North Reference?

[cocheese1111]

Which north reference is used for hunting Cache’s / benchmarks?

[rusty0101]

You will find Caches predominatly use north as the cache implementer perceives it. I suspect that this is predominately magnetic, however in places like alaska far western US and western Canada, you will probably find that they are using either true north, or adjusted magnetic north. (for any specific volume of area on the surface of the earth there is an average declination between true north and magnetic north. In Minneapolis MN, the declination is less than 10 degrees.) Since the Magnetic north poll is somewhere near the north end of Hudson bay, CA, there is a large segment of North America where Magnetic North points no where near North.

 

Benchmarks use True north. There are two observational ways to get true north, and several mechanical and electronic methods available. The easiest observational method, (though growing increasingly in-accurate) is to locate the North Star at night. (increasingly inacurate as earth's precession is moving the true north poll off of Polaris.)

 

The second method, though it takes longer, and provides less immediate precision, is to take a stick, or post, and mark where the end of the shadow of that stick is at some time in the morning, then again at some time in the afternoon. The line between those two points is extreamly close to true West-East. A line perpendicular to that line is a True North-South line. Regardless of where on the globe you are (asside from within the arctic and antarctic circles) the first mark will always be West, the second always East, and if you stand so the West is on your left, and east on your right, you are facing North.

 

I won't go into the mechanical and electronic methods, explaining how a gyroscopic compass, or laser compasses work may be fun, but gets to be a bit too much like work. Suffice it to say that once one of these has been set up to reference True North, it will provide you with a True North pointer as long as it has power.

 

For the purposes of finding either a cache or a benchmark, this is overkill. For the distances provided in benchmark descriptions, you are generally ok using a compass adjusted to within 5 degrees. This is not the case with orienteering, where you may have to use a compass to navigate several miles, and find a target to within 1 or 2 degrees, but in almost all cases, benchmarks, reference marks and described landmarks are all within 100 meters of each other.

 

Have fun.

 

-Rusty

[survey tech]

Rusty is right in general, except for the fact that Polaris is actually moving toward the north celestial pole at the present time, not away from it. Precession will eventually take it far from north and then right back around again in its infinitely repeating cycle, but this apparent motion is so gradual that no one alive today will ever see it more than one degree from true astronomical north. Due to this chance of fortune for those of us on earth during this period of history, determining true north on a clear night is very easy, and this has been the primary means of doing so for surveyors and navigators in modern times. The second method, using shadows, is not likely to give good results unless time is corrected to local apparent noon, which is too complicated to bother with for such a method which will only give approximate results even if done properly. Despite the fact that magnetic declination can cause deviation from true north of over 20 degrees in some parts of the continental U.S., a compass is still probably the best daytime alternative to observation of Polaris in most cases. Its easy to correct from magnetic north to true north, since the declination difference is provided on all USGS quad maps.