would have said so sooner but was distracted

this was a perfect conclusion to a perfect extended lesson for me
& i hope everyone got so much out of it all as i did

gratefully
m

>
>Michael:
>> curiously my old unreliable dictionary that i like to keep next to
>my old
>> unreliable atlas was misleading me by qualifying a spheroid as a
>particular
>> sort of ellipsoid that is generated by revolving an ellipse around
>one of
>> its axes
>>
>> it defines ellipsoids generally as geometric surfaces whose plane
>sections
>> are all either ellipses or circles
>>
>True enough. It's just that all the reference ellipsoids used here
>are oblate ellipsoids of rotation produced in exactly the way your
>dictionary defines "spheroid" - ellipses spun around a principal
>axis. So I guess I was misleading you by holding to the geographical
>specifics.
>The more general definition of ellipsoids allows for *triaxial*
>ellipsoids - which would in fact provide a marginally better fit to
>the geoid by having an elliptical equator, too, to bulge into that
>New Guinea high and sink under the Indian Ocean low, but a better fit
>won at the expense of horrible maths necessary to generate map
>projections. So, yes, in principle you could have options 2 and 3 in
>your original list as "spheroid" and "triaxial ellipsoid", but I
>don't know if anyone has ever used a triaxial reference ellipsoid for
>the Earth (once the maths got complicated and the computers got fast,
>it was better just to go the whole hog to the geoid), but the
>triaxials *are* very important when mapping potato-shaped small moons
>of other planets.
>
>Grant
>
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