source: git/trunk/doc/extend.sgml @ 7bb8184

<refmeta>
<refentrytitle>extend</refentrytitle>
<manvolnum>1</manvolnum>
</refmeta>

<refnamediv>
<refname>extend</refname>
<refpurpose>produce an extended elevation from a &x3d; file
</refpurpose>
</refnamediv>

<refsynopsisdiv>
<cmdsynopsis>
<command>extend</command>
<arg choice="opt">--specfile configuration .espec file</arg>
<arg choice="req">&nbsp;input .3d file</arg>
<arg choice="opt">output .3d file</arg>
</cmdsynopsis>
</refsynopsisdiv>
  
<refsect1><title>Description</title>

<Note>
<Para>
The <command>extend</command> program can also work on
<Application>Compass</Application> <filename>.plt</filename> (as can
<command>aven</command> and any other &survexsuite; program which reads &x3d;
files).
</Para>
</Note>

<Para>
If no specfile is given, extend starts with the highest station marked
as an entrance which has at least one underground survey leg attached
to it.  If there are no such stations, the highest deadend station in
the survey (or the highest station if there are no deadends) is used.
Extend puts the first station on the left, then folds each leg out
individually to the right, breaking loops arbitrarily (usually at
junctions).</Para>

<Para>If the output filename is not specified, extend bases the output
filename on the input filename, but ending "_extend.3d".  For example,
<command>extend deep_pit.3d</command> produces an extended elevation
called <filename>deep_pit_extend.3d</filename>.</Para>

<Para>
This approach suffices for simple caves or sections of cave, but for more
complicated situations human intervention is required.  More complex
sections of cave can be handled with a specfile giving directions to
switch the direction of extension between left and right, to
explicitly specify the start station, or to break the extension at
particular stations or legs. </Para>

<Para>
The specfile is in a format similar to cavern's data format:</Para>

<programlisting>
;This is a comment

; start the elevation at station entrance.a
*start entrance.a  ;this is a comment after a command

; start extending leftwards from station half-way-down.5
*eleft half-way-down.5

; change direction of extension at further-down.8
*eswap further-down.8

; extend right from further-down.junction, but only for
; the leg joining it to very-deep.1, other legs continuing
; as before
*eright further-down.junction  very-deep.1

; break the survey at station side-loop.4
*break side-loop.4

; break survey at station side-loop.junction but only
; for leg going to complex-loop.2
*break side-loop.junction complex-loop.2 
</programlisting>

<Para>
This approach requires some trial and error, but gives useful results
for many caves.  The most complex systems would benefit from an
interactive interface to select and view the breaks and switches of
direction.</Para>

</refsect1>