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Mag or True?


Guest CoyoteQuick

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Guest Geo Quest

When I'm geocaching I have my GPS set to magnetic north and I shoot my bearings using my Silva Trekker. I don't worry about declination. When I hike or backpack I am carrying USGS 7.5 min quads and set my gps to true north and correct for declination on my compass. For me, the debate is easy. If you use a map true north is the only way to go since the map is oriented TN. Without a map who cares?

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Guest JAMCC47

Greetings:

I totally agree with the forester, dont go in the woods without a map and compass and know how to use them. I spend the summer as a volunteer with the USFS in the mountains of CO and use both a map (pre-set with UTM GRID lines)a compass and 2 GPS's. We are concerned that in the coming summer we will probably be involved with a search and rescue for some Cacher that got lost. I have 2 caches planted up in the hills and from what I've seen the searchers just go after the numbers, never even bother to look at a map. The comments that come back are very revealing of where they have been, they all talk about how steep the trail is the unseen switbacks etc. When I mail them a copy of the map (after their quest) they are amazed to find a FS road that takes them right to the Cache. The other cache can only be solved if you use a map and compass and are able to figure out where you are. The attempts on this cache are rare, most go for the drive by and get out cache. A GPS when used with a map that has 500 meter UTM GRIDS is so helpful when you are in the boonies. Good hunting.

 

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JoseCanUSea

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Guest Rich in NEPA

quote:
Originally posted by JAMCC47:

... I spend the summer as a volunteer with the USFS in the mountains of CO and use both a map (pre-set with UTM GRID lines) a compass and 2 GPS's. ... A GPS when used with a map that has 500 meter UTM GRIDS is so helpful when you are in the boonies.


 

JAMCC47, I'm a bit curious about the maps you are using and where you got/get them. Are they USGS topos? Did you drawn in the UTM grids youself? I do this on all of mine, and (thankfully!) most of the new USGS 7.5-minute quads have UTM lines at 1km intervals printed on them. Which ones have 500-meter intervals? Thanks much.

 

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~Rich in NEPA~

 

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? A man with a GPS receiver knows where he is; a man with two GPS receivers is never sure. ?

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[This message has been edited by Rich in NEPA (edited 11 January 2002).]

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Guest Rich in NEPA

quote:
Originally posted by tenami:

Imagine if we had only one choice.


 

Well, if the Earth's rotational axis and its magnetic axis coincided, we would need only one choice! eek.gif

 

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~Rich in NEPA~

 

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? A man with a GPS receiver knows where he is; a man with two GPS receivers is never sure. ?

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Guest JAMCC47

quote:
Originally posted by Rich in NEPA:

JAMCC47, I'm a bit curious about the maps you are using and where you got/get them. Are they USGS topos? Did you drawn in the UTM grids youself? I do this on all of mine, and (thankfully!) most of the new USGS 7.5-minute quads have UTM lines at 1km intervals printed on them. Which ones have 500-meter intervals? Thanks much.

 


I use the TOPO program. With 500 mtr intervals, its all automatic, when you set the format. Works well with a UTM Roamer Scale. If I set the map at 1:12K then I set the UTM'S at 250 meters. That way the accuracy of the roamer is increased. In other words the standard 100 meter mark is 50 meters. And each digit is 10 meters, instead of 20 meters. If I realy want a close up I'll go to 1:6K, which is the layout I prefer for the last hunt to the cache.

 

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JoseCanUSea

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For geocaching, I use the same combination of map/compass/GPS techniques I’ve long used for backpacking and other back-country travel and navigation. (I also train law enforcement, search & rescue, firefighters, and outdoor recreationists generally in land navigation.)

 

I tend to go with magnetic bearings, as my first choice I a compass is a British M73 prismatic, which is not adjustable for declination; nor is the Brunton/Silva 54 baseplate, which is also excellent. I use a good protractor to plot bearings and simply do the math in making the conversions.

 

Instead of the standard large-sheet 7½ - minute topographic quadrangles, though, I use MapTech Terrain Navigator to produce what I call “MiniTopos,” which are the same quadrangles reduced in size to 8½” x 11" at a military scale of 1-50,000 instead of the standard 1-24,000. I print these out on either water-resistant paper or water-proof synthetic sheets and set the program to grid them for UTM. Thus no romer scale or other device is necessary to plot a GPS coordinate on the map; you can make your plot with a pencil point, your fingertip or even just a glance. (Because the scale is a precise 1-50,000, you can still use a romer scale if you choose to do so.)

 

Scaled down, the maps are much smaller and easier to deal with then the original, large-sheet topos - though they have identical detail - and they’re much more durable in bad weather. An added plus is that the good, sharp lines provided by the UTM grid make for excellent compass triangulation plotting.

 

With a good compass and attention to detail, it’s not difficult to get good old-fashioned compass triangulation accuracy in map plotting down to one tenth of one percent of the map scale.

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I'm a little late in the running, and I haven't read all those posts, so this may have already been covered.

 

Always use magnetic north, even when using a map. Any topo map worth the paper its printed on will have an easy to use figure that shows the declination of a compass.

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quote:
Originally posted by BobSatchel:

...Any topo map worth the paper its printed on will have an easy to use figure that shows the declination of a compass.


 

I don't believe this is true at least with USGS 7.5 Quads. The declination shown on the map was what it was when the map was originally created. The current declination could be quite different. If your GPS doesn't automatically correct the declination and you have to set it yourself, there are sites on the web that can give you the current declination for the location you desire.

 

Good luck.

 

Alan

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Someone mentioned that having you gps set to mag or true can affect the speed readout on the gps unit. That should not happen should it. Its all relative. Am I completely wrong on this?

 

I want to die like my grandfather did, peacefully in his sleep. Not screaming and yelling like the passengers in his car.

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I'd like to point out one reason why I like to use a map of the cache area. Sorry if this has been mentioned before.

 

Sometimes, just following a trail that leads off in the general direction of a cache will lead you astray, or will take you close to the cache, but not necessarily the closest point to the cache relative to the trail. I like to minimize off-trail hiking as much as possible at parks and stuff because I don't like to trample over the wildlife. With a map, I can figure out what point of the trail I should be at before I start bushwhacking.

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Topo maps are oriented toward True north. The setting on my GPS is set to True for direction. My magnetic compass is adjusted to compensate for magnetic declination to give True direction using an offset on the numeric scale. All azmuths used are True. No need to convert form true(map) to magnetic for magnetic compass or mag to true to use compass direction on map. I use my magnetic compass often in many different applications. You must calculate current magnetic declination using data given on the map that is being used. Of course one must be alert to any conditions that may infulence the accuracy magnetic compass.

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...since I looked at the map, posted by Geoffrey above, showing that the declination where I live in Sweden is 0º (yes, zero, nil, nothing). No wonder I've been orienteering since 1972, and still have never seen a compass equipped with declination adjustment.

 

I'm writing this just to show you how different a problem may seem in another perspective.

24148_200.jpg

 

Anders

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I set my GPS to True and set the declination correction (W+,E-) to True on my magnetic compass so that the compass is indicating True direction that corresponds to the map( USGS 7.5 Min Quad). My GPS,compass,and map all set to True direction. No more conversion to be done from compass to map or vice versa. The topo maps will provide Mag declination and the yearly correction to use. Grid North(GN) may differ from True North and is another correction may be necessary depending on user needs. Is anyone using MILS instead of degrees? (6400 Mils=360 Degrees) I use my compass often in many applications in combination with the GPS when land navigating.

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