Magnet Vs Truse North With Gps Projection

[+KVOM]

My old GPSr (Magellan ColorTrak) allows setting a waypoint via a projection from another waypoint, given bearing and distance.

 

I've used this to get coordinates of offset caches. Recently I went after a cache where the difference in bearing of true vs. magnetic north was significant, since the distance was a half mile. My GPSr allows setting the bearings to magnetic or true. How does it compute the difference? Does it have a table of magnetic variation, or is there something I'm missing?

 

I doubt it makes much difference except for using this feature.

[+Renegade Knight]

I don't know but I have a WAG. There would have to be a table and that table wouldn't be that hard to construct with the information thats out there.

[+EraSeek]

My old GPSr (Magellan ColorTrak) allows setting a waypoint via a projection from another waypoint, given bearing and distance.

 

I've used this to get coordinates of offset caches. Recently I went after a cache where the difference in bearing of true vs. magnetic north was significant, since the distance was a half mile. My GPSr allows setting the bearings to magnetic or true. How does it compute the difference? Does it have a table of magnetic variation, or is there something I'm missing?

 

I doubt it makes much difference except for using this feature.

GPS's think in terms of True, but if you set your unit to Magnetic it's firmware will determine the declination from your units current position. Most units (Garmin anyway) will display the declination when set to magnetic.

[Kerry.]

Most units have internal magnetic tables (which do require updating from time to time) for auto mode and also allow specific user input.

 

Coordinates, except for projecting another waypoint have no interest if things are set to true/mag etc. Projected waypoints certainly should not be projected using magnetic bearings etc.

 

Cheers, Kerry.

[Neo_Geo]

True or Magnetic North makes NO difference whatsoever regarding where in the world a coordinate is.

 

The only thing True or Magnetic setting changes is which direction the compass needle points.

 

N89° 99.999 W179° 99.999 is going to be the same location no matter what direction your GPSr thinks North is.

[+EraSeek]

True or Magnetic North makes NO difference whatsoever regarding where in the world a coordinate is.

 

The only thing True or Magnetic setting changes is which direction the compass needle points.

 

N89° 99.999 W179° 99.999 is going to be the same location no matter what direction your GPSr thinks North is.

Yes. No doubt. However if you project a waypoint at a distance of .25 miles and 340 degrees north from your position, whether it is in Magnetic or True will make a big difference unless you have a declination of 0.

[Kerry.]

Magnetic north is something that isn't absolute or fixed so why would anybody want to project on magnetic north anyway. Magnetic north wanders around quite considerably on any given day.

 

So why would anybody want to project a coordinate based on something that is very variable. It's rather meaningless.

 

Cheers, Kerry.

[+EraSeek]

Here is the average daily movement of the Magnetic North Pole :

 

 

Yes, a lot if you are close to it, but not that great if you are projecting waypoints in the US or Aussiland.

 

Never the less, I always use True.

[+mrp]

So why would anybody want to project a coordinate based on something that is very variable.

 

Well as others have pointed out, it actually doesn't vary that much in a practical sense (90 km at a distance of 5000 km is only +/- 1/2 a degree, and I can't make a bearing measurement more accurate than a about 2 degrees anyway.

 

I carry a compass, because it's handy to be able to use it for shooting bearings and checking my orientation. I don't like to have to remember the declination for my area, so I let the GPS do that, and it's a pain to switch. When I've seen bearings/projections given in multi-caches or puzzle caches, it's always been magnetic, with only 1 exception, and that used UTM grid north (which is different than either true or magnetic.)

 

About the only time I use true north is when I'm navigating using polaris, and I don't cache at night that much.

 

-- Pneumatic

[Kerry.]

So why would anybody want to project a coordinate based on something that is very variable.

 

.... I don't like to have to remember the declination for my area, so I let the GPS do that ....

So what are we saying that the declination in all and every GPS is the same, I doubt that. UTM grid north is different but without confusing things it should be the grid bearing (from grid north) that UTM projections are based on. Magnetic north has no mention in anything to do with UTM projections what so ever.

 

Even plane bearings are based from grid north but for long distance projections the I would be involving azimuth based from true north.

 

Cheers, Kerry.

[+IMLost]

Magnetic north does matter if you are using a map and compass. Yesterday I was out laying out a new multi took bearing with the GPS, came home laid the points out on a map by coordinate then checked the bearings on the topo with my compass and was off by 12 degrees. This cache requires that they know how to use a compass or learn how. If I give the bearings in true north and they use a compass they will be way off and if I give the bearings in magnetic and they us their GPS they won't find it either. I am kinda torn as to which bearing to provide in the cache.

 

The declination in my area is 12.37 degrees.

Edited February 8, 2004 by IMLost

[+Volwrath]

Magnetic north does matter if you are using a map and compass.  Yesterday I was out laying out a new multi took bearing with the GPS, came home laid the points out on a map by coordinate then checked the bearings on the topo with my compass and was off by 12 degrees.  This cache requires that they know how to use a compass or learn how.  If I give the bearings in true north and they use a compass they will be way off and if I give the bearings in magnetic and they us their GPS they won't find it either.  I am kinda torn as to which bearing to provide in the cache.

 

The declination in my area is 12.37 degrees.

Part of knowing/using a compass is accounting for declination. You should use true north in all your multis IMO. of course the declination in my area is 2 deg, hardly enough to worry about.

 

As far as declination goes, its going to get real interesting when the pole shift occurs.

Edited February 8, 2004 by Volwrath

[+IMLost]

Part of knowing/using a compass is accounting for declination.

Very true this is going to be a hard cache either way I can't really give details because I don't want to give away how to solve this one to any local cachers. Maybe I'll use tru north and give an encrypted hint for declination or maybe not.

Edited February 8, 2004 by IMLost

[+EScout]

All 3 of my placed multis use bearings or projection. I use true north in them. More important is for the cache placer to designate which he is using.

Declination here is 13.5 deg East, and I use a compass all of the time and it shows true north. One of my compass' has a declination adjustment. For the others, I get a "Sharpie" black marker and put a mark on the bottom at 13.5 E and just line up the needle over this for true north bearings.

Edited February 8, 2004 by EScout