>If you put a survey in a map it will show the centreline...

Andrews suggestion is the only way I have done it to date, due to the (generally helpful) behaviour of Therion whereby (for pdf exports) selecting nothing selects all, and selecting any map object suppresses all selected survey objects.

But it would be nice to be able to forcibly suppress scrap output, maybe something like this in a layout would do it;

layout LayoutSurveyOnly

      symbol-hide group all

      # hides all survey and scrap symbols except passage foreground and background colours I think

      symbol-show group centreline #shows centreline related symbols

      debug station-names #shows all station names

      scale 1 1000

endlayout LayoutSurveyOnly

That layout in combination with no selections (which by default selects all surveys and all scraps/maps in a source file) might do the trick. 

It worked for me, although it is slow to compile large projects due to Therion having to process all the scraps. As written above the layout generates centrelines with all the offsets and previews defined in maps, but hides all other drawing symbols.

You can see below the effect on one of my projects.  The light grey shading is the effect of the default white passage foreground colour overlying the preview below shading.

Thanks for seeding the idea Michael

Bruce