Provided by: survex_1.2.5-1_amd64 bug


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


       extend [--specfile configuration .espec file]   input .3d file [output .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.

       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), svxedit(1)