Help With Compass

[+TwoCyclists]

Hi, I've recently bought a compass to take on my geocaching adventures. I think I have the basics pretty well mastered. I guess this might seem a daft question, but what information do I read from my GPS and transfer to the compass. I have tried taking the bearing.....is this as precise as it gets? Any help would be greatly appreciated!

[+cottonmouth]

When you take the bearing you should either use the magnetic north setting on your GPSr or take the declination from a map. And that would be about as good as it gets. If the declination is significant it may make a difference.

In my neck of the woods the declination is 1° which can be neglected for short distances.

[Mighty Tiggers]

In North America, the declination can be 11.5 or more. There is a difference between TRUE NORTH, MAGNETIC NORTH, and GRID NORTH. For the purposes of this discussion, consider "declination" the difference between magnetic north and grid north. Why is there a declination at all and why does it change, depending on your location in the world? Well there are a lot of factors. One example, is that significant iron-ore deposits under ground can actually affect a magnetic compass and will increase or decrease the declination.

 

A ballpark estimate is to line up the north/south grid lines with your compass, and then make an adjustment for the declination. Usually, the legend on your map will tel you how to convert from magnetic to grid and vice versa. By the way, the Lambert Conformal Conical Projection System maps (aka Lat/Long maps you buy in the store) aren't as accurate as other systems in use - but they are the most common. The Universal Transverse Mercador (UTM) system is a bit more accurate, and is used by the military. You can get UTM maps from the U.S. Geological Survey office or a number of online sources, and they can be extremely detailed (depending on the map scale). They don't have UTM maps for every part of the world, available to the public, but check the website for more info...

 

http://www.usgs.gov/

[+reveritt]

Personally, I don't use my compass much. It's mostly for backup in case the gadget stops working. I do sometimes use it on a long hike when I have a map on which I have marked the location of the cache. I refer to the compass (instead of the GPSr) while walking to make sure I am going the right way. This is particularly helpful under dense pines when my GPSr is getting a weak signal.

 

Another good use is when I have stopped walking and want to take a bearing to the target. My GPSr tells me the proper bearing, but if I'm no longer moving, I need to use the compass to tell me where to look/walk.

Edited January 17, 2005 by reveritt

[+Dave1976]

I use one of those pin on bubble compasses that you can buy for about three dollars and I only use that when I am standing still then just turn my gps to rough estimate and that has always worked for me.

[+Thot]

Hi, I've recently bought a compass to take on my geocaching adventures. I think I have the basics pretty well mastered. I guess this might seem a daft question, but what information do I read from my GPS and transfer to the compass. I have tried taking the bearing.....is this as precise as it gets? Any help would be greatly appreciated!

In Washington State the difference between True North and Magnetic North can be more than 20 degrees, so be sure your GPSr is set to Magnetic North. If it's set to True North and you follow a bearing out 80 feet you can be off as much as 30 feet.

 

I didn’t understand what you meant by “I have tried taking the bearing.” You align the bezel on the compass to the bearing and then when the needle points north the bearing mark (or arrow if you have one) points in the direction of the bearing (or the cache if that’s what you’re doing).

Edited January 17, 2005 by Thot

[Aushiker]

G'day

 

Geoscience Australia has, in my view, a very good publication out, called Map Reading Guide: How to Use Topographic Maps. Well worth taking a look at it but it is a 5 mb PDF file.

 

Regards

Andrew

[+the hermit crabs]

You mihgt interested in the links in this post by Tahosa & sons.

[+JohnnyVegas]

Unless your copass can be set to show true north (Most cannot be) leave you GPS on Magnetic north or they will not match up.

[HIPS-meister]

I find that the GPS's interpretation of "a magnetic compass bearing" is usually quite accurate. The almanac data on magnetic declination is usually quite close to what you will actually encounter at the site, and there is rarely any practical need to convert them to absolute-north.

 

A compass that you intend to use with your GPS ought to be comparable in quality to one that you would use in orienteering. You should practice with it so that you can reliably obtain, follow, and check bearings to within +/- 4 degrees of precision .. which is actually fairly generous.

[+Alan2]

Hi, I've recently bought a compass to take on my geocaching adventures. I think I have the basics pretty well mastered. I guess this might seem a daft question, but what information do I read from my GPS and transfer to the compass. I have tried taking the bearing.....is this as precise as it gets? Any help would be greatly appreciated!

What GPS are you using? If you have one with a built in magnetic compass, you don't need a separate compass except for backup in case the GPS fails.

[+scott9282]

As far as GPSr units with built in compasses, they are only accurate when moving at about 10-15 MPH according to the Geocaching for Idiots book. A compass gives you good steady bearings to your target area.

 

This is done as follows:

 

When you get to about 300' from the cache, take a bearing. This is done by reading the screen that shows the bearing the cache is to you, NOT your heading. Your compass should be set the same as the GPSr, true North or magnetic. Take your bearing, and look along that line to see what features there are around.

 

Now go off on about a 45 degree angle from your line of travel until you are again about 300' from the cache, take another bearing from the GPSr and shoot that one on the compass. Look along the line, you should see one of the features, a tree, boulder, whatever, that you saw with the first bearing. Head for this feature, your cache should be around it somewhere.

 

Another good reason to carry a compass is it is not susceptible to battery failure. With even a rudimentary idea of where you are, with a compass you can navigate your way back to civilization by at least being able to walk a straight path to avoid circling in the woods.

[+Alan2]

Hmmm.

[+Warriorrider]

As far as GPSr units with built in compasses, they are only accurate when moving at about 10-15 MPH according to the Geocaching for Idiots book.

Unless you have an electronic magnetic compass such as the MeriPlat and the 60cs.

[+Poindexter]

scott9282 Posted: Feb 16 2005, 09:19 PM 

As far as GPSr units with built in compasses, they are only accurate when moving at about 10-15 MPH according to the Geocaching for Idiots book.

 

Not true. Either the book is wrong or you have misinterpreted it. Receivers with built in "magnetic" compasses are quite accurate and you do not have to be moving at all. Even ones that do not have magnetic compasses do not require you to be moving at that speed for the heading to be accurate. A steady walking speed of 2 to 3 mph is sufficient as long as you have good satellite coverage.

 

Declination (variation) is the difference between True North and Magnetic North. They are different because the magnetic north pole is in a different location than the True north pole. Local variations caused by ferrous material is called deviation.

Edited February 17, 2005 by Poindexter

[+diverhank]

Forgive me but I am perplexed on how a compass would help me better in finding a cache? I'm a NOB but I realize that the GPS arrow does the bee dance when you slow down or stop.

 

What I've been doing to combat this is when I'm 500 ft or more from the destination, I'd walk briskly and generally the arrow is rock solid in one direction. I'd pick out a terrain feature like a tree or a fence at the end of the arrow and I'd get my bearing from that. I'd use the GPSr for distance only when I get closer. This technique works well for me...

 

When there are too many trees near the cache, and GPSr signals are bouncing around in hundreds of feet, I'd approach from two (sometimes three) different directions, pick the terrain features...I can visualize the lines in my mind's eye and where these lines intersect, that would be where the cache is. I only had to use this technique once but it worked like a charm.

 

Would a compass present a better solution? How? thanks.

[+fw_ord]

Would a compass present a better solution? How? thanks.

 

Most of the geocaching I do is in rugged outdoors terrain. I believe that a compass is essential under these conditions. Typically I go with a friend where one of us uses the compass and one of us manages the GPS receiver.

 

There are several reasons for this, the simplest being that often the terrain does not allow us to easily walk a straight line far enough for the GPSr to create a reliable track heading. Sometimes because of satellite blockage by terrain or foliage it takes several minutes for the GPS derived position to settle down and as soon as the receiver starts to move the position accuracy is lost. Another reason is that often in thick forests in the summer, we find that we only get good position fixes in nearby open locations, but not at the cache. (Though you have to ask how the cache provider got the coordinates, winter?). In this case we determine the waypoint bearing and then pace off distances from our known position to the cache. Recently we came upon a cache in a ravine and the GPS receiver was not working well. I went to a nearby hill top and got a bearing and distance to the cache from there. We then went directly to the cache about 30 yards away.

 

We also move faster cross country by navigating with a compass. By sighting landmarks with a compass based on the bearing to a cache, we get accurate points to hike to. The bearing and compass are far more accurate than trying to read the simulated compass needle on the receiver. When trying to cover a lot of ground, rotating the receiver even a little bit and trying to use it as a compass results in very large errors over long distances.

 

Though, there can be some problems. My buddy had a beautiful new compass given to him by a German colleague. We were really struggling with our navigation that day until we realized the compass bezel was marked in units of radians and the GPS receiver was giving waypoint bearing in degrees. After setting the GPS receiver to output bearing in milliradians everything worked better.

Edited February 18, 2005 by fw_ord

[+Alan2]

Most of the geocaching I do is in rugged outdoors terrain.  I believe that a compass is essential under these conditions.  Typically I go with a friend where one of us uses the compass and one of us manages the GPS receiver.

 

If you get a GPS with a built-in magnetic compass you can get rid of your friend

[+elderstuart/laine]

you could try this site "Bearings (navigation)," Microsoft® Encarta® Online Encyclopedia 2005

http://uk.encarta.msn.com

[+robert]

Forgive me but I am perplexed on how a compass would help me better in finding a cache? I'm a NOB but I realize that the GPS arrow does the bee dance when you slow down or stop.

except for a GPS with a built-in magnetic compass will continue to point to the cache when you stop.

[+fw_ord]

If you get a GPS with a built-in magnetic compass you can get rid of your friend.

 

Since our wives won't hike and camp with us and our sons have grown and moved on, I'm kind of stuck with him unless I find a way to enjoy sittting around a campfire grilling steaks and drinking beer by myself.