An "alidade" can be a simple device for measuing azimuth angle on horizon only.
An alidade can be a considerable value to anyone boating on salt water. I once goofed
and forgot to attach safety chain to my outboard, and let a ten year old steer.
He yanked tiller so viciously, outboard motor suddenly lifted up and went to
bottom. If I had had an alidade, I could have taken bearings that would have
made it possible to find it.

alidade
n.Fr < Sp alidada < Ar al EiQada, two sides or sideposts < EaQud, upper arm6
1 a part of an optical or surveying instrument, consisting of the vernier, indicator, etc.
2 a surveying instrument consisting of a telescope mounted on a rule, used in topography Also alidad