Hello Fernando and other Fellow Dialers
In spite of all the varieties of errors magnetic compasses were and still
are very useful. Like Fernando I also used to possess more than one as I
could not resist buying a nice thing for about $10. My offsprings worked
hard however to bring that number down to two or three.
Now in sports stores you may buy really good and robust compasses produced
in Sweden and Finland. They are about $10, not very big but very accurate.
The dial is marked every 2 degrees so with some skill, and when making a
series of bearings, you may reach 1 degree accuracy. Do not hold such close
to your eye but simply press its back side to your stomach while keeping
the thing horizontally and try to take a bearing by turning yourself in the
direction of your target. After few exercises you will gain confidence in
this method. These compasses are filled with liquid which greatly
stabilizes them and are guarranted to endure low pressure up to 5000 m
above sea level, which is not always true. At least one of mine got bubbles
in Yosemite at about 3000 m above sea level only (BTW, quite properly you
reach this elevation driving your car as in America you should). These
compasses are so cheap that it does not pay to send them back to producer
if they break in spite of lifetime warranty. Their magnetic needle is so
thin that it is physically nearly impossible to magnetize them otherwise
than along the needle axis.
When you buy a compass check first of all if the needle moves freely and if
it stablilizes very fast. Then put the compass on any table inside the
store, note the direction of the magnetic, rotate the compass few times and
see if the needle comes back exactly to the same direction every time.
Generaly good compass should be sensitive, stable and quiet, just like a
good wife.
A more expensive, excellent hand bearing compass you may order at
http://www.celestaire.com/catalog/c_page34.html
Other remarks. Yes, compass shows its own compass North, rather than
magnetic North, let alone the true geographical North. Nearby objects made
of steel bring in the compass error that deflects compass readings from
magnetic readings. The only way to know this error is to measure it. In
case of dialers this is no big deal as most of us know the geographical
position and are cabable of calculating azimuths of celestial bodies at any
given moment. Charts of magnetic variation are published, they also give
variation changes with time so you may calculate variation for the given
year. Even good touristic maps provide information on magnetic variation
and its changes. After some practice all those things become natural.
All best
Slawek
P.S. The Celestaire Company I gave the URL to, has quite a choice of marine
and air navigation instruments and publications with prices generally lower
than at marinas and coastal stores. The most funny thing is that the
company is located in Kansas, right in the middle between the two Oceans.
Their strategic planning was really flawless. About a year ago I bought a
sextant from them and am really happy with it.
At 10:34 PM 2/8/99 +0200, Fernando Cabral wrote:
>Silas
>
>I did not know until recently, when I found I have more than 30 compasses:
>I am a compass collector. Just for the fun of it, I started comparing
different
>
>makers, models, styles. At first it came as a suprise to find that different
>compasses will show completely different readings. 2, 3, 5 or even 10 degrees
>east or west are quite common.
>
>Also, using the same compass you can get quite different readings as
>a consequence of temperature (I guess), pressure (altitude) and, especially,
>how you hold it in your hand. That is, if it is completely level or not,
>inclined to one or the other direction, so on and so forth.
>
>Temperature and pressure will affect it especially if it has air bubles in
>the liquid it is supposed to be suspended. I think the temperature will
>also affect the way the needle pivots around it axle because of the friction.
>
>Also, I've found that sometimes, when you turn your compass 180 degrees,
>it will not show 180 degrees but some degress more or some degrees less.
>
>The bottom-line is: there are too many factors influencing the reading.
>As if that wasn't enough, a layperson can not use it properly, can
>not even take parallax into acount, can not take into account local
>forces like iron in the floor or in the wall.
>
>Here in Brazil we say that if counsels were good we would not give
>them for free, we would charge for them. :-) but here my free advice
>goes: don't ever ask your client to read a compass. 999 times in 1000
>they will give you completely wrong readings. In the 1 case left
>reading will be within 2 or 3 degress off.
>
>- fernando
>
>
>
>--
>Fernando Cabral Padrao iX Sistemas Abertos
>mailto:[EMAIL PROTECTED] http://www.pix.com.br
> mailto:[EMAIL PROTECTED]
>Fone: +55 61 321-2433 Fax: +55 61 225-3082
>15º 45' 04.9" S 47º 49' 58.6" W
>19º 37' 57.0" S 45º 17' 13.6" W
Slawek Grzechnik
32 57.4'N 117 08.8'W
http://home.san.rr.com/slawek
