Actually, most have it correct. THe depth is measured from the keel, however, there are a number of points along the keel where these "depth sensors" are. Yes, theyre called depth sensors. So, depending on what angle is on the boat, the sensors will have varied readings.
The sensors work by reading sea pressure, then converting it to feet. There are two types of depth gauges, shallow and deep. Its important to secure the shallow depth gauge at a certain depth to keep from over pressurizing the system.
The way that a submarine, or any other vessel, reads the ocean floor depth is by a fathometer. This device sends a sound pulse to the bottom and calculates the time it takes to bounce back to the boat and converts it to feet or fathoms.
There are any number of things that are "more important" than another depending on the situation at hand. You can imagine that most times the bottom of the ocean is very important because its many 100's of fathoms deep. And normally we dont want to hit the bottom, especially when its 12,000 feet down!!
Hope that helped
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