Jump to content

How I Use A Compass To Avoid Walking In Circles


HIPS-meister

Recommended Posts

When searching for almost any cache, one of my most trusted tools is a good magnetic compass. I do not try to follow my GPS all the way to the cache. Here's what I do, and it works every time:

  • Walk to a point where the GPS tells me that the cache is about 300 feet away (the length of an American football field), and I can see the place where I expect the cache to be. I try to place a prominent landmark directly behind me that I will be able to see easily from the cache location.
  • Stop and wait about one minute, with good view of the sky, for the GPS to settle.
  • Note the magnetic compass bearing on my GPS screen. Jot it down. (I do not have a built-in compass on my GPS and do not use the bearing-pointer. Furthermore, I ignore the distance figure and use only the bearing number.)
  • Now, I walk on a sideways line where I'm once again next to a prominent landmark, in good clear view of the sky, and several hundred feet both from the cache and from my first position. I repeat the above procedure. I now have a large triangle in space... two known positions and the third, as-yet unknown, position of the cache.
  • Now, I put my GPS away, because from this point on it won't help me further. I walk to where I think the two bearing lines intersect. Often the location of the cache is apparent at this point and my search is over.
  • To be sure of my position, I can take back bearings on each of the two prominent landmarks I was standing next to earlier. (If you're not sure how to do that, see below.)

The reason why this compass-bearing technique works so well is easy to conceptualize. Anytime your GPS tells you "where you are," the simple truth is that it cannot actually be sure. On a good day under great conditions it can fix your position within about a 15-foot circle, but when you consider the fact that the cache-setter had similar accuracy that blossoms into a 45-foot circle... if you're close to the cache point. But, what if you're far away? 300 feet away?

 

Imagine a box, about 300 feet long and about 15 feet wide at each end. Any two arbitrary lines drawn through both ends of that box will be about no more than about +/- 3.5 degrees "spread" from each other: the box is, after all, long and narrow. The farther apart the two ends of the box are, the narrower the "spread" becomes. The compass-bearing given on your GPS, which will already be adjusted for magnetic declination, can be expected to be accurate "+/- 3 degrees or so" at that distance from the cache. This can tell you exactly which rock to look under, even in a field of stones. It can enable you to find a cache under the densest forest cover.

 

---

Now, about taking "back bearings." What I simply meant is that, while standing near to where I think the cache is, I dial-in the compass bearing that I took from the first landmark, which was behind my back when I took the reading from there, and then sight back toward that landmark. But this time I line up the south-seeking of the needle. I walk from side to side until the bearing is exact, which tells me that I'm standing right on top of the bearing-line I took earlier, but facing precisely in the opposite direction. Now, while remaining on that line, I take a back-bearing on the second landmark. When both back-bearings match exactly, I'm standing where the two lines meet: at the cache.

Link to comment

Maybe I misunderstood what you wrote, but you are getting the bearings from the GPSR aren't you? So, am I correct that you think this method makes the GPSr results more accurate than they are?

 

If so, I sincerely doubt you're correct.

 

Let’s imagine, for the sake of analysis that the GPSr is off by 1 mile. It seems to me you will be a mile away from the cache doing all your bearing & compass maneuvers. By making the error small it makes it less obvious that, in effect, you are doing the same thing -- homing in on the GPSrs incorrect location. Again, maybe I didn’t understand what you wrote, but it sounded like you also believe this method corrects for inaccurate posted coordinates as well as an incorrect GPSr reading. How large an error on the part of the cache owner do you feel it will over come? 50 feet? 100? 500?

 

Again, maybe I misunderstood what you were trying to say.

Link to comment

Let's assume that the owner of the cache was as careful as she could be. I think we have to assume that. Unless we are the FTFer, furthermore, each previous find is more-or-less an affirmation that the published coordinates are correct. (Note: if you download cache lists to your whatever-electronic-gadget and keep them there for a long time, you would not see any corrections, so check before you leave!)

 

Here is an example, then, of how the technique improves accuracy. Let me resort to a little absurdity for the sake of making a graphic example. :huh:

 

Let's say that the cache owner is sitting precisely on top of the cache ... wearing a 15-foot hula-hoop. :o She set the cache under truly ideal conditions, so what the GPS actually told her was not that the cache was (as it, in fact, is...) precisely underneath her feet. It told her that the cache was somewhere inside that fluorescent-purple hula-hoop she's wearing. (Yeah, I said "absurdity!") You, meanwhile, are looking for the cache, wearing a similar (lime green) hoop. You're also having an improbably-good day so your hoop is no larger. Your GPS, once again, is not telling you that you are standing precisely where you are, but only that you are somewhere inside your hula-hoop. So, while you listen to your patner calmly explaining to the curious muggles about the important competition that's taking place next month in Waikiki, :o you ask yourself a question.

 

Question is... just how close can those hoops be, so that they overlap in the slightest amount? All of the location encompassed by the two hoops combined are "correct answers" as far as the GPS is concerned. If she stands still and you walk around and you observe the ground-area that could be encompassed by your hoop, you'll see that you could walk completely around her, with her hoop extending 15 feet out from her and your hoop extending 15 feet out from you. The two circles combine to create an effective circle 45 feet wide. That's a big, big area to wander around in!

 

If, instead, you move 300 feet away and sight your compass at her (a purple hula-hoop is hard to miss; so is a green one...), you'll notice that a only very small spread of possible bearings will serve to encompass her hoop. If you put your hoop on the ground and take your bearings from anywhere inside that hoop, you'll still see that the spread of bearings is only +/- about 3.5 degrees. And that is a number you can use!

 

You use your GPS to give you the compass-bearing to your cache. (Note that I am not referring to a "built-in compass." I'm talking about a number that any GPS will give you.) Since the GPS does not know precisely where you are and the owner did not know precisely where she was, you know it'll be off +/- a few degrees and you can't know by how much (until you find the cache of course). But this is nonetheless a very small area in which to look; about the size of her hoop alone. And if you then triangulate by taking several such readings, all from a distance, each one of the readings will begin to agree on just one spot.

 

This technique also works well in the woods because you can take a reading where the sky is visible and use that reading where it isn't.

 

You need a compass that has a movable bearing-ring around the outside, with an arrow printed on it. Dial in the bearing, then turn your whole body until the north-seeking end (or, for back-bearings, the south) fits in that arrow; "boxing the needle." Now you are facing precisely down (or precisely against) that bearing. +/- a few degrees, you are looking at the cache.

 

There's another number your GPS can tell you that you also need to be very aware of, and that's DOP (Dilution of Precision). Basically that tells you how "confident" the GPS is that you really are so-many feet away. If DOP=1.0, the GPS is quite sure. If DOP=2.0, the true reading could be twice as big. DOP=3.0 isn't good for anything. Your GPS probably draws the accuracy-circle by multiplying the distance by DOP, which is why the circle keeps getting bigger and smaller. Once you are "inside the circle" and the cache is also "inside the circle," your GPS is simply guessing. And therefore, so are you.

 

Thot, to answer your question directly: All of our information is, obviously, coming from the GPS receiver. (This isn't orienteering or letterboxing.) But, we are applying the information it gives us in a different way, so as to factor-out the combined error. Nothing will ever enable us to know where in the owner's hula-hoop the cache may be. And, nothing will enable us to know our own position more precisely. (Say...) By taking our bearings from a distance, we reduce the influence caused by the fact that both of the positions are unknown by causing the two influences to overlap one another. If we also triangulate, we gain more precision by merging the data obtained by two independent readings. We are setting up a condition by which the various independent sources of error begin to cancel each other out, greatly increasing the probability that our answer is correct and that our cache will be found where we expect it.

 

"No, it's on this end of the log. It's that light-post. It must be on this part of the bridge." You can do all that, with your GPS in your pocket!

Link to comment

Over the years, I've tried a number of techniques to zero in to the cache location. Variations of the one you suggest included. The technique that I've settled on is as follows:

  • follow the little arrow thingie until I am +-10 feet or so.
  • look around for the cache, concentrating on piles of sticks next to trees.

:o

Edited by sbell111
Link to comment
Heheh.  I like sbell111's method too.  I do find it interesting that I put this same method into my FAQ when the FAQ first came online.  This same method was documented in this book. :blink:

But, it doesn't get you closer than the coordinates do. It's just another method of zeroing in on the location the GPSr indicates. It does help when the target location is covered and you can get outside the cover to take the readings and then triangulate on the location using this method.

 

But, the OP is saying this method gets him better results than the GPSr and it even corrects for errors in the posted coordinates.

Edited by Thot
Link to comment
Heheh.  I like sbell111's method too.  I do find it interesting that I put this same method into my FAQ when the FAQ first came online.  This same method was documented in this book. :blink:

But, it doesn't get you closer than the coordinates do. It's just another method of zeroing in on the location the GPSr indicates. It does help when the target location is covered and you can get outside the cover to take the readings and then triangulate on the location using this method.

 

But, the OP is saying this method gets him better results than the GPSr and it even corrects for errors in the posted coordinates.

 

how can it correct for bad coordinates -

 

it is these (bad) coordinates that he is triangulating on.

 

So he will end up zero'd out on the bad spot same as I will if I use the coord screen to take me to the 'exact' spot he is aiming at.

 

so my question is "HUH?"

Link to comment
Thanks for posting that.  I learned a lot from it and am going to try this technique this coming weekend if I can get out 'caching.  How much of an error gets introduced if you can't get 300 feet away?  Say you can only get 100 feet away?

If you want to get technical and trigonometric, you can figure the spread using the arctan(x) (inverse tangent) function. You know the length of the two sides of a triangle and you want to know the angle that produces that triangle. :blink::DB)

 

But if you're not a mathematics-fiend, :D then the simple answer is that you want to get as far away as you reasonably can. You'd like to be 200 feet or more away when you take your sightings but sometimes you can't be. In that case, triangulating (from several different points) becomes more important. And let's face it... not every cache hiding-place presents you with the same degree of difficulty or demands the same amount of precision.

 

For instance, last weekend I located a few new caches that were downhill off a mountain road... briar-patch city! But as I walked along that road, taking readings from about 100 feet away, it became clear that each reading was zeroing in on a certain <<feature>>. So I walked down to that <<feature>>, looked <<adjective>> it, and found the cache immediately. I knew that the cache would be found there because each reading was consistently pointing me toward that <<feature>>. The GPS would have been "vegged out" by the trees but it had no problems at all working from the roadway.

Link to comment

But, it doesn't get you closer than the coordinates do.  It's just another method of zeroing in on the location the GPSr indicates.  It does help when the target location is covered and you can get outside the cover to take the readings and then triangulate on the location using this method.  But, the OP is saying this method gets him better results than the GPSr and it even corrects for errors in the posted coordinates.

how can it correct for bad coordinates -

it is these (bad) coordinates that he is triangulating on.

So he will end up zero'd out on the bad spot same as I will if I use the coord screen to take me to the 'exact' spot he is aiming at.

so my question is "HUH?"

No, let's be clear about what I am saying here! :blink: If the original posted coordinates are wrong, then quite obviously there is nothing that can be done to "correct for" that. You can't make a silk purse out of a sow's ear, no matter how hard you try. If the data you have is incorrect, then it's impossible to create the correct data out of thin air... period.

 

We all know, however, that incorrect coordinates are rarely posted, and when they do get posted they're very quickly corrected. If someone else has found it... so can you. I've encountered bad coordinates only a handful of times, and all of these were, naturally, on FTFs.

 

"Following your nose until you're ten feet away...?" Ahh, there's the rub. Your GPS cannot tell you that you are "ten feet away!" Oh, it can say that on the meter, but the truth is that it can't know your location with that kind of precision. The moment your distance to the cache reaches the current effective-error of your position reading (you're "inside the circle"), you have reached the limits of what GPS can do for you (right here and right now), and you're now "Wandering Willie." Think about those hula-hoops... about how, and why, 15-feet becomes 45.

 

Oh well... enough repeating myself. I hope this helps! Happy geocaching!! :D

Link to comment
If the original posted coordinates are wrong, then quite obviously there is nothing that can be done to "correct for" that. 

The need for that statement should illustrate how difficult it is to understand what you're saying.

 

As I said in my original post I'm having a heck of a time figureing out what you are saying. Particularly since you did not answer the questions I asked in my first reply. So I’ll ask again.

 

Do you believe you can get closer to the location of the cache with this triangulation method than with the GPSr alone?

 

Said another way, if the GPSr is off by 30 feet do you believe your method will get you closer to the cache than 30 feet?

 

For the sake of this question assume all of the activity takes place in an open area with no trees or other things that interfere with the GPSr.

 

In your reply please answer my question above first. Please make the first sentence of your reply either, “Yes my methods gets me closer to the cache than I can get with my GPSr alone,” or “No my methods cannot get me closer to the cache than I can get with my GPSr alone.”

 

In my first reply I gave an illustration where the GPSr is in error by one mile. I did this to magnify the error enough that readers can visualize that your method would have you going through your triangulation method a mile from the cache. You refused to address this hypothetical saying a GPSr can’t be off that much. I also asked about large errors in the posted coordinates. Again you refused to address this saying posted coordinates are never off by much. These replies beg the question. By refusing to address hypothetical situations with large GPSr and posted coordinate errors you simply make it hard to visualize that your method does nothing more than take you to the GPSr’s coordinate location in a much more difficult and roundabout way.

 

It's simpler and more direct to do what most people do -- follow the GPSr to the coordinates.

 

The need for that answer should illustrate it’s difficult to understand what you are saying.

 

If you wonder why I pursue this, it’s because you seem to be convincing some novices that this method gets them closer to the cache. It won't, and I hope to prevent them wasting time going through your machinations which are pointless.

 

I want to make clear that the triangulation method will help under certain conditions. For example, when the cache is located in a covered area where you cannot get good readings. By moving to open areas you get good GPSr answers you can transfer these to the cache location using the triangulation technique. Under those conditions the triangulation technique is useful. It can sometimes be helpful in an area of restricted movement where you cannot move easily to the GPSr coordinates.

Edited by Thot
Link to comment

Yes, triangulation works very well if you can't get good signal near the cache site and you are able to get good signal within line-of-sight of ground zero.

 

However, if I'm reading you right, it's almost as if you're saying the further away from GZ the more accurate the reading. Not true, you just have tighter tolerances.

 

First, we use Magellan SporTraks. Yes, they are slower, but we rarely are in so dense of foliage that we can't get decent signal. Plus, they auto average.

 

Second, our technique is a bit different. I try to appraoch as straight in as I can without unecessary bushwacking and slow about 75' out to account for lag. I've learned to better judge where to stop. I then clip the GPS to a tree limb or to my walking stick and let it sit still while I go off and help Sissy hunt the cache.

 

If we haven't found the cache within a few minutes I go back to the GPS and find the bearing to the cache and sight using my manual compass. We use her GPS while hunting the cache while mine is averaging and all the while getting a more accurate notion of where in the real world it is.

 

This technique has worked very well for us. Most of the time it is not needed because you can usually very easily find a cache in the woods. Though one comes to the mind that was very hard. The technique eventually pointed the GPS to within a couple of feet of the cache. I learned that I can have high confidence in the reading the longer it averages. The confidence in the other person's reading is the kicker unless I know them and their hides. Some folks' reading and my technique gives me the confidence that I can mark a spot, put one hand on it and the cache be within arm's reach.

 

The thing to remember is within the tolerances we're talking about, a GPS does not adjust for error between where it thinks it is and where the marked waypoint is. The error will always be the same direction and magnitude. In essence, the error whether you are standing on top of the waypoint or 200' out, it's the same--given all variables the same otherwise.

 

Hope this helps.

Link to comment
However, if I'm reading you right, it's almost as if you're saying the further away from GZ the more accurate the reading.  Not true, you just have tighter tolerances. [...] The thing to remember is within the tolerances we're talking about, a GPS does not adjust for error between where it thinks it is and where the marked waypoint is.  The error will always be the same direction and magnitude.  In essence, the error whether you are standing on top of the waypoint or 200' out, it's the same--given all variables the same otherwise.

CoyoteRed, you are saying exactly what I mean: tighter tolerances.

 

Nothing can make the owner's GPS information more accurate than it is. You cannot calculate either the right answer or a better answer by any means. No matter what you do, this uncertainty will always remain. (Cache owners strive to minimize it.) I didn't want to dive into trigonometry and I don't want to dive into the mathematics of probability, but this is a probability-problem. Nothing will ever change the essential nature of the problem, which is of course one of the challenges of our sport. You're faced with a problem with two degrees of uncertainty; now, here is a useful method that reduces it toward only one.

 

What distance (and triangulation) does do is this: it helps to factor out the effect of your uncertainty as to your own position! Your readings will not "vary widely" as they do when you get close; they will be consistent.

 

It is a bit unfortunate that GPSes tell you not only "you're X feet away," but "you're X feet away in Y direction." Once you get "inside the circle," both pieces of data are bogus. Use the information from a distance, and when you get closer, put your GPS in your pocket.

 

---

btw: I realize that what I am explaining is a bit difficult and that I may not be explaining myself clearly, but I'm not "refusing" anyone or being evasive. (Nor, particularly, trying to be some know-it-all!) :mad: "It works for me, and here's why; hope it helps!" Other fellow cachers, please chime in!

Link to comment
...But as I walked along that road, taking readings from about 100 feet away, it became clear that each reading was zeroing in on a certain <<feature>>. So I walked down to that <<feature>>, looked <<adjective>> it, and found the cache immediately. ...

Was that <<feature>> a pile of sticks next to a tree? :mad:

Link to comment

GPSrs do not tell you X distance away at Y direction if you turn off maps and navigation modes. Within 100' of a cache, I switch to satellite mode and glance at signal strength and actual position coordinates. When closing within 50' of the cache, I note general direction (landmark, etc) and put the GPSr away and look for the cache (no kidding, the "where would *I* hide one here technique works really well!). Hips does the same thing, but from 300 feet out. Last weekend I did a really long cache (I think the total hike turned out to be over 5 miles), I used the GPSr pointer to find a landmark in the distance and put the GPSr away (much over 300 feet, even). This just makes sense, for it is better to watch where you are walking and pay attention to your surroundings then have nose glued to GPSr. Plus, if the GPSr dies, Bush shuts the system down, or sun spots clobber signals, you have SOME clue as to landmarks you recognize to get back out.

 

Once I get to the cache, I put the GPSr down at the actual cache location and note distance and direction the GPRr tells me the cache is located. It is ALWAYS amusing. If it shows more than 40' for a micro, or more than 60' for an ammo can, I'll let the cache owner know.

Link to comment

Back in May 2002, I posted the following in a 'How do you zero in' thread:

 

I tend to use the following steps to locate a cache:

 

1) Follow the arrow until it approaches zero feet It might still say ten feet away or whatever. I know I'm close because the distance and directions will start to go batty. I mark this physical point. Drop my pack, set a stick at the location, remember the approximate spot, whatever. At this point, I also verify the difficulty rating. This can give me a clue as to how it is hidden (Not always, however. A few weeks ago, I found one that required what I would call 'beginner rock climbing'. It had a terrain rating of 1.5.)

 

2) I look for the obvious hiding places in the area (within about 30 feet of my marked location). Piles of sticks, rocks, or leaves, downed trees, stumps, the base of trees, in bushes. 95% of the time, this results in a find.

 

3) If I still haven't found it, I'll back off 75 feet or so and shoot a bearing. Pull out the hand-dandy compass on march the distance shown on my GPSr. Mark the location as in number 1, above. (Some cachers do this step first. I prefer to snag the easy ones without this step, as explained above. If I have problems, I'll do this step.)

 

4) Follow the procedure in number 2, above.

 

5) Pull back 75 feet in a different direction, follow the procedures in step 3 & 4 with the 'marked location' being the intersection of the two bearing lines.

 

6) Expand my search area to about 50 feet from the marked location.

 

7) Decrypt the hint. This will typically give me the bit of info I need to find it. Beware of obfuscation.

 

8) Look everywhere. Expand search area to 75 feet of marked location.

 

9) Go to the next cache on my list. If I come back on another day, I will typically find it with no problems.

Link to comment

You are deaing with basic triangulation. There are a number of factors that come into play the for most is that more the legs of the triangle are equal distanct the more accureate the measure of the distance. This is simple suveyiong I learned in college way back when.

 

The main problem with this is that you GPSr is giving a bearing to where it thinks, okay caclulates the coordinates of the waypoint, i.e. cache. But if the GPSr caculates the coordinates as being 50 feet from where the cache actually is then your triangulation will tell you that the cache is 50 feet from where it actually is. SO basically Thot is correct in it is NO more accurate then just following the pointy arrow and start looking when your 10+feet out.

 

The typical GPSr is not ment to give you accuracy to the cm but 10meters is typically good enought to find you car or tent or the mouth to the harbor. That is the main falicy with your esplination is you assume to much accuracy on the part of the GPSr. But it is not accurate. Some days it is very good others it is well SUXs. I have done caches where I zeroed out and was 1 foot from the cache otherw where it zeroed out and was 50 feet from the cache. Go figure. But the GPSr does NOT know where the cache is so the bearing it give is just to where the coordinates are for the cache which can vary by even if you have 3meter accuracy up to 6 meters, 3 for each GPSr yours and the placer, so you have a potential for a total circle with a diameter of 12 meters, or about 50 feet.

 

cheers

Link to comment
The main problem with this is that you GPSr is giving a bearing to where it thinks, okay caclulates the coordinates of the waypoint, i.e. cache.  But if the GPSr caculates the coordinates as being 50 feet from where the cache actually is then your triangulation will tell you that the cache is 50 feet from where it actually is.  SO basically Thot is correct in it is NO more accurate then just following the pointy arrow and start looking when your 10+feet out.

 

Ahh, I guess the point is subtle after all. There is a difference. Without triangulation, you, who do not know your own position within (say) 50 feet, are looking for a cache whose position is also not known within (say) 50 feet. You assume you know when you are "10 feet out," but you don't and you can't. This point cannot be over-emphasized! This is what misleads people: one is taking at face-value a piece of information that cannot be known at face-value. One is using a measurement that in fact describes the approximate center of a region of some size, as though it were a description of a single point. It is not.

 

Since you cannot eliminate either source of uncertainty, you compensate for it, employing the available information in a different way. Reliance upon angles from a distance, rather than location-indicators and map-pointers, will yield markedly better results. A high-quality compass is needed, with careful attention on how you are using it, since "+/- 4 degrees" is not a very large spread to measure. I simply encourage you to try it for yourself.

Link to comment
I wonder if the few minutes taken by doing all the triangulation, etc. saves more time than just going to the slightly more general spot and starting to look.

Since the triangulation gets you no closer to the cache than just going straight to it, the answer is a clear; no, it doesn't save time -- it takes longer. Quite a bit longer.

Link to comment

I notice once again you didn't answer my direct question, so I'll repeat it again:

 

Do you believe you can get closer to the location of the cache with this triangulation method than with the GPSr alone?

 

Said another way, if the GPSr is off by 30 feet do you believe your method will get you closer to the cache than 30 feet?

 

Please make the following answer the first sentence of your next reply. Either, “Yes my methods gets me closer to the cache than I can get with my GPSr alone,” or “No my methods cannot get me closer to the cache than I can get with my GPSr alone.”

Link to comment

As Coyote Red said, your method assumes the farther you are away from the cache the more accurately you can home in on its location. By this logic, if you were a mile away with a compass sextant you could probably narrow the location down to an inch.

 

You say:

A high-quality compass is needed, with careful attention on how you are using it, since "+/- 4 degrees" is not a very large spread to measure.

I think you mentioned this yourself, but just to be sure. If you have a +/- 4 degree error in pointing your compass, at 300 feet away it creates a 42 foot error. So, if you knew the exact bearing to the true location of the cache and you can’t point your compass better than closer than +/- 4 degrees you would be off by 42 feet. If you are good enough to aim it +/- 2 degrees you will be off by 21 feet. Maybe you're better than me, but I doubt seriously I can sight a compass closer than 2 degrees.

 

But, all this is a digression, because however far away you get, you are always sighting along a bearing given by the GPSr. This is, you are pointing your compass to where the GPSr’s thinks the coordinates are.

Edited by Thot
Link to comment

No - you won't get any more accurate readings. What you'll get is a larger area of acceptability in locations the further you move away.

 

I like diagrams, so here we go.

 

Let's assume for this portion that you're using a highly accurate Trimble GPS. Let's also assume that a little yellow etrex can only get you within a 30 foot radius of the actual location. Finally, let's assume that the cache is under the tree, and yet the coordinates that the hider's GPS gave are about 10 feet off, right next to the big black monolith. The coordinates are within the accepted tolerance of 30 feet - so they're just slightly off.

a46fbbf4-ac27-4e68-8544-9bf61405cb79.jpg

 

If you extend your range out of the 30 foot radius, you'll be triangulating on the bad coordinates, and the compass will be pointing to the monolith. That still doesn't get you any closer. You're just triangulating on bad coordinates.

bae9437b-92e4-4ab7-bce7-c64c0e6b41ae.jpg

 

Now let's switch the Trimble and the little yellow etrex. The hider is using the Trimble and placed ultimately accurate coordinates. Your etrex may be placing you anywhere within the grey circle, (here's Thot's point) backing up to get a new bearing is still going to place you anywhere within the grey circle - but not necessarily in the right spot.

 

HOWEVER (and this is something I've wrestled with since I started GPS-ing), if I back up 100 feet, and the GPS says it's at a bearing of 42° from spot A and 170° from spot B that gives me an AREA to search. 42° is not really 42.0000000°, but something between 41.5° and 42.49999999° - a range due to rounding. Also, a compass is not highly precise either. I doubt that I would be able to site a very precise 42° let alone 42.00000°. Therefore by backing out the circle, I get an area of searchability based on the tolerance of what I consider "42°". It's an ever-widening angle from my GPS/Compass. Combine that ever-widening angle with another ever-widening angle and you get a four-sided polygon area of searchability.

38cb364a-d86f-439a-a2e3-70687fe8c8c8.jpg

 

To take it to the absurd to see if that works, let's back up further - about 100 miles. I used Geocalc and found that if I am standing at N 41° 16.612 W 087° 30.978 and project a waypoint 100 miles at a bearing of 42.49°, I get N 42° 20.270 W 86° 11.836. If I project it at a bearing of 41.5°, I get N 42° 21.292 W 86° 13.320. These two points would both be considered 100 miles away at 42°, but are actually 1.73 miles apart.

 

So by backing FARTHER up, what you consider the targetted bearing has a wider range, and by taking two areas of wider range, you might get a larger (and more accurate, but not precise) location of the cache.

0d80f964-ccf3-4aff-b8e8-063162ffcdda.jpg

 

Eventually, even at the lesser distances, you will back up far enough that your 42° from point A and 170° from point B WILL point to the right location, but only because the area of convergence of the ever-widening angles overlap with the area of the 30 foot radius. But you'll probably be far enough away that it will just be "over there next to that clump of trees" and sure enough, your GPS would have gotten you there in the first place.

 

What does that mean to the average cacher?

Using a compass to triangulate on a location based on the bearings taken from your GPS will not get you to any more accurate location than just using your GPS.

 

However, after having been lost in a forest once because my GPS couldn't get a signal and I didn't have a compass to tell my direction, I will never go into the forest without one again.

 

Also, I take the compass along for when the GPS's arrow won't actually point to the bearings that it's giving me. If my GPS is saying that the cache is at 165° and 50 feet from my location, but the because I've stopped and the GPS's arrow is actually pointing at 24°, I like to have my compass to assist in pointing out which way is REALLY 165°.

Edited by Markwell
Link to comment

I don't know where some of you guys go caching but backing off 300 feet around many caches near here will yield zero view of any object near the cache. Also by the time I step around trees, rocks, streams, thorns, the occasional rattler and spend 5 minutes chasing down my 2 year old - my compass and bearings will be useless. However, the GPSr still cheerfully points me in the general direction of an area that I will have to seach within 40 foot of to compensate for the general tilt of the universe.......sigh. AKA - look for the out-of-place pile of {fill in objects here}

Link to comment

The deal is your GPSr can only caclulate a bearing and distance to a given waypoint. THis calculated value can vary greatly. Here is a test. GO set a waypoint and mark it with a little flag or stick in the ground. Come back a week or two later. Have your GPSr tell you where the point is and see how close you come. Depending up on the satellite constilation you may be close but mostlikely your be far off, ie up to 20-30 feet.

Your assuming again you GPSr has a hig degree of accuracery it doesnt not to the extent your talking about. Your technique will not get you any closer then just followint the arrow/pointer display.

I use a compass where we will be covering distance to take a sighting on a distanct object and then walk to it and repeat.

cheers

Link to comment

You're method sounds great if it works for you. I'll just follow the arrow 'till I zero out and then be sure to not give away the cache location while you're still triangulating. :unsure:

 

Seriously though. I've done a few caches where you physically couldn't get two bearings due to hills, rivers, cliffs etc.. So just be sure you don't forget how to do it the old fashioned way.

Edited by WildGooseChase
Link to comment

Markwell: "Using a compass to triangulate on a location based on the bearings taken from your GPS will not get you to any more accurate location than just using your GPS."

 

HRRRM. You are certainly correct. I have no idea why one would triangulate based upon GPSr bearings. However, one could bring up a topo map, find two prominent terrain features, determine magnetic bearing to those, then go to the field and spot those, use triangulation off them with a magnetic compass (called a "re-section") to determine the precise spot on the ground. To the inch! Much more accurate than a GPSr ever will be.

 

The problem is, if the cache owner did not determine the coordinates this way, we STILL have to deal with the 22-30 foot best accuracy of THEIR GPSr. Ack! As several very experienced cachers (or those extremely proficient at navigation techniques) have pointed out time and again, based upon all of this, it is so much quicker to use a GPSr, expect at least a 40' inaccuracy, and enjoy the stinking fun of the stinking hunt.

 

Oh, markwell, I am not suggesting you are stinky.... (nor anything in the second paragraph, for that matter). :unsure:

Link to comment

Take the beautiful graphic from two posts ago, now, and draw a second gray circle, slightly overlapping the first. This is the error concerning your present location. The cache owner promises that the cache is somewhere inside his circle... but are you actually inside the owner's circle yet? This is what you do not know, and this is where the second degree of uncertainty arises.

 

On the average, the two circles will overlap by some amount, so a 30-foot owner's circle might expand to only a 50 or 60-foot search area. The two sources of uncertainty blend in with one another to some degree. But you are still searching an area larger than you could.

 

And once you get in that area, following that little pointer, guess what! Once you get inside the circle, every one of those 360 degrees is equally probable. The pointer reading is meaningless. If it does not appear to be so, consider this: you could be relying, without even knowing it, on some of the averaging-data that your GPS has accumulated as you walked in a straight line. Have you ever noticed that your GPS seems seems to become more uncertain the longer you pace back and forth like that?

 

For many caches (and, heaven help us, light-poles...) there is a lot of ancillary information that you can draw upon. If you see an obelisk and a large tree, and nothing else, then you probably don't need the additional information. Heck, you can leave your GPS in your car. But what if the area is a riprap pile of stones at the edge of a river, hundreds of feet long, and in that area you are searching for one fake rock... :unsure:

 

I have answered the question and will repeat it once more: The purpose of the triangulation technique is not to make the owner's area smaller than it is; nor to divine what cannot be known. (These are technological limits of the GPS system, period.) What it does do is to reduce the amount of area that you will otherwise find yourself searching, reducing the area to approach that of the original. To repeat: It is not possible by any means to determine whether the cache lies under that obelisk or under that tree. What triangulation can do is to limit your search space so that it is no-larger or not-much-larger than that gray circle.

 

The proof is in the puddin'.... try it for yourself. If you don't want to, or if you routinely search for caches where the location is self-evident once you get there, fine! There are plenty of caches out there like that. But the caches I place, and the ones I love to find, are not like that. The caches I love to find don't have visual clues. In my neck of the game, the cache might be in a film-canister with a fake piece of grass glued to the top, sitting flush to the ground. And this is where your compass might come in handy. When people go out caching with me, I notice :unsure: that they start following me, because I'm not wandering: I'm walking, purposefully, in a straight line.

Edited by HIPS-meister
Link to comment
You are certainly correct. I have no idea why one would triangulate based upon GPSr bearings.

 

From the original poster...

 


  •  
  • Walk to a point where the GPS tells me that the cache is about 300 feet away (the length of an American football field), and I can see the place where I expect the cache to be.  I try to place a prominent landmark directly behind me that I will be able to see easily from the cache location.
     
  • Stop and wait about one minute, with good view of the sky, for the GPS to settle.
     
  • Note the magnetic compass bearing on my GPS screen.  Jot it down. (I do not have a built-in compass on my GPS and do not use the bearing-pointer.  Furthermore, I ignore the distance figure and use only the bearing number.)

 

Color added for emphasis.

Link to comment
...But what if the area is a riprap pile of stones at the edge of a river, hundreds of feet long, and in that area you are searching for one fake rock... :unsure:...

What a coincidence. I found a cache just like this just a few weeks ago. I was patient and let my old 3+ settle for a few minutes and it took me right to the cache. I was actually standing right over it.

Link to comment
The deal is your GPSr can only caclulate a bearing and distance to a given waypoint. THis calculated value can vary greatly. Here is a test. GO set a waypoint and mark it with a little flag or stick in the ground. Come back a week or two later. Have your GPSr tell you where the point is and see how close you come. Depending up on the satellite constilation you may be close but mostlikely your be far off, ie up to 20-30 feet.

Have tried this test many times and although I have seen readings as much as 25 feet off - I TYPICALLY (most of the time) see readings within 15 feet. Give the GPSr a few minutes to average at each test and it is typically within 6 feet. An error margin I can live with.

 

Also.....the most {much more than half} of caches I have found have been within 15 or so feet of posted coors even given all the possible errors. I do not find myself weaving and stumbling my way - at least not until I see I am within 30 feet or so of coors.

 

BTW - I am the type that has absolutely no interest in looking for a micro with grass glued to the lid and pushed into the ground to look like 99.9999% of the surrounding area. I KNOW for sure that my GPSr isn't accurate enough for that task. But to each his own......

Link to comment

5d6c86ab-efb2-458e-a08d-03c74366161a.jpg

 

OK...

Area of grey is the uncertainty area of the hider.

Area of blue is the uncertainty area of the seeker.

Area of orange is where these areas intersect.

Seeker goes outside of the bounded area and triangulates to what their GPS shows as Ground Zero. It still points to the wrong spot. How does that decrease the area of searching? Wouldn't the seeker still search in the blue area? They don't know where the overlapping orange is. For all they know the blue area that they should avoid could be southwest of ground zero instead of northeast like the diagram shows.

 

Believe me - I've tried the triangulation, and I think ANECDOTALLY this works, like averaging several coordiantes. I just would like to see if there's any empirical or logical basis for this.

 

I believe that you CAN use triangulation if you're trying to get your GPS to zero out. But just zeroing out your distance on the GPS doesn't guarantee that the cache is at that location.

 

I mentioned in my last disortation above that I use the compass when I can't get the arrow to point in the right direction. But in the end when I DO get my GPS to zero out, it's probably not going to get me any closer to the cache than someone else that just got within the 30 foot radius area.

Edited by Markwell
Link to comment
Take the beautiful graphic from two posts ago, now, and draw a second gray circle . . . . <snip>

For the third time you have refused to answer, “Yes my methods gets me closer to the cache than I can get with my GPSr alone,” or “No my methods cannot get me closer to the cache than I can get with my GPSr alone.”

 

Apparently you are unwilling to answer this simple straightforward question. Why is that?

Edited by Thot
Link to comment
As Coyote Red said, your method assumes the farther you are away from the cache the more accurately you can home in on its location. By this logic, if you were a mile away with a compass sextant you could probably narrow the location down to an inch.

 

You say:

A high-quality compass is needed, with careful attention on how you are using it, since "+/- 4 degrees" is not a very large spread to measure.

I think you mentioned this yourself, but just to be sure. If you have a +/- 4 degree error in pointing your compass, at 300 feet away it creates a 42 foot error. So, if you knew the exact bearing to the true location of the cache and you can’t point your compass better than closer than +/- 4 degrees you would be off by 42 feet. If you are good enough to aim it +/- 2 degrees you will be off by 21 feet. Maybe you're better than me, but I doubt seriously I can sight a compass closer than 2 degrees.

 

But, all this is a digression, because however far away you get, you are always sighting along a bearing given by the GPSr. This is, you are pointing your compass to where the GPSr’s thinks the coordinates are.

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.

Link to comment
5d6c86ab-efb2-458e-a08d-03c74366161a.jpg

 

OK...

Area of grey is the uncertainty area of the hider.

Area of blue is the uncertainty area of the seeker.

Area of orange is where these areas intersect.

Seeker goes outside of the bounded area and triangulates to what their GPS shows as Ground Zero.  It still points to the wrong spot. 

Very nice colorful graphics. You have illustrated the situation very well. Exactly as you show, all that the triangulation he advocates does is point him to where the GPSr thinks ground zero is. The very same place you go if you follow the GPSr directly to ground zero.

 

I took the liberty of altering your graphic slightly. The center of the gray area would be the posted coordinates and the cache could be anywhere in the gray circle. It does not necessarily have to be in the orange overlap/intersection.

 

markwell0sw.gif

Link to comment
No - you won't get any more accurate readings.  What you'll get is a larger area of acceptability in locations the further you move away.

 

<snip>

 

So by backing FARTHER up, what you consider the targetted bearing has a wider range, and by taking two areas of wider range, you might get a larger (and more accurate, but not precise) location of the cache.

0d80f964-ccf3-4aff-b8e8-063162ffcdda.jpg

I agree with your drawings. But, I’m not sure I understand your point.

 

The farther you get away the larger your area of uncertainty (the orange area in your drawing). And the larger this area of uncertainty the more likely it is to contain the cache. Clearly, the larger the area you encompass the more likely it is to contain the cache. If you make this area of uncertainty larger and larger it will eventually have to include the cache, even if it’s 100 yards away. If by “more accurate” you mean the area is more likely to contain the cache, I wouldn’t consider that accuracy. But, by that definition just start out assuming you are going to search a 100 foot circle around the point you GPSr thinks is the posted coordinates and you will almost always have an “accurate” search area.

 

Incidentally, your orange area should be centered on the black dot, not on the cache.

Link to comment
The farther you get away the larger your area of uncertainty (the orange area in your drawing).  And the larger this area of uncertainty the more likely it is to contain the cache.  Clearly, the larger the area you encompass the more likely it is to contain the cache.  If you make this area of uncertainty larger and larger it will eventually have to include the cache, even if it’s 100 yards away.  If by “more accurate” you mean the area is more likely to contain the cache, I wouldn’t consider that accuracy.

That was my point. You will get a "correct" answer, but that it won't necessarily be helpful. Example: The Lincoln Memorial is 620 miles, at a bearing of 104° from my front door stop. That is accurate, but not really helpful. Is that 103.5000° or 104.49°. It could be anywhere between N 38 59.049 W 77 00.271 and N 38 50.400, W 77 04.574 - a distance of 10.7 miles.

 

But, by that definition just start out assuming you are going to search a 100 foot circle around the point you GPSr thinks is the posted coordinates and you will almost always have an “accurate” search area.
Again - accurate, but not precise. Lowering precision improves accuracy. It doesn't make it correct or even helpful, it just increases the likelihood of being right, which isn't necessarily a good thing.

 

Quick analogy (since we like Starbucks this week): A new employee comes on and says he can be more correct with the prices by guessing at them after having bought Dunkin Stunod coffee for years. He tells customers that the coffee is more than 3 and less than 7 dollars. He's accurate, but not precise. His lack of precision increased the likelihood of his being accurate, but it lacked the precision necessary to complete the task. He has a price list that is both accurate and precise enough for what he needs to do. We have a tool that is both accurate and precise enough for what we need to do - the GPS.

 

My point is that yes, you can be more correct by losing the precision, but that isn't necessarily better. You're just increasing the area of searching.

 

All of this is beside the point. The real point is that by triangulating based on your GPS's listed magnetic bearings is only going to get you to the center point of where your GPS thinks a particular set of coordinates is at that time. It will not increase the likelihood that you'll be within the area of the other person's coordinate circle (the fluorescent-purple hula-hoop). You'll only be finding the center of the green hula-hoop.

 

In other words, I'm agreeing with you, Thot. Using the magnetic bearings from the GPS and triangulating on the position will not get you closer to the waypoints provided by the hider. It will get you to the point that your GPS says you're 0 feet away from the cache. If you wait 15 minutes and try it again, you'll zero out again - probably in a different location. That's the nature of the GPS, and the challenge of the game.

 

Incidentally, your orange area should be centered on the black dot, not on the cache.
Remember for the orange area, I changed my premise. This assumed the hider used ultimately precise coordiantes. The monolith and the black dot didn't need to be in the picture.
Link to comment

While he is waiting for his GPS to settle down, I have closed the 300 foot gap, started looking for URPs, USPs, or "boxes under bushes, and logged my find.

 

I have cached with a pair that has over 1700 finds (Team Perks), , and I myself have over 300 finds. What works best for me, and them, is to start looking at the most likely spots for a geocache when you get 20 feet accuracy on your GPS.

Link to comment
Incidentally, your orange area should be centered on the black dot, not on the cache.
Remember for the orange area, I changed my premise. This assumed the hider used ultimately precise coordiantes. The monolith and the black dot didn't need to be in the picture.

My error. I see now you did change. A cursory review of that post had me on your original meaning for the dot.

Link to comment
It is a bit unfortunate that GPSes tell you not only "you're X feet away," but "you're X feet away in Y direction." Once you get "inside the circle," both pieces of data are bogus. Use the information from a distance, and when you get closer, put your GPS in your pocket.

I'd like it to have something along the lines of a parking sensor... start beeping as I get closer, progressively shorter times between the beeps. When I'm right on top of the cache, it will emit a steady tone.

 

That would make it a lot more enjoyable.

 

:D

 

But seriously, the GPS are accurate enough, no need to take all the fun out of it by having it point right at the cache site. :ph34r: With good coords it'll take you right there pretty much each time anyway.

 

;)

Link to comment

If the GPSr was exactly correct all the time and each cache had exact coords posted how much fun would it really be? Part of the fun for me is have the GPS get me close and then for me to have to look around and think like the hider. "That stump looks like a good place"...stump is empty then move on to the next likely looking spot. I know I get bored easily and if this were too easy I would quickly tire of it. :ph34r:

Link to comment
I'd like it to have something along the lines of a parking sensor... start beeping as I get closer, progressively shorter times between the beeps. When I'm right on top of the cache, it will emit a steady tone.

Actually, I believe there are caches like that out there. (Lots of hams, like myself, in this sport.) Apparently there are also caches you can only find with a wireless-enabled PC running specific software.

 

"Too much time .. on my hands .." -- Styx :ph34r:

Link to comment
Take the beautiful graphic from two posts ago, now, and draw a second gray circle . . . . <snip>

For the third time you have refused to answer, “Yes my methods gets me closer to the cache than I can get with my GPSr alone,” or “No my methods cannot get me closer to the cache than I can get with my GPSr alone.”

 

Apparently you are unwilling to answer this simple straightforward question. Why is that?

Confrontational, aren't we? :ph34r: The answer, quite plainly enough, must be "yes."

 

The main thing is that a compass is not as subject to tree cover, nearby buildings or towers, etc as the GPS. The ability to do "back-bearings" to confirm that you are where you wanted to be also helps.

 

We were active in orienteering and route-finding long before GPSes were public and enjoy mixing the two technologies; finding this very useful. "O" fans have been using these techniques in the woods long before the first Navistar satellite was launched! (Obviously, the fact that we are using GPS is an adaptation of those techniques.)

 

Look once more at the overlapping-circle (Venn) diagram above, then ponder what would happen if the two circles, instead of being overlapped like that, were a few hundred feet apart. Consider only the influence on the bearing figure; ignore the distance. The "possible bearing to the target" is small at a distance; grows larger as you get closer; becomes a full 360-degrees in the orange (overlap) region of the diagram above (the exact size and orientation of that area being, as the artist did note, arbitrarily depicted on that diagram).

 

In the other diagram, the one showing incoming bearings as triangles that overlap the circle... consider that these shapes are actually rectangular. (You don't know exactly where you are, hence the apex is not a single known point.) The farther away you are, the more those rectangles approach being triangular and can be considered as though they actually were triangular.

 

You will not get a figure that is closer or more certain than what the cache setter was able to obtain, but you will obtain a figure close to it, which is a considerable improvement. As has been stated, if it was too easy, we would probably switch to a different sport/pastime/hobby/game ! :D

 

Cache on! By whatever means works best for you! ;)

Edited by HIPS-meister
Link to comment
Guest
This topic is now closed to further replies.
×
×
  • Create New...