NAME

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

SYNOPSIS

       extend [ --survey=SURVEY ] [ --specfile=ESPEC_FILE ] [ --show-breaks ] INPUT_3D_FILE [ OUTPUT_3D_FILE ]

DESCRIPTION

              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.

SEE ALSO

       aven(1), cavern(1), diffpos(1), dump3d(1), sorterr(1), survexport(1)

                                                 09 January 2019                                       EXTEND(1)