In a submarine, when the Captain tells the Dive Officer, “Take us down to 200 feet”, that is 200 feet relative to what? An arbitrary designation of sea level? The actual surface of the ocean? Are tides and wave action figured in? And what about the bottom end. Are they measuring to the keel, the midpoint, the top of the conning tower, or some other arbitrary point on the sub?
I realize they are most likely using a pressure gauge to determine their depth, but I’m still curious about the reference points being used. I’m sure that on a large submarine the difference from keel to top of conning tower must be somewhere in the vicinity of 50 to 60 feet at least, which would make a substantial difference in such depth measurements. So, what is the Straight Dope?
From the surface. The depth is measured to the keel (the bottom of the boat). On modern submarines the distance from the keel to the top of the tower (sail) is probably even more than you said but you are right in general.
U.S. submarines have both mechanical pressure gages and electronic gages/meters. The mechanical gages have several different ranges (i.e. shallow gages and deep gages) allowing for increased precision near the surface. The electronic indicators are usually digital. All depend on water pressure, but the mechanical gages do not require electrical power.
All depth indicators are calibrated to read depth from the surface to the keel of the submarine, assuming a calm surface.
On a Los Angeles-class sub, the distance from the keel to the top of the sail is 50 feet. (The beam/diameter of the sub is 33 feet.)
(Incidentally, the U.S. Navy insists on spelling the word “gauge” as “gage.” The latter spelling is listed in the dictionary as an acceptable alternate, but non-Navy people often assume it is a misspelling.)