readings, magnets, and geology

[praezis@geo.unibe.ch]

Hi all

Pat wrote:
> If the reading is between 0 and 179 degrees, subtract 20 degrees and add
> 200.
> If the reading is between 180 and 359, add 20 degrees and substract 200. 
There is another method which makes comparisons easier. It somehow only seems to
be known in Switzerland (and in some engineering fields): simply use grades
instead of degrees (400 instead of 360). Quick, easy, nice, figurative, decimal.
(Of course those who still read foot and inches are used to non-linear numbers :-))

Paul wrote:
> Does 
> anyone have data to support this idea that about 1 meter (or maybe less) is 
> where the effect falls off to the order of precision of a hand-held survey?
I cannot speak for lava tubes, but for steel-reinforced concrete in a touristic
cave. There, one meter distance is the absolute minimum, as a general rule, we
had to use either theodolite or other triangulating methods.

Bill wrote:
> I wonder how many of the failures to agree in
> Lilburn (a marble cave), were the result of the many granite boulders which
> have washed into the cave, rather than errors on the part of the instrument
> people.
Granite (unless it is a very special one with a gigantic Magnetite content) is
practically non-magnetic. The initial composition of the molten rock is usually
quite different between Lava (which has more Mg, Fe) and granite (which has more
Al, Ca, Na, K). I'd even go as far as to say that the marble itself is more
likely to bear magnetic minerals in significant compounds than is the granite... 

Hope I could help a bit...

Many greetings

Praezis


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