PDA

View Full Version : North Reference



kay dee exer
3rd November 2005, 10:51 PM
Hi guys, I have an eXplorist 500 and have a question regarding the North Reference setting. Should it be set on TRUE or MAGNETIC ?? Just wondering what the differences are between them ?? Thanks for any help you can offer. :D Cheers Trev...

arthurking83
4th November 2005, 06:10 AM
I always use TRUE as a reference, as it's easier when you reference to a map (which is almost always set to TRUE).
MAGNETIC is a tricky one as you have to allow for Declination or Variation, which is different in each part of the world.........etc.

There is no 'better one' to use, but TRUE is my preferred.

If you were referencing your North bearing to a compass then you would most likely use MAGNETIC!

cheers.

festy
4th November 2005, 06:17 AM
Yep, i agree. When using a map (paper or digital) use true north, as that's what the map will use. When following a compass bearing, use magnetic north.

kay dee exer
4th November 2005, 10:48 AM
Thanks guys.. :D

The Explorer
5th November 2005, 04:34 PM
I always use TRUE as a reference, as it's easier when you reference to a map (which is almost always set to TRUE).

Keep in mind that there are actually three ways of referring to north on a map. True (to the actual north pole of the earth - where lines of longitude originate), Magnetic (to the magnetic north pole which moves around so you have to adjust from true to get current bearing) and Grid North ..which is the alignment of the UTM grid lines...not necessarily to true north ...depends where you are (referred to as Grid Convergence). Most maps (used for on land navigating - well most of the 1000+ paper maps I have) have GRID north-south and east-west lines (UTM Grid lines e.g. MGA 94) not TRUE north-south and east-west (ie lines of longitude and latidude) as athurking83 & Festy seem to be suggesting (though at some locations, on large scale maps they will be ~ the same).

Most maps have the various alignments of all three on the map somewhere as a set of three north arrows. The easiest one to use would be grid north (as these lines will typically be shown on the map) but as Grid bearings are not output by GPS/Compass you have to use either True (if using GPS) or Magentic (if using GPS or Compass) If you use TRUE North you will still have to, in most cases, apply the grid convergence factor using a compass rose if you want to navigating using a map..and same procedure is require if wish to use Magnetic North - you just need to figure out the mag variation from grid north either using figures shown on map or.....

My old Magellan Nav1000 (ok it wasn’t mine - it was works..and cost about $5K back in early 90's) use to tell you magnetic variation at your location...the latest Magellan’s don’t (as far as I can see) You can figure it out though by plotting a course from your current position to a point on the same UTM easting some distance to the north (ie make up a dummy waypoint with the same easting as your current position but say 5km further north). If the GPS gives a magnetic bearing to this point of say 5° it means Mag North is 5° west of Grid North. If you get a bearing of 355° then Mag north is 5° east of Grid North.....if this is all a bit confusing just do what I do...follow the arrow on the GPS:)

Cheers
Greg

arthurking83
5th November 2005, 05:17 PM
What Explorer says is very true, but I thought would overcomplicate matters :confused:

Most of the maps I've seen in Aus, True north and Grid north are around 1° or so in variation, so I assumed this would have been "good enough" for navigating by GPS.
But I suppose I shouldn't assume, and mislead.......etc.

There are some cheap books available on reading maps (~$15)
and there is a thread on how to read maps (http://www.gpsaustralia.net/forums/showthread.php?t=498).
Page 12 of 18 of the pdf explains magnetic variation, grid convergance......


cheers.

kay dee exer
5th November 2005, 05:20 PM
I'll let you's know when my head stops spinning. Whhooooooooooooaaaaaa...


That is some serious mumbo jumbo. I best be getting me some reading material. Thanks again, I think... :D