Magnetic North or True North

[+GIDEON-X]

In general caching.....Bringing up a preloaded cache, hitting "Go-To"..Does it make any difference which setting (TN or MN) one uses.....to find the cache. In my experience it doesn't, Projecting waypoints is another story....What do U think?...........

[+team lagonda]

often wondered that myself..not sure..

[+egami]

In general caching.....Bringing up a preloaded cache, hitting "Go-To"..Does it make any difference which setting (TN or MN) one uses.....to find the cache. In my experience it doesn't, Projecting waypoints is another story....What do U think?...........

 

I don't know the intricacies of how they are programmed, but I would think that your map is based off TN and that regardless of your compass setting that when you were being shown N on a map that it'd be TN and not MN. In which case, no it wouldn't matter for that.

 

I am sure someone knows the answer.

 

*edit* Actually, I guess the "map" isn't what you were inquiring about...I thought that because a lot of time we'll overhead view the map to get to the cache until we're close then we'll switch to the screen that shows feet / direction.

 

So, in short, no...for finding the cache I don't think it matters. You are going to the same location regardless.

 

It might slightly alter what direction it tells you that you need to go based on the setting. What I was getting it, that I am not sure of, is if the map alters based on your setting or if it's statically set to show you TN.

Edited January 22, 2008 by egami

[+StarBrand]

The arrow on the GPS will point towards the cache location with either setting.

[+OienLabs]

There are three norths. Magnetic, true and grid. When you are going for a waypoint this does not matter. Using the gps as just a compass and projecting it will have an impact.

[+TexasGringo]

You use magnetic north....but Magnetic Declination varies across the USA.

 

The variations are called "Isogonic lines".

 

The one line where there is no variation between Magnetic North and True North is called the "Agonic" line....and in the USA it runs from about Florida up through the Great Lakes region.

[+egami]

Does GN even become a factor on a GPSr?

[+Mudfrog]

It does not matter whether you have your GPSr set on true north or magentic north when looking for the typical cache (following the arrow to a set of coordinates).

 

It's when you project or set a course to follow on the GPSr that it would make a difference. At some points on the globe, i believe Florida was mentioned earlier in this thread, the difference is negligible. In most places however, you would need to know which setting to use if going for a projected type (offset) cache.

[+Renegade Knight]

Does GN even become a factor on a GPSr?

UTM is grid based.

[+Cedar Grove Seekers]

You use magnetic north....but Magnetic Declination varies across the USA.

 

The variations are called "Isogonic lines".

 

The one line where there is no variation between Magnetic North and True North is called the "Agonic" line....and in the USA it runs from about Florida up through the Great Lakes region.

 

I live in the Great Lakes region, and MN and TN differ by 11 degrees here. That seems like a lot for being on the agonic line?

[+Markwell]

Magnetic North is only really required when you're following a magnetic compass.

 

If your GPS is set to True North, and it says to go 15°, and you pull out your magnetic compass and shoot a bearing of 15° - you may be in trouble based on your position on this type of map (this is an old one):

 

However, if your GPS tells you to go 15° and you follow the arrow on the GPS (and the numbers get smaller) you'll be fine.

 

 

So hears the scenario...

Here in Chicago, I have a cache 750 feet away. My GPS is set to True North, and the GPS says that the cache is 45°. So I take out my trusty compass and shoot a bearing of 45° and walk for 750 feet. BUT WAIT, I didn't take into account the magnetic declination, which around here is somewhere between 2 and 3 degrees (let's say 3). According to my calculations, after traveling 750 feet, I'll be off 41 feet. It's not a huge deal - many GPS units are off by that much.

 

OK, bigger declination and longer distance.

 

I'm hiking on Cape Cod, and there's a cache 3 miles away. My GPS is set to True North, and the GPS says that the cache is 45°. So I take out my trusty compass and shoot a bearing of 45° and walk for 3 miles. BUT WAIT, I didn't take into account the magnetic declination, which around Cape Cod is somewhere around -16 degrees. With that much difference in the declination and traveling that far, I'd be off by 0.78 miles.

 

But here's the question: Why wouldn't I take out my GPS in the 3 mile hike?

 

 

So - magnetic declination doesn't really enter into it. Follow the arrow on your GPS and watch the numbers go down.

Edited January 22, 2008 by Markwell

[+scuba dude]

For every day caching, they don't matter. If you are using some combination of paper map and compass (or the E-compass feature of some GPS) then it would. When using a true compass, you need to convert your grid to magnetic north when projecting a point at some distance and direction from a known location. Failure to account for the variation could put you off by feet or miles depending on how far your plan on traveling. With a GPS, you could just set the internal compass to 'grid' and be done.

 

SD

[+TexasGringo]

***I live in the Great Lakes region, and MN and TN differ by 11 degrees here. That seems like a lot for being on the agonic line?***

 

What part of the Great Lakes area? Look at the chart that Markwell put up and the Agonic line is the darker "0" Line. If you are at the Great Lakes in Ny...then you would be 11 degrees (or more) difference.

 

----------------

 

Don't worry about Isogonig and Agonic lines....just follow your compass...or GPS.

Edited January 22, 2008 by Drooling_Mongoloid

[+OienLabs]

Does GN even become a factor on a GPSr?

UTM is grid based.

 

That's it. Plotting a course on the map, then transfer the bearing to the gps. True, mag or grid?

[+Cpt.Blackbeard]

I agree with those who say it makes no difference.

I'm about the same as Chicago here on declination so I don't worry about it either, but when I travel out west I consider it there while using map and compass.

With the GPS now though I doubt I'll ever use map and compass again, knock on wood.

[+Renegade Knight]

Does GN even become a factor on a GPSr?

UTM is grid based.

 

That's it. Plotting a course on the map, then transfer the bearing to the gps. True, mag or grid?

How about input the destination's coords in UTM then hit "go to" ? The calculated bearing would be based on Grid North but putting the final coord in the GPS makes it a moot point.

Edited January 23, 2008 by Renegade Knight

[+egami]

Does GN even become a factor on a GPSr?

UTM is grid based.

 

I know that...

[+The Jester]

Does GN even become a factor on a GPSr?

UTM is grid based.

 

That's it. Plotting a course on the map, then transfer the bearing to the gps. True, mag or grid?

Depends on the map. The USGS topographic quads are set-up on True, so that is what you would use.

[+Renegade Knight]

Does GN even become a factor on a GPSr?

UTM is grid based.

 

I know that...

Sorry, I thought you asked if it was ever a factor in a GPSr. I didn't know you already knew it was, and why.

[+Jeep'en Jumpers]

GIDEON-X is up to something, don't know what but you could expect something like this.

 

http://www.geocaching.com/seek/cache_detai...=y&decrypt=

Edited January 23, 2008 by Jeep'en Jumpers

[+HarleyPiper]

True, the arrow will point to the cache with either setting. That said, I prefer to keep the GPSr set to show the magnetic bearing to the waypoint. I occasionally carry a compass when caching, and that makes it simple to match bearings from the GPSr and the compass in situations when GPS coverage suddenly goes south (heavy cover, up against vertical obstructions, etc.)

[+tokencollector]

When using a true compass, you need to convert your grid to magnetic north when projecting a point at some distance and direction from a known location. SD

 

OT slightly, but . . . Or use the declination on your compass to adjust to map north (TN or GN). Or, with the right compass, adjust the compass to account for the declanation. [i bought a set of Ranger CL compasses to help teach orienteering to my scout troop (and to help with caching before I got a GPSr with an electronic compass)]

Edited January 23, 2008 by tokencollector

[+egami]

Sorry, I thought you asked if it was ever a factor in a GPSr. I didn't know you already knew it was, and why.

 

Thanks for not answering the question twice in a row.

Edited January 23, 2008 by egami

[+egami]

Does GN even become a factor on a GPSr?

UTM is grid based.

 

That's it. Plotting a course on the map, then transfer the bearing to the gps. True, mag or grid?

 

Ok, but does that actually factor in on the GPS side internally? I guess what I am saying is you can determine bearing with a compass and a map...I can see where GN is a factor on map, but I don't see where it's really a factor on the GPS, per se.

 

I am talking functionally as a factor on the GPS electronically or computationally. Not just from using it in place of a compass to determine bearing. Maybe that is where the confusion lies.

Edited January 23, 2008 by egami

[+Team Cotati]

In general caching.....Bringing up a preloaded cache, hitting "Go-To"..Does it make any difference which setting (TN or MN) one uses.....to find the cache. In my experience it doesn't, Projecting waypoints is another story....What do U think?...........

 

No, it doesn't.

[+the hermit crabs]

GEDION-X is up to something, don't know what but you could expect something like this.

 

http://www.geocaching.com/seek/cache_detai...=y&decrypt=

Possibly, but not necessarily. I think that this thread is an offshoot from a couple of posts in the Projections thread in the "Getting Started" forums. Someone there asked about how to do projections, and GIDEON-X said rather emphatically that the GPS must be set to Magnetic north. I disagreed, and said that the finder should do the projection with either True or Magnetic, depending on how the hider set it up. (And I added a mini-rant about cache owners who require projections but who don't specify whether to use True or Magnetic.)

 

Most (but not all) of the waypoint projections in puzzle/multi caches around my local area use true north.

[+TexasGringo]

I wanted to add in this forum thread (by Trainlove)

 

that all those other lines are called isogonic lines, i.e. I live near the 15°W isogonic line.

 

If one looks at an aeronautical chart one sees the isogonic lines 15, 15.5, 16, 16.5... but if you look at a VOR circle the Magnetic north there looks to be different a bit, that's because around here the 15° isogonic line looks to almost be 15° west of north itself, just plain luck.

[+eagletrek]

I wanted to add in this forum thread (by Trainlove)

 

that all those other lines are called isogonic lines, i.e. I live near the 15°W isogonic line.

 

If one looks at an aeronautical chart one sees the isogonic lines 15, 15.5, 16, 16.5... but if you look at a VOR circle the Magnetic north there looks to be different a bit, that's because around here the 15° isogonic line looks to almost be 15° west of north itself, just plain luck.

 

 

Oh no!!!!!!!! Not more lines to worry about!!!! I keep on tripping over those dang contour lines. Now I got to worry about all them isogonic lines, also?????

[+TexasGringo]

***Oh no!!!!!!!! Not more lines to worry about!!!! I keep on tripping over those dang contour lines. Now I got to worry about all them isogonic lines, also????? ***

 

Watch your step and don't trip over them...

[+tokencollector]

I am talking functionally as a factor on the GPS electronically or computationally. Not just from using it in place of a compass to determine bearing. Maybe that is where the confusion lies.

 

As I understand the way a GPSr works, setting it to TN, GN, or MN should not alter the functionality of the GPSr.

 

for a GPSr without an electronic compass, the bearing to a cache can be useful in conjunction with a compass when you are not moving. In that case, the difference between TN and MN could be significant in deciding which direction to walk.

 

Determining position is a function of tringulating your position relative to the GPS satellites. For the calculations involved in triangulation, the direction of north is irrelevant. It should not matter how you orient your GPSr, your postition should remain constant (within drift).

 

Alignment of the compass, in a GPSr with a non-electronic compass, is a calculated quantity. I was there, I am here, I travelled along this heading, therefore north must be there.

Edited January 25, 2008 by tokencollector

[dagger dog]

only on the map, usgs are true they show the declination on the rosette, the red lines are grids, the borders are true.

 

the old addage east least, west best was used to remember whether to add or subtract the declination, depending on which side of the agonic you were located.

 

also the magnetic lode in the Canadian north that deflects the compass needle also "drifts" so the declination changes over time . the usgs maps have the "drift" calculations in the legend.

 

what modern marvels will be seen in the future? all the old ways are slowly fading away. whooda thunkit?

Edited January 25, 2008 by dagger dog

[+Renegade Knight]

Sorry, I thought you asked if it was ever a factor in a GPSr. I didn't know you already knew it was, and why.

 

Thanks for not answering the question twice in a row.

To answer what appears to be your question in more detail so that you can better picture the answer.

 

The GPS uses the model tied to the system it's using. That's a Grid for UTM. The internal math may or may not be based on the grid. They could convert everything to WGS84 and then convert it back to the UTM grid. Regardless to use the UTM grid they do have to have it modeled in the GPS firmware.