Re: [gpsd-dev] CEP(50) 0.285 meters

[Fred Wright]

On Sat, 23 Jul 2016, Fred Wright wrote:
> On Fri, 22 Jul 2016, Fred Wright wrote:
>
> > There's a way to compromise on just the ellipsoidality issue by using a
> > spherical formula to get the "distance angle" but then using the actual
> > ellipsoidal radius (which has a non-iterative formula) at the location in
> > question rather than a constant for the angle to distance conversion.
> > Very roughly speaking, the error in this approach is proportional to the
> > product of the distance and the eccentricity.
>
> I misstated a bit.  The relevant scaling parameter is the normal, not the
> radius (assuming geodetic latitude, which is the kind normally used).
> This is the perpendicular segment from the local tangent plane to the
> equatorial plane.  Except at the equator and poles, it doesn't quite pass
> through the center.

Oops - correction to the correction.  The normal isn't to the equatorial
plane, it's to the polar axis.  Thus, it's normally *larger* than the
equatorial radius.  If this seems weird, it's because geodetic latitude is
based on the angle of the normal, not the angle of the radius.  Geodetic
and geocentric latitude differ maximally at approximately 45 degrees.

Fred Wright