Provided by: survex_1.2.26-1_amd64 bug


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


       extend    [--survey=SURVEY]    [--specfile=ESPEC_FILE]    [--show-breaks]    INPUT_3D_FILE



              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.

See Also

       3dtopos(1), aven(1), cad3d(1), cavern(1), diffpos(1), sorterr(1)