How I Use A Compass To Avoid Walking In Circles

[+AtoZ]

Some how you are missing the POINT. YOUR GPSr IS NOT ACCURATE NOR IS THE GPS system precise to the degree you are talking about. Do you understnd that FIRST AND FOREMOST. Your GPSr will give you a bearing and distance to where it has calculated the waypoint to be. then if you move sideways a distance it will give you another bearing and distnace. NEITHER of these is based upon the actually coordinates of the cache but where the GPSr has calculated the coordinates of the cache to be. CAN you see that.

YOUR GPSr IS NOT PRECISE NORE ACCURATE. There is a give stastics analysis that show the probable of a the calculation but is based upon taking a stastically signifinate number of readings etc...

But you method is NOT any more accurate then just following the arrow. If you were using a theolite and had knew precisley you coordinates and then the bearing to a point from two diffeant locations then yes what you suggest is okay. But your GPSr is just giveing you a rough estimte. Try this find a benchmark and go to it every day for a week and see how much you GPSr varies. The GPSr only gives a ROUGH position.

cheers

Edited February 10, 2005 by AtoZ

[+Thot]

If the GPSr was exactly correct all the time and each cache had exact coords posted how much fun would it really be?  . . .  if this were too easy I would quickly tire of it.

You raise an interesting question. I’ve thought about this. There are precision GPSrs today that will get you very, very close. I’ve read claims of under an inch (2 cm). Not the handheld gadgets cachers use but, larger, expensive units. I assume it’s only a matter of a few years until hand held units will begin to have much more accuracy.

 

Once the placer and the hunter’s combined error is reliably under a foot (decimeter ?), will the game become mundane and die out?

 

The first question friends ask me when I mention the game is, “If the GPS takes you right to the cache, what’s fun about that?” Today I explain the Easter egg hunt aspect and that seems to satisfy them there’s something worthwhile about it. Most of the fun to me is getting out and going to parks and wooded areas I would never have gone to otherwise. But, I wonder if that would be enough if there were no “hunt” aspect to the game.

[+Tzoid]

You don't have to post the exact coordinates to the inch, even if you know them. Especially if it would ruin the hunt.

[+sbell111]

You don't have to post the exact coordinates to the inch, even if you know them. Especially if it would ruin the hunt.

This is not really an issue. The cache pages have the coords to three decimal places. Therefore, the best you will get is locating the cache within something like 40 or 50 square feet.

[+nfa]

Hi,

 

If every cacher on the planet was given a free upgrade to a 1 inch accurate GPS unit tomorrow, there would still be over 140,000 caches placed that are only accurate to (at best) 10-30 feet. So there will be challenges to the game for years to come.

 

nfa-jamie

[HIPS-meister]

There are many things besides technology which makes a cache fun to look for. A challenging, clever "hidden in plain sight" (HIPS) hiding device, for example, is one thing I particularly enjoy. (I like to swap stories about clever hiding techniques or inventive caches, even if I'll never get the chance to visit them.) Locations of historic significance, or obscure places in one's own town that you drive by every day but didn't know about, are also of particular interest to me. This is reflected in the caches that I set and that I find.

 

The beauty of geocaching as a sport, to me, is that it really comes down to a personal, one-on-one challenge "from the cache owner to you." When someone finds one of my caches and says that it was devious, or that s/he didn't know about this place until now... I smile and say, "You're welcome!" Each person who enters this sport can find a particular niche that they enjoy. For some people, that's "getting numbers," whereas I personally am rather disinterested in that. For some people, that's having a walk in the park with their children and family, or a hike in the woods. To each his own.

[+Markwell]

If you triangulate based on the magnetic bearing reading on your GPS, you will only be getting to the point that your GPS says you are zero feet from the coordinates. That does NOT mean that you are actually zero feet from the coordinates since your GPS is just guessing very closely. It is NO DIFFERENT than if you just followed the GPS until the distance says 0 feet.

 

One more diagram (taken to the absurd for illustration):

Premise: You're in a box canyon and the readings "draw air quickly as if through a straw."

 

The hider stands on the five gallon tub under the tree and gets a set a reading of N 34° 02.382 W 118° 43.386. However, the canyon walls mess with the coordinates and their GPS gives them a bad location, about 60 feet from the actual site. They're actually standing on N 34° 02.390 W 118° 43.380. The bluish-grey circle with the dot in the center are the bad coordinates that they received from their GPS. The actual cache is really under the tree.

 

You come in to the box canyon with your GPS. It's signals are also confused, but confused differently. It says that N 34° 02.382 W 118° 43.386 is in a different location today. It's off to the southeast by about 45 feet. Therefore, when you are standing on N 34° 02.379 W 118° 43.378 (the center of the orange circle), your GPS says you're actually at N 34° 02.382 W 118° 43.386.

 

If your GPS is not telling you that you're in the right location, no triangulation from any distance, whether those triangulation lines are triangles, rectangles or polymorphus rhombatangles, will triangulate you to the cache at the base of the tree. It will only triangulate you to the center of the orange circle - which, not coincidentally, is where your GPS says the coordinates are at a distance of "0.00 feet".

Edited February 10, 2005 by Markwell

[HIPS-meister]

Oh fellow gentlebeings, isn't it plain enough that you can never be more certain of the cache's position than the owner was? I never said otherwise, and never will.

 

Every one of you, when you draw your diagrams, draws a triangle from your GPS (a single point) toward the cache area. And there's the rub: your own location is an uncertain area, too! Just as the cache owner did not know her position as a single point, neither do you!

 

When the GPS draws a bearing-line from "where you are" to "where the cache is," it is necessarily estimating "where you are." You do not visually see this on your GPS so it is very easy to overlook it. It is easy to treat the problem as simply, "the bearing I see on my GPS is 'off' because I do not know where the cache is. I know where I am (wrong!) but not where the cache is..." Wrong.

 

Both points, on each end of the projected bearing line, represent uncertain, unknowable positions. Not just one of them (the cache position), but both of them. In Markwell's graphic above, put the GPS inside the blue circle at the upper left of the graphic: the GPS could be anywhere inside that blue circle, just as the cache could be anywhere inside the orange one, and give you exactly the same reading. Markwell's graphic is drawn as though the position of your own GPS could be known with certainty; in other words, as though the problem had one degree-of-freedom rather than two.

 

How do you reduce the influence caused by the fact that both positions are unknowable? Separate the endpoints, as far away as you reasonably can. Take bearings that are distant, and rely upon your GPS bearings less-and-less as you approach.

 

When you get to the point where both you and the cache are "inside the circle" that is drawn on your GPS screen... stop relying upon your GPS pointer.

 

It works.

[+StarBrand]

Ok, I have read the posts and read the above re-explanation and have come up with my own drawing:

 

The circles are the areas of "uncertainty" the rectangle is the area of uncertainty that my GPS is both at and pointing at as I understand your explation..

 

My question is: How does your technique improve any of this??

 

[HIPS-meister]

Thanks for the drawing, StarBrand. Let's use your drawing as being "you're fairly close to the cache," because the rectangular box you've drawn is fairly fat.

 

First of all, blot out the "right answer," the obelisk, and the tree. Also blot out the dot which represents "where you are." You do not know: where you are, where the owner was, or where the cache is. The true extent of your knowledge is only the circles... their size and their approximate distance from one another. Nothing more!!

 

Take a straightedge and start drawing lines at random. The rule is that each line must begin somewhere in the first circle and must end somewhere within the second one. After you have drawn a hundred lines or so ... observe the "spread" of those lines. How widely do they "fan out," from the most extreme clockwise line to the most extreme anti-clockwise line? Offhand, I'd say that there looks to be a spread of about +/- 30 degrees here...

 

Okay, then, what does that mean? It means that there is a range of about +/- 30 degrees in what a properly functioning GPS could be telling you right now. 60 possible values, and one (but only one) of them is right. You don't know which one. But the odds that you're looking at the right answer right now (and don't know it yet) is, worst case{*}, about 1 in 60.

 

Now shorten the box so that the two circles are adjacent but not touching and repeat the procedure. Now the spread is probably more than 80 degrees. The lines, which represent a possible solution to our problem, "fan out" much more. There are now about 160 possible values the GPS could give us: the odds of our GPS being right are now about{*} 1 in 160.

 

Now, move the two circles so that they overlap. Now the spread becomes 360 degrees because, within the region where the two circles do overlap, a line can be drawn in any and all directions and still meet the requirement that one endpoint lies in one circle and the other endpoint lies in the other. That's as bad as it can get, of course.

 

[ {*} Note: Your GPS receiver has lots of clever "averaging" technology designed to dampen the variance that you see... which is why you need to let your GPS "settle down." What it's doing is gathering a bunch of data-points which it is then averaged to give you the answer you see. Which is why your actual observed performance will probably be better than these worst cases.]

 

Now, let's go the other way. Stretch the box out so that it's, say, three feet long. Once again, draw lines that start in one circle and end in the other, and observe the spread of those lines. You will see that the range of bearings is now much smaller. A properly functioning GPS will return one of these bearing values, and now, if we have a spread of (say) +/- 4 degrees, there are only 8 to choose from. The probability that our answer will be the right one jumps up to 1-in-8. The longer the box becomes (within reason) the narrower the spread will be and the better the probability that our GPS is, in fact, telling us the right answer will also be.

 

So what does this mean?

• Bearings taken from a distance are more usable than bearings taken from a close distance.
  
• The closer you get to your destination, the more unreliable the answers can become.
  
• The very clever electronics in your GPS are using averaging and other fun tricks to try to make your actual observed answers better than these. Those electronics generally do succeed.
  
• Even though these techniques might be complex overkill for urban caching among obelisks and trees, there are other times when they will make a rather dazzling difference in your success.
  

Q.E.D.!

[+StarBrand]

Miscommunication - I am where I am as are the coors reported and the cache at some fixed points on the WGS-84 datum - the circles are the possible range of coordinates being reported by a GPS. Any point within my circle can point to any point within the reported circle (my error). In addtion the cache hider had an error circle but I don't know where it is in relation to the reported circle. Again, How does drawing any arrows from my circle to the reported circle help locate an overlapping error circle in an unkown position?? (clear as mud) Aren't I always still at the mercy of GPS error (mine vs hiders)??

[HIPS-meister]

StarBrand, I "borrowed" your diagram, not using it as you intended but using it nonetheless. I think if you'll work through my example now .. do it on paper .. you'll see my point at last.

[+BEyeCyclists]

Unless I have missed it, no-one has answered my question that asked whether triangulation is worthwhile if one knows the cache co-ordinates are correct.

 

So let me attempt to answer my own question. Let us assume (1) the published cache co-ordinates are exactly correct and (2) my GPS is always known to give a current position within r distance of the true current position.

 

Suppose my GPS gives the bearing of the cache from my current position as A, and assume I am at distance d from the cache where d is large compared with r (e.g. d > 10 * r). Then

 

(3) The true bearing of the cache from my GPS reported current positiuon is exactly A

 

(4) The true bearing of the cache from my true current posiution is A +/- delta, where delta is the angle subtended by r at a distance of d.

 

On the map, draw two lines centred on the GPS reported position at bearings A-delta and A + delta. (We cannot draw lines from the true position because we do not know where that is)

 

(5) The cache lies between these two lines.

 

Now go to a second point so that the bearing of the cache is approx A plus or minus a right-angle and repeat the exercise.

 

(6) Now there are 4 lines on the map and they form an approximate square

 

(7) This "square" definitely surrounds the cache.

 

(8) the area of the square is 4*r*r

 

(9) Search the square and you will find the cache

 

(10) Now do a much simpler exerciise. Walk until the the GPS says the distance to the cache is zero. Draw a circle on the plan of radius r around the position as reported by the GPS. The cache is within this circle.

 

(11) The area of the circle is only 3.14 * r * r, so exercise 10 gives a smaller search area than the bearings.

 

(12) But exercise 10 only took one position and we are comparing it with an exercise using two positions. Suppose after a while we go to a new position where the GPS reports a zero distance to the cache. We now have two intersecting circles and we only need to search their intersection, which is an area even less than 3.14 * r * r.

 

Conclusions:

 

(13) If the cache co-ordinates are exactly correct, it is better to not bother with the bearings, but just to search a circlular area around the first place where the GPS says the cache is.

 

(14) If the cache co-ordinates are significantly more accurate than the GPS then again it is better to not bother with the bearings, but just to search a circle around

the first place where the GPS says the cache is.

 

I may be completely right, I may be completely wrong, I may be partially right, I may be partially wrong, or the correct answer may be somewhere in the middle. [apologies to Spike Millegan].

[+BEyeCyclists]

As soon as I posted that I realised a mistake. Forget the map. I should have said draw lines on the ground from the actual current position. The conclusions are the same as is my doubt.

[+Jeep_Dog]

Unless I have missed it, no-one has answered my question that asked whether triangulation is worthwhile if one knows the cache co-ordinates are correct.

 

If the cache owner placed the cache using an resection technique with a magnetic compass, and thus knew the EXACT coordinates of the cache, then yes, indeed, triangulation (intersection) technique utilizing a magnetic compass would certainly be a better way to locate the cache.

[CoyoteRed]

Getting back to what I think the original premise is, I content that all things being equal and you can get as good of a signal (a combination of signal strength and satellite geometry) as from where you are triangulating, the method presented is not as accurate as simply being near where your GPS unit puts you.

 

I'm not trying to refute triangulation when you have poorer signal at ground zero.

 

The reason I say it is not as accurate is because you are introducing a step that is prone to a certain amount of error, namely using a compass.

 

Secondly, AFAIK, the internal workings of a GPS unit does not take into account any errors between where it thinks it is and the waypoint entered--especially on the scale we're talking about. If this is the case, then when you are 300' out your actual position in your circle of error corresponds exactly with the actual position of the entered waypoint in the projected circle of error.

 

The hider's circle of error has nothing to do with this exercise which seems to be trying to get a more accurate position by triangulation.

 

Further more, because you are only getting a one dimension at a time when triangulating and considering a GPS's electronic position drifts in the real world, your second bearing will be taken while pointing to a different real world location!

 

Therefore, triangulating using a GPS and compass is not as accurate as using a GPS alone at ground zero when you have the same signals at all points. The only time triangulation will be more accurate is when signal at ground zero is very poor.

 

FOLLOWUP: Yes, I understand about using a compass at ground zero and do so myself. I leave my GPS in one spot near ground zero and let it average. I use a manual compass to get a bearing and distance to the waypoint from the GPS's location. I am much better able to judge 20' than 200'.

 

Hope this helps.

[+BEyeCyclists]

Unless I have missed it, no-one has answered my question that asked whether triangulation is worthwhile if one knows the cache co-ordinates are correct.

 

If the cache owner placed the cache using an resection technique with a magnetic compass, and thus knew the EXACT coordinates of the cache, then yes, indeed, triangulation (intersection) technique utilizing a magnetic compass would certainly be a better way to locate the cache.

Thats a different conclusion to mine, so which of the numbered steps in my "proof" is incorrect?

[+SilverMarc]

This is the funniest thread, I must admit!

 

Even the directions of my Garmin say that once you get 20 feet or so from the zero point, ignore the GPS compass and just follow the distance down to zero.

 

This has worked well for my finds to date, even the ones that others have claimed were "way off" the given coordinates.

 

--Marc

February 11, 2005 @ 10:02 AM

N40° 46.565' W073° 58.756'

[+Jeep_Dog]

Thats a different conclusion to mine, so which of the numbered steps in my "proof" is incorrect?

Perhaps all of them?

 

Again, you asked if we knew the exact coordinates of the cache, would triangulation be worthwhile. Your proof starts off with a GPS known given, which is not an "exact" coordinate. The only precise way to determine a location is, once again, if the cache owner utilized an intersection techinique.

 

 

In this example, the cache owner placed the cache then took a look around. He saw across the lake hill 747, and shot an azimuth (bearing) to the peak of the hill that determined 10 degrees magnetic. He also clearly saw the peak of hill 721, and shot an azimuth to that point utilizing a compass and determined 70 degrees magnetic. He converted the magnetic azimuth to a grid (map) azimuth (I won't get into a declination discussion, there's plenty of threads on that topic) and plotted the back azimuth lines to the cache, noted the coordinates, and utilized them for the cache.

 

Now, I come along as a hapless cacher, knowing the cache owner is very anal retentive and determines coordinates using this method. I'd note a prominent terrain feature on the maps (such as those two hills across the lake) I'd walk around that loop road, pick up a 070 magnetic azimuth to hill 721, pace off distance, and close to the cache break shoot an azimuth to 747 until I determined I was precisely 190 degrees off that hill, and look down, because the cache would surely be there.

 

AGAIN, I assumed your premise we knew the exact location of the cache.

 

Is this quicker than using a GPSr? It depends on how good you are with a compass. Folks that have been orienteering or been avid outdoors people can use a compass in this way without much of a second thought, and would find this technique quicker, and more accurate.

 

Oh, sidenote. The above photo is from the vicinity of one of my caches. It is moved, since the cache is a multi, so no spoiler here. At any rate, I took a GPSr reading at the cache with 7 birds, WAAS enabled, and my GPSr stating it had 7' accuracy. I also used a technique as above to determine three azimuths to prominent features. These three I converted to grid, plotted on a map, and X marked the spot. I then plotted the GPSr coordinate, and what would you know, the GPSr was one meter away from the intersection coordinates, exactly where I'd expect it to be with 7' accuracy.

[+AtoZ]

I don't know what to say that hasn't been said before, REPEADELY. But your GPSr only points to a caculated location not the exact location. Your GPSr is already doing a triangulation, well actully a quadlilliation if you really want the truth to come up with a CALCULATED VALUE. There is no absolute with the GPS system. But if it makes you feel good then good, I guess.

cheers

 

/em wacks the dead horse one more time.

[+BEyeCyclists]

You say "Your proof starts off with a GPS known given, which is not an "exact" coordinate. The only precise way to determine a location is, once again, if the cache owner utilized an intersection techinique"

 

Sorry Jeep Dog, I don't see the relevance of this. I did not assume the cache owner used GPS to determine the co-ordinates, I just made the hypothetical assumption that the co-ordinates given for the cache are correct. You say you can find such a location better by triangulation from recogniseable physical features than you can by GPS, and I do not dispute that. But assume there are no features on the ground you can triangulate from. The question is: is it better to use triangulation from distant locations known only by GPS, or is better to simply search the circle around the first point where the GPS says the cache is? I still contend the latter is better as it gives a smaller search area.

[+sbell111]

OK, OK, you win.

 

I must be playing the game incorrectly. I thought this game was simple and fun. Somebody hides a box of trinks under a pile of sticks next to a tree. They use a little electronic gizmo to figure out where it is. They post this looong number on the web where I find it. I take the long number and enter it into my little electronic gizmo. I follow the little arrow on my gizmo until it says I'm there. Then I look around until I see a pile of sticks. Under the sticks is a box that I open. I trade trinks and sign the log. Once I get home, I log it online.

 

Clearly, I have not been playing correctly. As it turns out, the game is really complicated. Too complicated to be fun, in my opinion. I guess I'll quit playing.

[+Jeep_Dog]

LOL sbell. Same here. While I know how to apply triangulation techniques, I follow my little GPSr arrow until 100 feet out, then meander along the same bearing and keep my eyes open for a cache.

 

It's worked so far.

 

Oh, yeah, I generally have fun rooting around with my daughter trying to find them. Yeah, that's right, fun.

[+Jeep_Dog]

Sorry Jeep Dog, I don't see the relevance of this. ... The question is: is it better to use triangulation from distant locations known only by GPS, or is better to simply search the circle around the first point where the GPS says the cache is? I still contend the latter is better as it gives a smaller search area.

Actually, I don't see the relevance, either. I'm still trying to figure out why I responded to this thread to begin with.

 

In fact, this all hurts my head so much I'm not using a compass OR a GPSr the next couple of caches.

[HIPS-meister]

The sister sport of orienteering virtually consists of the kind of backwoods direction-finding that Jeep_Dog describes, and I encourage you someday to try it. Learning how to maintain a bearing course through light or dense woods, around obstacles and so-forth armed only with a compass and a map is both great fun and a great exercise. (These days you usually have a GPS in your knapsack "just in case".... turned off.)

 

It is truly difficult (I see now) to explain why this technique works and why it works so well, until you experience for yourself a situation where it is applicable. When you are seeking a cache where radio reception is poor, or there are no visual clues to help you pick among dozens of hiding places, then, and perhaps only then, will the value of this method truly become clear. If you are able to seek out the caches that you're looking for by following the little needle, then by all means keep doing it: it works for you. When it finally doesn't, re-read these posts.

 

A modern GPS is a clever piece of electronics. It will work so well in many situations that it will catch you by surprise when you get yourself into a situation where it does not. For instance, the location-averaging feature... If you walked into the woods from an open clearing and entered the woods only recently, your GPS still has a lot of clear-sky readings to work with. But, if conditions have been marginal for the last twenty minutes or so of your walk, the GPS memory is full of "junk" and the little needle-display might turn out to be Wandering Willie. This is where you'll be glad to have a compass in your pocket.

 

The reason why distant-triangulation works in conjunction with GPS is that it gives you a narrow course to set... a course that is narrower the farther away you are. But you still need to know how to set a course in the woods, and how to use a compass to make sure you're on that course even if the cache is out-of-sight of where you're taking your reading! (It should be emphasized: get well away from your car! That's quite a hunk of steel and it can fuddle your compass from many feet away...)

 

Basic orienteering skills give you one additional benefit: they teach you to observe the woods. And they teach you how to be aware of your position, bearing and distance as you approach a distant target. This is not the same application of a compass that we use in Orienteering: this is a bearing-line in which both endpoint positions of our GPS-lines are "uncertain." But it can still be far better than what you have to work with when relying upon the GPS alone.

Edited February 13, 2005 by HIPS-meister

[+BEyeCyclists]

"The reason why distant-triangulation works in conjunction with GPS is that it gives you a narrow course to set... a course that is narrower the farther away you are."

 

Yes, your method gives you a constant straight course to set, aso opposed to a course that wanders more and more as you get nearer the cache. So the distance to travel to some point where at one point of time the GPS said the cache is, is minimised.

 

Its actually quite hard to find any spot where the GPS says at that point of time the cache distance is zero, in fact it is probably impossible. Your method gets round this problem and I reckon that is the reason it works. The fact that it was 10 minutes ago that the GPS said the cache is here, and now it says it is somewhere else, is irrelevant.

[+sbell111]

I realize that this is off-topic, but I was wondering why this thread is still alive. The issue has been beaten to death.

 

People fall into two categories, those that perfer to shoot a bearing to get to ground zero and those that follow the arrow.

[+Alan2]

Your post reminded me of something I tried 3 1/2 years ago after I got my Vista.  (The Vista has built in magenetic compass as well as ability to project waypoints by keying in distance and bearing).

 

I switched to meters.  Then when I got to lets say exactly 50 meters away accorinding to the Vista, I projected a waypoint 50 kilometers plus 50 meters according to the bearing displayed.  I made the porjection when the GS showed minimum EPE error.

 

Then I started walking as the arrow pointed until the display counted down to 50 kilometers and I was at the cache.

 

One thing this eliminated was the "bee dance" that occurs when you are at the cache.  An advantage I believe.  But I'm not sure if the accuracy due to the projection and angle and trig actually gets you to the cache better than just plugging in the cache coordinate.

 

I never came to a conclusion but I thought I'd repeat the concept here to see what you math wizes think and if any of you ever tried this.

Before you lock the thread, does anyone have inpout from my earlier post that seems to combine following the arrow with a form of "triangulation"? Actually this method might work better with non-magnetic compass type GPS's.

 

Thanks.

Edited February 13, 2005 by Alan2

[HIPS-meister]

Not personally owning such a GPS, I don't know how accurate it will be for this purpose. What I use is a compass which has a sight mechanism specifically designed to let you get precise bearings on distant objects. But a compass with a flip-top mirror will do well enough. The thing you need to be able to do, and to do with "+/- 4 degrees of precision," is to sight upon a distant object and be able to ascertain its bearing from you with that kind of precision. My impression of the electronic-compass features is that this is really not what those are designed to do. They'd be great, say, in a canoe or a kayak, because they'd give the GPS another independent source of bearing information (and, combined with the position data, would enable it to ascertain the presence or absence of tidal and current drift).

 

If you are navigating cross-country through the woods, you move from one point to another, constantly shooting bearings ahead of you and back bearings behind you, to help you keep on-course. Since there is going to be some amount of error each time you repeat this process, care and precision are necessary. This is especially true when your direct-line course is blocked... by geography or obstacles or simply by a hill that you can easily walk over but can't see through.

 

As you're walking along the bearing you've chosen, the electronic compass would be a fine reference to make sure you're going in generally the right direction. And it may well be more precise than that... I don't know, because I can't look inside the GPS and see for myself where that little needle (or ball or whatever-it-is) is actually pointing. But you need to be able to sight across it, and "eyeballs" aren't good enough for precision work.

 

Anything can disturb the compass. For example, you should take your watch off. You should stand 20 feet or more away from any car. Since I can't see the compass needle inside of a GPS, I have no way to know if the needle is being affected or not. And, not owning such a unit, I don't have any experience with it.

[+Alan2]

I;ve checked my Vista magnetic readings with my $35 Suunto and the readings match within a few drgrees when you line up the arrows on both in the same direction.

 

Actually sighting with the Vista is very neat, probably as good re better than a prismatic compass. There are two arrows that you sight along and line up with to the distant object. You click the switch and the bearing is stored in the Vista. You can then navigate matching bearings and headings or if you want you can project a waypoint to the heading stored. The latter is neat when you can estimate some distant object and store a waypoint for it. Then you can navigate to it and even when it goes out of sight blocked by hills, or as you go around water obkjects) , your GPS of course still has the approximate location stored as the waypoint which you continue to follow.

 

However, my previous post is not about compasses but about setting the waypoint artificially 50 KM beyond the actual location to better take you to the cache. Please re-read the full method I posted. Any ideas about its effectiveness?

[HIPS-meister]

It seems to me that the technique described was coincidentally useful in one particular case. If you "project" an uncertain measurement out 50 feet (or any distance), how can it be 'better' than what you started with? How can it gain accuracy? I can't speak to this with any authority or experience. I don't know.

Edited February 14, 2005 by HIPS-meister

[+Alan2]

Well, it does eliminate the "dancing bee" effect of the arrow as you approach the cache. That seems to help in orientation at the cache.

 

The other question I have is whether the angle craeted by the 50KM plus 50 meter projection allow a better track. If someone else can put some math and graphics to this, maybe we can get a decent analysis.

[+BEyeCyclists]

Having read this thread, here is the method I will use in future when the conditions suit. It is probably what a lot of people already do anyway.

 

I will follow the arrow on the GPS so long as it leads me in a straight line. As soon as this is no longer the case, stop, note the current distance and heading of the cache from the GPS then head off in that second straight line direction for the given distance, ignoring the GPS. That point will be the middle of my search area.

 

The above can be carried out with or without a compass. Without a compass you can use the direction of the arrow on the GPS just before you stopped i.e. just after it changed direction from a straight line path.

Edited February 14, 2005 by BEyeCyclists

[+BEyeCyclists]

 

I switched to meters.  Then when I got to lets say exactly 50 meters away accorinding to the Vista, I projected a waypoint 50 kilometers plus 50 meters according to the bearing displayed.  I made the porjection when the GS showed minimum EPE error.

 

Then I started walking as the arrow pointed until the display counted down to 50 kilometers and I was at the cache.

 

One thing this eliminated was the "bee dance" that occurs when you are at the cache.  An advantage I believe.  But I'm not sure if the accuracy due to the projection and angle and trig actually gets you to the cache better than just plugging in the cache coordinate.

 

I never came to a conclusion but I thought I'd repeat the concept here to see what you math wizes think and if any of you ever tried this.

I think this is a good method - what it does is to take you direct, without wandering, to the location which the GPS considered to be the cache location at some time in the past When you get there the GPS will probably consider the cache to be somewhere else but that does not matter - what the GPS considered the location a while ago is just as good as what it considers the location now.

 

The only slight modification I might make is just to go in a straight line until the GPS arrow starts to take me off my previous constant heading, rather than go to 50 metres or whatever.

[HIPS-meister]

... I will follow the arrow on the GPS so long as it leads me in a straight line. As soon as this is no longer the case, stop, note the current distance and heading of the cache from the GPS then head off in that second straight line direction for the given distance, ignoring the GPS. That point will be the middle of my search area.

To a certain extent, the technique you describe exploits the fact that, when the GPS sees you proceeding on a certain bearing at a certain speed, it will assume that you have continued to do so even when the data-stream says otherwise (or when it has been cut-off, as in a tunnel). If you proceed in a bee-line toward the cache location as the GPS started to calculate it from a comparatively long distance away, this computational feature of the GPS will give you improved accuracy -- as it was intended to do.

 

Should you "wander," and/or should you have turned-on your GPS for the first time only when you were getting fairly close, this feature would be of little value. Then, having become accustomed from past experience to your GPS appearing to be highly accurate, you may be startled and frustrated by the fact that it "inexplicably" isn't. You may follow the bee-line to an area, fail to find the cache right away, look down at the GPS and notice that "suddenly it's not so sure anymore." And, lo(!), it isn't. This may be why.

 

I encourage you to try this and to see what works for you.

Edited February 26, 2005 by HIPS-meister

[+BEyeCyclists]

when the GPS sees you proceeding on a certain bearing at a certain speed, it will assume that you have continued to do so even when the data-stream says otherwise

 

You may be right but are you sure a GPSr does this? Do all GPS receivers do it? How does the GPSr know when I am travelling in a straight line - after all, each position has an unknow error associated with it. How can it distinguish between me jumping around and the signals jumping around? At sea straight-line travel is the norm, but this is not true when walking.

 

Peter Seaman

[+Markwell]

Neither of my GPSs assume that I'm continuing in a straight line if it can't capture reception. If I've lost the signal it doesn't report where I am or my proximity or direction to the target waypoint. It says "No Signal"

[+Prime Suspect]

Neither of my GPSs assume that I'm continuing in a straight line if it can't capture reception. If I've lost the signal it doesn't report where I am or my proximity or direction to the target waypoint. It says "No Signal"

Neither do of either of mine, nor have I ever seen this documented anywhere. The only GPSr I know of that will do anything like this is the high-end Garmin auto navagation system. It uses an electronic compass and speed sensors wired into the car to dead reckon your location when it loses sat lock.

[+EScout]

Yikes! What a discussion.

This is how I use a compass. I hike with my GPSr on the "compass" page, with the Go To active to the waypoint. One of the bottom fields is Bearing. At forks in the trail, ridges, etc. I stop, read the bearing and use my compass to help me decide how to proceed. When the 100 foot alarm beeps, I often stop and use my compass, depending on the terrain.

Your compass is a good tool, and since you have to carry one anyway, I think it is helpful to be using it along the way in case you need it to get back (dead batteries, drop GPSr on a rock.)

[HIPS-meister]

The single most crucial point ... that is repeatedly being overlooked and/or discounted ... is that there is not one but two unknowns at work in this problem: (1) your location; and (2) the position of the cache. It may seem unimportant that your GPS and the owner's GPS simultaneously played by the same rules, but it truly is the crux of my point.

 

When you look at your GPS and it says, "I am here!" it is very easy to think that you are at a known position and that only the GPS is at an unknown position. It is very easy to think that you are at a known point and that the cache is located in a nearby region. It is also very easy to think that, "even if I am 'also in an unknown region,' it really doesn't make any practical difference." My point, and it is an "ah haa-a-a-a-h..." point, is that it actually does.

 

If you normally look for caches in areas where there are only a few realistic possibilities to choose from ("this light pole or that one") once you get close to the area, then your reliance upon visual clues and learned experience make any excessive precision in advance navigation of very little practical use to you. But, when you are devoid of such "additional clues," it can make all the difference.

 

You cannot, by definition, improve upon the accuracy of the cache-owner's GPS. You cannot make your final search-area encompass an area any smaller than the one dictated by the owner's GPS. Your objective is simply to limit your search-area so that it is, insofar as possible, no larger than the owner's area. And this is what the compass technique very-effectively does.

 

Unfortunately, I don't think it will benefit anyone to belabor this point further. Enough has been said.