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EXTEND(1) EXTEND(1)

extend - produce an extended elevation from a .3d file

extend [ --survey=SURVEY ] [ --specfile=ESPEC_FILE ] [ --show-breaks ] INPUT_3D_FILE [ OUTPUT_3D_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 deep_pit.3d produces an extended elevation called deep_pit_extend.3d.
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 *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
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.

aven(1), cavern(1), diffpos(1), dump3d(1), sorterr(1), survexport(1)
13 September 2018

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