17. 4. 2020 v 21:12, Roger Schuster <roger@r-schuster.de>:

To avoid the white artifact (see file kolkhoehle_artifact.pdf) it is necessary not to hide the survey stations! As soon as you use either the attribute -visibility off or use symbol-hide point station in your layout you will have a white trapezoid on the final pdf map. Only hiding the survey line is possible. As long as the stations are visible the resulting map is fine (see kolkhoehle_ok.pdf). Btw. symbol-hide group cave-centerline causes an error.

As you may see, your .th2 file include only those three stations (and sketch). Probably it is bug in Therion that it not allows to hide stations from the final output. I may imagine the reason - MetaPost works with one scrap in time to interpret and draw that particular scrap. When it has nothing in scrap it will produce error.