Sprinkle an area at the bottom of a hill in say brown to represent dead ground.
The trouble is that there is no such thing as “dead ground” in the absolute sense of being a specific place. Ground is “dead” when viewed from one point because the shape of other ground in between gets in the way, but move to another point and view from there and it all might be visible.
Since the point of observation is in three dimensions, not just two, it may even be possible to see into otherwise dead ground just by getting higher up. One of the problems of siting fire trenches during the early 20th century was that mounted officers might choose a position for its fine fields of fire, and, when the men had dug trenches there, it would be discovered that points easily visible to a bloke sitting on a horse cound not be seen at all by eyeballs peering over a trench parapet at ground level.
All the best,