[Therion] Colouring an elevation based on surveys that are not being displayed

[Tarquin Wilton-Jones]

Hi folks,

When you colour a survey by depth (color map-fg altitude), it colours it
based on the scraps that have been selected. This is normally useful.

When you produce an output with only one part of the survey (eg. a
zoomed-in section of a more complex part of the survey), however, it
would be more useful to be able to keep it the same colour as it was in
the larger survey, so that a viewer can quickly see which part of the
cave they are looking at, and see that the colours match. (My own
use-case is different, but the solution would be the same.)

Is there, for example, a way to say this:

layout local
  color map-fg altitude
endlayout
source "cave.th"
select subsectionMP at subsection.cave
altitude-relative caveMP at cave
export map -proj plan -layout plan -o "cave.pdf"

Alternatively, is there a way to add scraps belonging to the highest
points and lowest points into the map, but not have them render, while
still being used for altitude colouring (this seems like a hack rather
than being a dedicated solution).

Alternatively (and I know this will seem a little out of place here, but
it is an alternative solution for what I need), is there a way to import
a map-image, and crop it to only include a portion of the image?

Cheers!

Tarquin