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post #21 of 23 Old 10-17-2011
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Originally Posted by emoney View Post
On my powerboat, I checked my GPS vs. the speedometer (hey, it's measure in mph so I couldn't say knotmeter) and there was a 5mph difference betwixt the two. Question was, which one was correct?
Assuming no current, your speedometer might need to be calibrated. If there is current, it is hard to tell. Also they are measuring different things boat speed (through the water) versus SOG.

BTW - you should be able to change to the units of measure from miles, knots, kilometers, etc. on both the GPS and the speedometer.

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post #22 of 23 Old 10-17-2011
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Originally Posted by emoney View Post
On my powerboat, I checked my GPS vs. the speedometer (hey, it's measure in mph so I couldn't say knotmeter) and there was a 5mph difference betwixt the two. Question was, which one was correct?
Short answer: I'd trust a satellite before I'd trust a powerboat's Pitot tube.

Long answer: When you look at the way SOG is calculated from GPS positions, there is some error at slow speeds because the instantaneous positions have some error. But that would show up as random noise in your readings, which is easily damped out by good software. As a practical matter, if your GPS is showing a stable speed to the first decimal (x.x), without frequent changes after the decimal, then your signal is good and/or your software is damping it. I would consider the GPS to be highly reliable, and far more reliable than the pitot tube, which I believe could be severely affected by turbulence, salinity level, and temperature. In fact, I think its potential non-random bias probably exceeds the tidal currents in most waterways.

I'm not familiar enough with pinwheel-type speedometers used on sailboats to make a comparison, but I doubt they would be more accurate than a GPS for SOG.

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post #23 of 23 Old 10-17-2011
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The thing with 'knotmeters' is that they are 'calibratable', however that doesn't mean they've been accurately calibrated. Out of the box they should be OK but the only way to be certain is to run a measured distance in the absolute guarantee of no influencing current.. time the runs and do the math.

Not many boaters go to that effort..


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