From: Julian Todd (julian_at_goatchurch.org.uk)
Date: Fri Jan 19 2001 - 15:37:37 CET
Received: (from mdom_at_localhost) by karto.ethz.ch (8.9.3/8.9.3/SuSE Linux 8.9.3-0.1) id QAA16846 for cavexml-outgoing; Fri, 19 Jan 2001 16:41:09 +0100 Received: from rhenium (rhenium.btinternet.com [194.73.73.93]) by karto.ethz.ch (8.9.3/8.9.3/SuSE Linux 8.9.3-0.1) with ESMTP id QAA16842 for <cavexml_at_cartography.ch>; Fri, 19 Jan 2001 16:41:09 +0100 Received: from [62.7.43.190] (helo=gordon) by rhenium with smtp (Exim 3.03 #83) id 14Jdf2-00071m-00 for cavexml_at_cartography.ch; Fri, 19 Jan 2001 15:41:08 +0000 Message-ID: <021701c0822d$e9dba8c0$0a3c10ac@gordon> Reply-To: "Julian Todd" <julian_at_goatchurch.org.uk> From: "Julian Todd" <julian_at_goatchurch.org.uk> To: <cavexml_at_cartography.ch> Subject: Cross Section libraries Date: Fri, 19 Jan 2001 14:37:37 -0000 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 5.00.2314.1300 X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2314.1300 Sender: owner-cavexml_at_karto.baug.ethz.ch Precedence: bulk
>> Try to think about something as "library of cross section" in detail
>> defined somewhere in a file and use only reference to this library
>> and actual proportions ("LRUD") to such profile in particular point.
>>
>> In real cave there are not endless kinds of profiles.
>
>This idea was suggested to me by a caver who had been surveying since
>about 1960 or so, but was never and still isn't very computer literate.
>It had certain intuitive appeal to him, so I assume there are
>other non-computer people who could think of cave passages
>in the same manner.
Unfortunately this idea doesn't stand up to scrutiny.
Consider the case of a keyhole passage, thinking of it as a
circular tube with a rectangular trench in the bottom. The
circle has a diameter, and the trench has a depth and width.
If your trench gets deeper you cannot distort your original/library
shape by stretching it vertically without your tube becoming an oval!
The complexity required to overcome this problem (in terms of
inventing other types of distortions from the linear kind and
including special labels on the nodes on which certain distortions
only apply) is prohibitive. (I have tried it!)
It is far easier simply to define each cross section in terms of a
series of line segments representing the sketch which the
surveyor did on his pad of paper. It will also cope when
someone finally makes a laser measuring device to
capture cross sections in a particular plane.
In reality in general every single cave cross section is distinct,
so there is no point in defining their shapes by cross-referencing
to a table rather than including the data directly in the definitions.
Of course, there is no reason not to include shape libraries in
the cross-section editor to speed up entry, but whichever you use
as your basis is not real data.
Julian Todd.
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