Thought the rest of you might be interested in this one, since I found the solution. All you would need, is to include the *surveys* in your map, then use "symbol-hide group centreline" to hide them from the render, so the map looks like this: map foo subsectionmap@subsection othersubsection anothersubsection endmap (Or if you need the centreline for some centreline-only section of the survey within the part you are trying to isolate, you would need to include a scrap or two containing a single survey station and no other symbols from the highest and lowest parts of the cave, and then use -attr to select and hide those stations again. Once a single station is included from a survey, the whole survey gets used in the altitude calculations.) By making Therion include them, it will use their altitudes to work out the colours, even if they will be hidden from the rendering.