The method which I would use would be to make a careful selection of the
land area using Quick Mask (unless it is a single colour - in which case
just use the Magic Wand). Once you have it selected, hit Ctrl+J to copy
the selection to a new layer and then create your drop shadow (hit the
fX icon at the bottom of the layers palette and choose 'Drop Shadow'
from the fly out menu).
(Posted from the UK)