On Wed 01 Dec 2004, Rev Simon Rumble <simon(a)rumble.net> wrote:
3D spatial reasoning is always the bit I don't do
well on in IQ
tests, so this stuff doesn't come easily for me.
I found this page very helpful once I'd read it enough times and
lain down on the sofa with my eyes closed once or twice:
Doesn't the ellipsoid used have an effect on the
There are two issues with ellipsoids, and this is the reason why back
on the list in June or whenever it was zool and Ivor were disagreeing
about whether WGS-84 is global or not. It is, and it isn't.
The issue you are thinking of is related to the earth not being quite
a sphere. So yes, you're right that latitude and longitude vary
depending on which approximation to the shape of the earth to work
them out. However, the earth is near enough to a sphere for the
purposes of locating a restaurant - see
for some calculations that Dave did.
The other issue is the one of once you have latitude and longitude,
turning them into square coordinates, and this is where we really do
care about the ellipsoid.
First consider a Mercator projection - not a transverse one. Imagine
the earth is made of a thin rubber shell, and the lines of longitude
are wires embedded in that shell. So the wires curve and meet in two
points at the poles. Now detach the wires from each other at the
poles, and straighten them out. The rubber will stretch to allow this.
Near the equator, there will be very little stretching. Near the
poles there will be a lot. So near the equator, distances will be
preserved but distances will change more the further you get from the
Now consider a transverse projection; keep hold of our straightened-out
globe and turn it on its side. What *was* the equator is now some
line of longitude, and it's near that line that distances are preserved.
So if the countries we mainly care about are round about there, then
hurrah, it's all going to work nicely.
If they're not, then rotate our wire thing around a north->south axis
(change the ellipsoid) until they are.
Does that help explain it?
Now as for which ellipsoid you should choose being as your guide is
worldwide - your guess is as good as mine. However it is possibly not
as vital for your distance calculations to be incredibly accurate.
Depends what you want them for really.