extend - produce an extended elevation from a .3d file
] [ --specfile=ESPEC_FILE
Note: The extend program can
also work on Compass .plt (as can aven and any other
Survex program which reads .3d files).
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).
If the output filename is not specified, extend bases the output filename on the
input filename, but ending "_extend.3d". For example, extend
produces an extended elevation called
If you pass --show-breaks
then a leg flagged as "surface
survey" will be added between each point at which a loop has been broken
- this can be very useful for visualising the result in aven.
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.
The specfile is in a format similar to cavern's data format:
;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
; change direction of extension at 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 survey at station side-loop.junction but only
; for leg going to complex-loop.2
*break side-loop.junction complex-loop.2
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.