Fw: survex questions

[Wookey]

----- Forwarded message from HughStL <hugh at stlawrence.free-online.co.uk> -----

Date: Thu, 28 Jun 2012 11:28:18 +0100
From: HughStL <hugh at stlawrence.free-online.co.uk>
To: wookey at aleph1.co.uk
Subject: survex questions
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Hi Wookey, how's things?

Couple of survex questions:

1. Getting Aygill survey together but have had to create fake
(cartesian) data for Precambrian from the old 1971 ULSA survey - Pete
Hall has done this. Only thing is, that old printed survey from which
Pete has faked his coordinates, has a True North arrow on it. From
what I can gather from t'internet, the magnetic declination back in
1971 was almost 9? West. As the rest of the Aygill survey is based on
compass magnetic bearings, how do we adjust the Precambrian fake
survex file to conform with the magnetic norm of the other files?

(there is a whole issue here of mag variations and compass
calibrations and so on, it's going to be a devil to unravel! Some of
the survex files go back to the late 80s and I rather suspect some of
the compass calibrations were just 'borrowed' from some other survex
file - ie. files copied and name and data changed but not things like
calibration! Oh shit....)


2. On a more basic understanding level, how should you write a compass
calibration into a file? For instance, I've just got some suunto
instruments off ebay - I went to the farm to check the reading against
the 'calibration test' that's written on the corner of the farm,
sighting on the signpost. Should be 328? but my compass was 331?.  In
a survex file, how do you write that compass calibration?  Do you
write it as +3 (ie. telling survex that the compass reads 3? big). Or
do you write it as -3, effectively telling survex to subtract 3? from
all the readings?  I'm not sure how the innards of survex works, and
therefore what instruction you should be giving it?

Let me know if you need to see any survex file.

By the way, the fake data file (cartesian) screwed up the whole survey
when it was included in the aygill master - it wouldn't produce a 3d
and gave loads of errors ('giving up, too many errors'). The solution,
after much trial and error, was to throw out all the 'export' stuff.
Then it worked fine!

cheers
Hugh



----- End forwarded message -----
Wookey
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