For most landlubbers, most of the time, this is a question that is quite simple, and a variety methods will all give you essentially the same answer. When you are standing on, or rolling over, the ground, the ground is always your reference point, so measuring Speed Over Ground (SOG) is pretty much the only thing that makes sense. It is unusual that you would even think of something else.
Sailors have a much more complex relationship with the word “speed.” They certainly understand the concept of a “Speed Over Ground” (SOG), but until the widespread use of satellite navigation systems, SOG was only available intermittently, and rarely accurately. The information that was available was “Speed Through the Water” (STW). Sometimes, these numbers are the same, if there are no currents in the water, other times they can be different. Sometimes very different, if the current speed is more than a small fraction of the boat’s STW.
The modern sailor has both SOG and STW easily available. SOG from the GPS satellites, and STW from a number of mechanical or ultrasonic methods.
Sailing along the east coast of the USA we have to contend with one of the largest flows of water in the world. Parts of the Gulf Stream move over 2 cubic miles of ocean water every minute to the north and east. Surface speeds of three, four, even four and a half knots are common.
Four knots is a significant fraction of most sailboat’s speed, so no matter if you are headed north and looking for an extra boost, or headed south and trying to avoid bucking the strongest part of the current, knowing what the current flow is can be a really important navigation tool.
Given a Course Over Ground, a Speed Over Ground, a Heading, and a Speed Through the Water it has always been possible to calculate the speed and direction of the local current–if you were handy with vector arithmetic and trigonometry. Modern sailing instruments can, of course, do this in real time and present the data to the navigator as it happens both graphically and numerically.
To me, this data is worth a LOT while making a long distance ocean trip. If I only know the Speed Over Ground, or only the Speed Thru the Water, I feel very under informed about my situation. Not knowing how the water I am sailing in is moving is missing a key piece of the puzzle.
Playing the currents can make a big difference in how long a passage takes. As an example, last year while we were migrating north, we had stopped off the coast of South Carolina to fish. While we were there, a much larger, and theoretically much faster, sailboat passed us heading north. That was the last we expected to see of them…
We used what we knew about the expected behavior of the Gulf Stream, and what our instruments were telling us about the current speed and direction, to stay in the fastest currents we could find. Three days later, as we approached Cape Henry at the entrance to Chesapeake Bay, who comes up on us from behind, but our bigger, faster friend. They actually called us on the radio, “How do you get here so fast?” Karen was impolite enough to laugh…