts the direction to the magnetic pole to the left (west) of Chicago. Therefore a clockwise measurement of true bearing angle will have to be increased by the westward offset (declination) to get the magnetic bearing the compass reads. A quick check is to imagine facing true north, then align your compass heading to that direction. Your direction of travel compass bearing will read +14, or whatever the declination value is for your location. The needle will be pointed slightly to your left. The opposite is true if you are located anywhere to the west of a line from Chicago to New Orleans.

With that mind picture, the clockwise measured magnetic bearing is obviously a larger value than the true bearing. No excuse for me not checking the initial assumption in the problem setup here on this thread.

 

[paul ron]

I hate to add another glitch but the zipper slider is generally made of steel and rides just at chest height. So holding the compass to your chest may be causing some problems with your readings. Check it out to be sure?

Also I just noticed something odd which I never paid much attention to untill I walked by my closet where we have a few fleece sweaters n winter jackets hanging. My compass was swinging everytime I passed a fleece but not teh regular jackets. Why?... maybe static electricity? I scanned em and only the slider was metal and clearly showed on the needle. My other jackets also have metal sliders but don't effect the compass as much as the fleece did.

Anyone care to speculate?

 

[Nessmuk]

I hate to add another glitch but the zipper slider is generally made of steel and rides just at chest height. So holding the compass to your chest may be causing some problems with your readings. Check it out to be sure?

Also I just noticed something odd which I never paid much attention to untill I walked by my closet where we have a few fleece sweaters n winter jackets hanging. My compass was swinging everytime I passed a fleece but not teh regular jackets. Why?... maybe static electricity? I scanned em and only the slider was metal and clearly showed on the needle. My other jackets also have metal sliders but don't effect the compass as much as the fleece did.

Anyone care to speculate?

Magnetic fields are generated by electric current in motion, not by the field of static electricity so that can't be the reason unless there is a continuous discharge going on. What kind of hangers do you use? Some experiments to try.... If you exactly replace the fleece with nonfleece on the same hanger in the same location do you still see the effect? Next, isolate the fleece in an area without any other magnetic materials (outside would be best, and not on a floor that likely has nails), then pass a compass over it, is there any deflection? Do you get the same result with other jackets? Is the effect repeatable in different locations?

 

[Jay H]

I hate remembering mnemonics because mnemonics is so hard to spell. Anyway, it is easier to picture what is happening fysically than remembering which way to add/subtract by using a phunny word.

I know the declanation stuff but I hardly ever deal with true bearings, I mark my maps with a magnetic north bearings or have TN print them out so I don't have to deal with convertions. But TN spits out bearings in true, so it's like the one time I have to convert and I must of had a brain fart then (ice cream induced ).

I think I'm safe from the bra strap interference. Maybe compass makers should put a warning label on them "Keep this navigational device from all bras and metal encrusted underwear"

Jay

 

[Nessmuk]

I know the declanation stuff but I hardly ever deal with true bearings, I mark my maps with a magnetic north bearings or have...

Yup, me too. As part of pre-trip map study I mark all my maps with a parallel series of MN lines, spaced about an inch and a half apart in the expected region of travel. It's a fast and (nearly) foolproof way of transferring a course on the map to a magnetic bearing in the field or vice-versa without ever using an actual number or conversion math. But if for some reason I unexpectedly find myself on an unlined map it's good to have the MN/TN picture in mind as a backup method.

 

[kmorgan]

Static Electricity will deflect a compass needle

Magnetic fields are generated by electric current in motion, not by the field of static electricity so that can't be the reason unless there is a continuous discharge going on. What kind of hangers do you use? Some experiments to try.... If you exactly replace the fleece with nonfleece on the same hanger in the same location do you still see the effect? Next, isolate the fleece in an area without any other magnetic materials (outside would be best, and not on a floor that likely has nails), then pass a compass over it, is there any deflection? Do you get the same result with other jackets? Is the effect repeatable in different locations?

While a magnetic field, per se, is generated by flowing electricity, a static charge is an imbalanced charge on an object. This is the same effect that allows you to "charge" a comb and pick up small pieces of paper, or rub a balloon on your head to charge it and have it stick to a wall.

Static charges will deflect a compass needle quite easily. To test this for yourself all you need is a nylon comb. Run it through your hair a few times and place it near your compass. The needle will be extremely attracted to the comb and you will be able to move it anywhere you wish.

You can also bend water with a charged comb. Turn on a water tap so that the water flows in a narrow stream. Comb your hair. Move the comb close to the water but don't let it touch. You should see the water stream move towards the comb!

Kevin

 

[Nessmuk]

Maxwell's equations of electrodynamics rule

While a magnetic field, per se, is generated by flowing electricity, a static charge is an imbalanced charge on an object. This is the same effect that allows you to "charge" a comb and pick up small pieces of paper, or rub a balloon on your head to charge it and have it stick to a wall.

Static charges will deflect a compass needle quite easily. To test this for yourself all you need is a nylon comb. Run it through your hair a few times and place it near your compass. The needle will be extremely attracted to the comb and you will be able to move it anywhere you wish.

I'll try it, but the physics just isn't there to generate a magnetic field from a charged comb with constant static charge on it when held stationary. There is no mechanism for a bar magnet (compass needle) to get deflected by a stationary electric field. If there is relative motion between comb and compass or if the charge is rapidly draining away then I suppose it could happen. Or if there is some other effect such as paint with dissimilar electrical properties on either end of the compass needle, but that's a reach.