This page contains various bits and bobs related to therion, the cave survey drawing software.
Simple test-case for how to define and use different area symbols. In this case blocks and debris.
Download these two files to a directory and run either run therion there, or load them into xtherion and run from there. Editing the definition for A_blocks_TEST or a_blocks_TEST lets you see how things change.
This doc is the metapost graphics language spec. which helps explain what's going on. The 'punked' rock-drawing function comes from therion. It's in therion/mpost/therion.mp in the therion source.
The problem of basing a complicated therion survey on a survex 3d file is non-trivial. This originally came up in 2005 with the Mulu data. And again in 2018 for Peak/Speedwell.
The Therion book has a helpful description of import -surveys [use|create|ignore], but it doesn't explain -filter. Here is 2005 example using the Mulu data from 2005, showing the use of filter