True North vs Grid North???????
Peter Fitzgerald
Can anyone explain the difference between True-North and Grid-North
(Grid Convergence) on topographic maps. The difference is only about
one degree but I am curious as to how it comes about.
I suspect it might have something to do with the Earth being round and the map being flat or
something like that.
Also, should you include this angle when you account the difference between Magnetic North and
Grid North on maps? I have been told I should but I can't see why. One degree or so probably
won't get me lost, but knowing the answer would help to fill in the gaps and maybe let me sleep
at night.
Any help would be appreciated.
William Metzenthen
In article <33A02B...@netspace.net.au>, Keith Moon wrote:
>Peter Fitzgerald wrote:
>>
>> Can anyone explain the difference between True-North and Grid-North
>> (Grid Convergence) on topographic maps. The difference is only about
>> one degree but I am curious as to how it comes about.
>
>the magnetic pole. Maps provide you with the means to calculate this
The original message hasn't arrived at this site yet, but I will try
to give an answer anyway. The difference between True-North and
Grid-North is easier to understand if we model the earth's surface
as a sphere. Then we can agree that we both know what true north is.
Grid north is the direction of the vertical grid lines on your map.
The two directions are not the same because of the projection (UTM for
normal topographic maps) which is used to map the curved surface (the
surface of the earth) onto the flat surface (the map). It's possible
to make maps with grids which align perfectly with true north but then
you have other problems.
Note that grid convergence varies over any given (normal topographic)
map and is usually only given for the centre of the map.
For bushwalking this is mostly quite academic; I can't think of any case
in bushwalking where I would need true north or have any means of
determining it independent of a map more precisely than the typical
magnitude of grid convergence.
--
-----------------------------------------------------------------------------
Bill Metzenthen | http://www.suburbia.net/~billm/
bi...@suburbia.net | for information on an 80x87 FPU emulator,
Melbourne, Australia | using floating point (particularly on Linux), and
| an implementation of fpce stuff for 80x86 Linux.
-----------------------------------------------------------------------------
Laurence Knight
Peter Fitzgerald wrote:
>
> Can anyone explain the difference between True-North and Grid-North
> (Grid Convergence) on topographic maps. The difference is only about
> one degree but I am curious as to how it comes about.
> something like that.
> Also, should you include this angle when you account the difference between Magnetic North and
> Grid North on maps? I have been told I should but I can't see why. One degree or so probably
> won't get me lost, but knowing the answer would help to fill in the gaps and maybe let me sleep
> at night.
> Any help would be appreciated.
The simple answer is that the magnetic poles do not match the geographic
poles, but rather move over time. I'm not sure where you live, but in
many parts the magnetic variation is of the order of 10 degrees which is
sure enough to get you lost if you aren't paying attention - certainly
enough to stuff you up if your are orienteering/rogaining.
LK
Laurence Knight
D'OH, I didn't read the question properly. A risk you run when zipping
through newsgroups.
LK
Noel E Cranswick
>Can anyone explain the difference between True-North and Grid-North
>(Grid Convergence) on topographic maps. The difference is only about
>one degree but I am curious as to how it comes about.
>I suspect it might have something to do with the Earth being round and the map being flat or
>something like that.
>Also, should you include this angle when you account the difference between Magnetic North and
>Grid North on maps? I have been told I should but I can't see why. One degree or so probably
>won't get me lost, but knowing the answer would help to fill in the gaps and maybe let me sleep
>at night.
>Any help would be appreciated.
True North is the direction that will take you to the north pole in a
straight line
Grid north is the direction marked as north on the grid maps.
The differecne is due to the curvature of the earth as the grid map is
flat (and the earth is not)
This is only of practical importance if you plan to travel straight to
the north/soulth pole from a bearing on your map without taking this
difference into account (ie: you'll miss it)
CYA,
Noel