Loop closure is automatically done by therion when it process the centerline data. All you have to do is to put in suitable equate commands.

For example, suppose that there is a main cave passage with two side passages, and you have surveyed the main passage, and the side passages separately. Then you have three survey data files, and have drawn the respective maps. When you assemble these together you must write the necessary equate commands (and the join commands for the scraps), as in the following code.

  input cave.th
  input cave2.th
  input cave3.th

  equate 2@cave 21@cave2
  equate 4@cave 23@cave2

  equate 3@cave1 31@cave3
  # equate 5@cave1 33@cave3 

  join scrap1@cave1 scrap2@cave2 -count 2
  join scrap1@cave1 scrap3@cave3 -count 2

The result is shown in the figure below. The side passages have been drawn at a offset to better distinguish them. The connection straight lines go from the station points in the offset maps to the station points in the main passage map. This is shown for "cave2" in the figure. If you do not close the loop, ie, you miss one equate command, the connection lines fail to reach the proper station points in the main map, as in the case of "cave3" in the figure.

In the extended elevation map the loop is unrolled and there usually remains a gap between the end points, because it is likely that the extents of the shots to the right does not equate that of those to the left. The two station points at the gap must be drawn at the same altitude, because they represent the same survey station.

You can specify a starting station for the extended projection. You do this with the "centerline" command extend start station, for example extend start 1@survey2. You may need to use also the extend ignore command. This commands defines the station points that therion should not use to compute the extended elevation map. Finally the command extend hide tells therion not to show the following shots in the extended elevation, and extend vertical describes the shots for which only the vertical displacement should be drawn.

For example, suppose that there is a survey with a side passage,

   ...
   2 3 10 90 0
   3 4 10 180 0
   ...
   21 22 10 90 30
   22 23 10 180 0
   23 24 10 90 -20
   ...

   equate 2 21
   equate 4 24

There is clearly a loop misclosure, and therion carries out the proper compensation of the shots, as far as the world position of the station is concerned. When it comes to drawing the extended elevation map, you must make choices and tell therion about them. For example if you want to put the points 2 and 21 at the same place, then points 4 and 24 will be different. You must tell therion not to use the shot 23-24 in drawing the extended elevation. You must also avoid putting a point of type "station" for the station 24 (option "-name 24") in the scrap, otherwise it will be placed to its correct position in the extended section, and the piece of scrap between 23 and 24 will be distorted. The extend command with the two stations of the shot is just what you need (notice the order of the stations: the command says to ignore the connection from "24" to "23" to build the extended map),

   extend start 6
   extend ignore 24 23

The result is shown in the figure below, in the lower part. The map of the side passage has been displaced above the main passage using the map offsetting feature of the map command. The first and last stations (namely 21 and 24) are connected to their position in the overlay by using map-connection points. Finally a map-connection line is drawn between points 4 and 24 (in the overlay). This is obtained by inserting a scrap in the source file, and adding the map m1 to the extended map.

  scrap s1 -proj extended
    point 0 0 station -name 4 -from 3
    point 100 0 station -name 24 -from 23
    line map-connection
      0 0
      100 0
    endline
  endscrap

  map m1 -proj extended
    s1
  endmap

If you want to draw the points "4" and "24" on the same spot and split the map at the other end, at the points "2" and "21", you need to change the starting station and the ignore shot. There must not be a point of type "station" for station 21 in the scrap.

  extend start 1
  extend ignore 21 22

The result is shown in the upper part of the figure.

If you do not write the extend ignore command therion uses all the stations to generate the extended elevation map. Therefore it stretches the shots so that the displacement between points 2 and 4 in the extended elevation, is the same as that between 21 and 24. The outcome are stretched shots and enlarged passages, which is probably not what you expected.

You might achieve the same result with extend ignore 23. In this case therion ignores all the shots connected to station 23. However if there are only three points you must specify the middle point and the end point where you want to ignore the extend, otherwise therion might choose the other endpoint. Beware not to write extend ignore 24 after the equate 4 24 command. Otherwise therion will ignore station point 4 when it generates the extended elevation map.

Another error that might be a cause of concern to you is to write extend ignore with a station point that is not the second to the last, or the second, in the side branch. If you do this all the points between the one you specifed and the end of the side passage can be extended in a wierd way. If you specify the second to the last point, the last point is displaced with respect to the corresponding point in the main passage. If you specify the second point, the first point is displaced.

It may happens that you write an extend ignore A B but the drawing comes out distorted as if the extend ignore command were not taken into account. This depends on the order of the shots that therion follows in building the map. Try to swap the two stations in the command, eg, extend ignore B A, and things will go into place.

It is customary to draw a dotted (or dashed) line connecting the two points to show that they denote the same survey station. To do this you must provide a line of type map-connection in a scrap containing the two points. Supposing they belongs to different surveys (which is the most common case) you write somethind like

  equate 1a@survey31 12@survey24

  centerline
    extend start 1a@survey31
  endcenterline

  scrap connetion1 -proj extended
    point   0 0 station -name 12@survey24 -from 11@survey24
    point 100 0 station -name 1a@survey31 -from 1@survey31
    line map-connection
        0 0
      100 0
    endline
  endscrap

The "map-connection" line does not need to be a straight segment, but can be any curve. However a line with straight (usually horizontal and vertical) segments is used to emphasize that it is a "connection" line. For example, the following is a "U" shaped line,

     line map-connection
         0 0
         0 20
       100 20
       100 0
     endline

The result is shown in the figure below, in which only the portion of the survey with the map-connection line is displayed.

Extended map
Extended loop (0)
Extended loop (1)
Extended loop (2)
Extended loop (3)
Extended loop (4)