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I'm trying to emulate the earth which is an oblate spheriod - flattened
sphere.
The true earth parameters are
equatorial radius = 6378137
polar radius = 6356752.314
which gives a flattening effect of 1/298.257
The correct result does not occur by scaling the y axis of a sphere to
create the flattening. What seems a likely solution is an ellipse that is
swept around its center.
Any body have any ideas?
DF
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davidafisher <dav### [at] copper net> wrote:
> The correct result does not occur by scaling the y axis of a sphere to
> create the flattening.
Why not?
--
- Warp
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Wasn't it davidafisher who wrote:
>I'm trying to emulate the earth which is an oblate spheriod - flattened
>sphere.
>
>The true earth parameters are
>
>equatorial radius = 6378137
>polar radius = 6356752.314
>
>which gives a flattening effect of 1/298.257
>
>The correct result does not occur by scaling the y axis of a sphere to
>create the flattening. What seems a likely solution is an ellipse that is
>swept around its center.
Sweeping an ellipse around its short axis gives exactly the same result
as scaling a sphere along the y axis. In both cases the result is a true
oblate spheroid.
The Earth isn't exactly a true oblate spheroid, but the difference is
far too small to see.
In fact, you have to be sure to place your eye within 0.2 degrees of
being perfectly aligned with the centre of your monitor to have a chance
of seeing the difference between the representation of the Earth as a
sphere or as an oblate spheroid. If your eye is off centre by more than
that, then the perspective error caused by viewing the screen at a
slight angle exceeds the difference between the two shapes.
--
Mike Williams
Gentleman of Leisure
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davidafisher wrote:
> I'm trying to emulate the earth which is an oblate spheriod - flattened
> sphere. The correct result does not occur by scaling the y axis of a sphere
The formula (http://mathworld.wolfram.com/OblateSpheroid.html)
certainly looks just like a sphere with a different scaling for
one axis. Of course, you can also use an isosurface, which has
the advantage that you can add terrain elevation later.
#declare RADIUS_EQUATORIAL = 6.378137;
#declare RADIUS_POLAR = 6.356752;
isosurface
{
#local ALPHA_INV = 1/RADIUS_EQUATORIAL;
#local C_INV = 1/RADIUS_POLAR;
function {ALPHA_INV * (pow(x,2) + pow(z,2)) + C_INV * pow(y,2) - 1}
max_gradient 2
contained_by { sphere {0,6.5} }
pigment {color White}
}
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davidafisher wrote:
> I'm trying to emulate the earth which is an oblate spheriod - flattened
> sphere.
This is the wrong group for this question. This group is for discussion of
the POV-Ray source code, not general questions of using POV-Ray. Questions
about using POV-Ray should be asked/made in either povray.general or
povray.newusers.
Thorsten, POV-Team
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