Mercator's occupation long in decline (13)

I believe the answer is:

projectionist

I believe this clue is a double definition.

'mercator's' is the first definition.
(map-maker responsible for the Mercator projection)

'occupation long in decline' is the second definition.
(film projector operator - an old-fashioned job)

(Other definitions for projectionist that I've seen before include "Cinema employee" , "Cinema worker" .)

I've seen this clue in The Guardian.

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