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"True North"


Guest spinn

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I am looking for some information on "True North" vs "Magnetic North". I was told the GPS receiver notes "True North" is that correct? How do I locate "True North"? Any help would be appreciated.

Thanks,

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Guest NeverLost

Hello spinn,

 

most gps units can be set to either but here is the way i do it becuase i use my 7.5 min topo quads a lot with the gps.

 

If you have a declination-adjustable compass, and adjust it properly

for the local declination, then it reads bearings relative to true north.

In this case, you should set your GPS to read true bearnings as well.

Then your GPS, your compass, and your maps will all agree with one

another.

 

Setting the compass *once* for the correct declination and then

doing everything with true bearings is much more foolproof.

 

Later,

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Guest sbukosky

If you like maps, go to a local airport, one that services general aviation. Ask for the local "sectional chart". It is the map that pilots use for referencing terrain using "dead reckoning" and waypoints. The map is oriented to true north. It will also have a line on it showing the magnetic deviation from true north. For example, in my area of Wisconsin it is 3 degrees west variance and as much as 15 degrees west variance around the west coast. Back when I was flying ragwings, I would have been in heaven if GPSR's would have been available!

 

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

Steve Bukosky N9BGH

Waukesha Wisconsin

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Guest jfitzpat

I won't get into Mag vs. True here (since I've said my piece and then some on the same subject in another thread), but another big issue in using maps with a GPSr is map datum.

 

Some USGS quads are NAD27 and some are NAD83. Most GPS units use WGS84 by default, so your position maps correctly on a NAD83 map, but can be off by up to 200 meters on a NAD27 map.

 

Most avionics charts use the Lambert Conformal Conic Projection for the vertical axis, which is different still. On the other hand, most VFR sectional charts are something like 1:500,000 scale, which makes the vertical error very small when physically plotted on the map.

 

Good Luck,

-jjf

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Guest NeverLost

jfitzpat,

 

yes i agree that you ''must'' set your GPS to the correct map datum for the particular maps you are using .

 

most all the maps i own of the 7.5 min.quads(own about 50 quads) for my area are NAD 27 so this is how i got my GPS set up.it is set to NAD 27 for the map datum.

 

to do map orienteering with a GPS unit this is a big help knowing that your position matches exactly to your real location on the maps you are using for reference.

 

i have plotted my position many times onto a map from the coordinance readings of my GPS and it is amazing just how close it is.

 

also on many occasions i have saved a particular track from my GPS unit(like when walking an old road bed) and later plotted this track to the map by either uploading the track waypoints to a topo program or doing it manually with a UTM overlay to get the waypoints and it ''will'' show you to be exactly in the road bed on the map.

 

so for me it is true north for the GPS setting with the right datum for the maps being used and a good declination-adjustable compass adjusted properly for the local declination and all agree with one another.

 

Later,

 

[This message has been edited by NeverLost (edited 02 January 2002).]

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Guest jfitzpat

Hmmm, I don't remember saying "must", personally, I use uncorrected Clark and do corrections in my head... icon_wink.gif

 

Sorry if I went on a bit, I just did the http://www.lostoutdoors.com site, which, among other things, plots waypoints or manual coordinates directly on USGS topo or aerial images, then took a trip for the holidays so datum and sectional chart navigation where both on my brain.

 

-jjf

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Guest jfitzpat

Hmmm, I don't remember saying "must", personally, I use uncorrected Clark and do corrections in my head... icon_wink.gif

 

Sorry if I went on a bit, I just did the http://www.lostoutdoors.com site, which, among other things, plots waypoints or manual coordinates directly on USGS topo or aerial images, then took a trip for the holidays so datum and sectional chart navigation where both on my brain.

 

-jjf

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