A Riddle…

There was much head scratching on board yesterday evening. I used the “odometer” readout on our GPS to determine our miles run for the day that I posted yesterday with such glee. Even under those nearly ideal conditions I was amazed at how well we did. For some reason, I went back to my chart, and actually measured the distance on the paper. Surprise! Not 166 miles, but 122, a much more realistic number.

After checking my paper plotting, and rechecking, and re-rechecking, I concluded that it was right and the total distance reported by the GPS was wrong. After much cogitation, I know why…

The boat has three separate GPS sensors that are connected to the network. One is built into the chart plotter in the cockpit, one is part of the AIS system on the stern, and one is part of the combined weather/compass/GPS at the top of the mast. The masthead unit has always given the most stable and consistent reading so that is what I was using.

While we did not go in a PERFECTLY straight line, we certainly did not wander around in our course enough to add any significant extra distance, so that’s not it. Soooo….

What do you think the answer is? All relevant information is in this post, and some information is present just to “unclarify” the issue for extra fun!

I’ll post my answer tomorrow.

[Trevor: Feel free to use this as an example in your GPS class.]

P.S. The skipjack tuna was delicious!

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3 Responses to A Riddle…

  1. Bill Kinney says:

    And we have a pair of winners!


  2. JBauer says:

    My math might be a little fuzzy on this but it seems like a 45 foot mast swinging through 20 degrees circular arc every minute over 24 hours would add 46 miles that would not be related to forward motion. I’m thinking in large wave action the masthead GPS might not be the best indicator of forward progress. Thanks for the riddle and looking forward to your explanation.


    • Trevor says:

      I have to agree. The lateral movement of the truck as a result of wave action is being translated into distance covered me thinks…..trust the paper!


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