Survex question

[Carlos "Guâno" Grohmann]

Dave,

Some friends here in Brazil have almost the same problem as you,
except they are working in laterite caves, but the magnetic issue
remains.
I don't know if the method they are using will be applicable to your
lava tubes, but here it goes:

The magnetic variation happens only in the caves, so they start the
survey a bit far from the cave, using normal topo methods
(compass/clino), to get one shot that is not affected by magnetism
(make a backcompass to ensure). This shot will be the reference for
the rest of the survey. From this point on, they use protractors
(360º) and nylon fishing lines. First you adjust your protractor with
the reference shot, that is, you put a line in the reference shot, and
match the angle in the protractor with the azimuth of the shot. Then
you pull a line to your next shot, and see the angle (azimuth) in the
protractor, and so on for the entire cave.

Maybe this helps.. (if my explanation is not too complicated)

Carlos




On 3/25/07, Olly Betts <olly at survex.com> wrote:
> On Thu, Mar 15, 2007 at 11:08:45AM +0000, Dave Nixon wrote:
> > Therefore I am thinking of doing the survey from
> > two absolute points on the surface (GPS derived)
> > and taking the survey underground by measuring
> > angles. Can survex accept data in this fashion to
> > create co ordinates output drawings?
>
> It doesn't currently.  Turned angle surveys are a feature we've always
> intended to add (since it is a standard technique), but I don't believe
> anyone has actually asked for it before, so it's never been very high
> priority so far.
>
> Am I right in thinking you're plan is to just rely on the GPS points
> to provide the "rotational reference", rather than measuring an
> "angle from north" for the leg at the start and/or end of the traverse?
>
> Do you (or anyone else) have any sample data already?
>
> Cheers,
>     Olly
>
> --
> Survex http://lists.survex.com/mailman/listinfo/survex
>


-- 
+-----------------------------------------------------------+
              Carlos Henrique Grohmann - Guano
  Geologist M.Sc  - Doctorate Student at IGc-USP - Brazil
Linux User #89721  - carlos dot grohmann at gmail dot com
+-----------------------------------------------------------+
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