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> Bill wrote:
>>
>> So it looks like it is calculating ellipsoidal, but not as well as Paul's
>> unit. (2229 on mine vs. 2232 on Paul's in his example)  Perhaps time for a
>> firmware update?
>
> You may be confused on this point, Bill -- my Magellan 315 does *not*
> compute distance on the ellipsoid. It uses a sphere. That's how I'm
> able to do spherical trig on it.

Paul,

Ah, thank you for the clarification.
>
 > When I first discovered that, I was a bit disappointed. But now I
> think it was perfectly reasonable for Magellan to simplify the
> computation to a sphere. At long distance, a few km one way or the
> other doesn't matter to me. The important thing is that the
> approximation converges on the precise value as I get close to the
> destination.

A kissing-cousin question to the above did run through my mind.  A
lady-friend's child needed help with a middle-school assignment a decade or
so ago. They were given lat and lon of a half-dozen locations around the
world (no datum given), and had to name the country and city.  Off we went
to the library.  What we found were two locations in bodies of water (no
islands nearby), most cities off by maybe 10-30 miles.  I did not learn if
the the instructor's lat and lon were in error and/or if different datum
could throw things that far off.  The teacher never responded to my
questions.

That and the differences in our distances (I assumed yours was ellipsoid)
caused me to question:

If the distance is off, has the unit placed the destination lat/lon at the
"wrong" GP and calculated correctly?

OR, has it used the correct GP and calculated the distance differently?

Bushwhacking and sailing experience indicate that when I get to my
destination it will be at its chart coordinates, therefore it is just a
matter of distance calculation.  0.13% difference between the methods would
be no big deal for a small craft over a 2000-mile-plus passage, but is
significant for cel nav working to tolerances of 0.00046%.

Thanks for making that clear,

Bill

   

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