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# Extended elevation control

Assume following situation (in plan view), when there is a loop in centerline between stations 1 and 4, small chimney near station 5 and entrance to the cave is at station 6.

By default, centerline extended elevation of this situation looks like this:

This is obviously not what we would like to get.

To control process of extended elevation, there is a special `extend` option in centerline command.

First of all, we would like to start our extended elevation at station 6 (where the entrance is). This can be done by specifying

    extend start 6

in the centerline. Now it looks better,

but there is still a problem with a branch containing station 5.

If we would like to start this branch from station 4, we need to forbid therion to extend station 5 from 1. This can be done by specifying

    extend ignore 1 5

This means, that shot from 1 to 5 will be ignored, when extended elevation is generated.

If we would like to extend branch starting with station 5 to the left, we need to specify also

    extend left 5

This will extend all stations from station 5 to the left.

As we have mentioned before, there is a small chimney above station 5. In this case, it is much more natural to draw this shot as vertical (because it is a chimney). To specify this, use

      extend vertical 5 5'

Or you can completely hide this leg from extended centerline using:

      extend hide 5 5'

## Stations in extended elevation scraps

Even if the station 1 is present more then one time in the map, therion automatically detects correct position of this station in each scrap and they are usually drawn correctly.

The only thing that is missing is connection line between stations 1 in these two branches. Therion does not automatically generate these lines because their shape usually depends on particilar map.

To draw such a line all you need to do is to create a simple scrap with this line. Here is an example:

    scrap sc -proj extended
      point 0 0 station -name 1 -from 5 -visibility off
      point 100 0 station -name 1 -from 2 -visibility off
      line map-connection
          0   5
          0  15
        100  15
        100   5
      endline
    endscrap

As you can see, even there are two stations with same name, they are distinguished by `-from` option, which specifies previous station in extended elevation. Using this scrap you receive the final map:

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