Re: Maps! (let's have a seperate thread, instead of using roll call)


<steviephilips@xxxxxxxxxxx> wrote in message
news:1149879701.520558.151350@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Correct, but you dont need to actually own the current sheet or have on
in your possesion to know that info, do you ?

Ahem...you do.


To calculate the current deviation, you DO need a sheet that is no more
than a few years old.

Or you could ask your mates or you could look it up.

And what precisely would they be looking up?

If you look up data say from an isogonic map this will give you the magnetic
variation for various locations. The variation here is between the magnetic
north pole and the geographic north pole. However, the geographic north pole
is largely irrelevant where maps are concerned! What you really need to know
is the magnetic variation between the magnetic north pole and the grid
system that is used on your map. For that you need the variation data for
your map! You will only get an up to date figure if you have an up to date
map.

Anyhow, in a country such as Britain the principal reason why one should
have/use an up to date map is that many features - particularly man-made
features - change over a relatively short period of time. Yes, the magnetic
variation changes, but for most practical purposes this is of secondary
importance. Physical landscape features don't change as much in the same
time period - except maybe in areas where there's a lot of
mining/quarrying/forestry activity - as do man-made features. One can argue
that in some situations a 30 year old map may still be largely okay to use.
Somewhere along the way though you'll need to navigate using a man-made
feature and a fair degree of them have changed; it'll be sod's law that the
one you choose will be the one that's changed. I don't think there's a major
pressing need to change/update maps every year or even at every revision,
but maybe every 5 or so years or if you know that a mapped area has
undergone major changes. The more remote areas of the country on the whole
undergo fewer changes than urban or more populated areas.


I currently use 2deg, and current rate of change is about 1/4deg per
year.

Ahem....

That's not very precise.

I take it you mean you use 2 deg W magnetic variation for the maps of
Britain that you use.


In 5 years time I might enquire if the rate of change has altered
significantly.

The most important geomagnetic fact you should know is that the Earth's
magnetic field is neither stationary, uniform nor perfectly aligned with the
geographic poles.

The rate of change is always altering! However, as you imply, in 5 years
it's not likely to be significant for your purposes.

We're quite lucky in Britain in that at our latitude the TMP (Transverse
Mercator Projection) that's used for many maps of Britain produces a
variation between geographic north and the grid (north) of on average less
than 1 degree (generally about 30-40 mins), so that for all intents and
purposes north on a map points to true (geographic) north. Furthermore, the
Earth's magnetic field in the location of Britain currently produces a small
single digit magnetic variation (between magnetic north and geographic
north). The average (IIRC) is currently about 3 degrees W, getting slightly
smaller as one moves east and slighty south, and bigger going west and
north, i.e. the magnetic variation in NW Scotland is more than in SE
England, though the range is never more than about 5 degrees. IIRC, if
you're out in Central -Eastern Europe the magnetic variation is zero.

Of course one should always remember local anomalies where the field varies
due to the nature of the underlying rock structure and the effects on
compasses can be significant, e.g. in areas where there are large iron ore
deposits or gabbro such as in the Cuillin hills.

Other places are less fortunate. In the eastern US the variation is roughly
0-10 deg W, central US 0-10 deg E, and western US 10-20 deg E. And it gets
worse as you go north into Canada! Of course in the region of the magnetic
poles a compass is next to useless. I can recall in Cape town using a
variation of 20-30 deg W and in the Smokies of Tennessee ignoring it
completely as it was zero. (If you're interested in the magnetic field
variations try searching for 'isogonic maps' which I guess you should find
on geological survey sites, e.g. BGS.)


I dont need the very latest map in my possesion to know what the
current magnetic deviation is, do I ?

Er... yes you do. The variation on maps is the variation between mangetic
north and the grid north. However, for practical purposes, no. There are
better reasons for having a more up to date map - changes in the man-made
landscape - than the relatively small change in the magnetic variation.

snip

I doubt having the latest map would stop them being stupid.

But do use as recent a sheet
as you can, and check it against the latest for changes.

Absolutely!


WHSmith is a
great place to spend 40 mins poring over the latest sheets making notes
of the area you are going to.

I would suggest not! If you spend that amount of time then it's more than
likely someone's going to approach you and lean on you to buy it!

Try a public library, if they have maps, instead.


Yeah sure, and an interesting use of time to those that have the time

:-)

GAGS


.

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