[Proj] Lat-Lon values under different ellipsoides
Jan Hartmann
j.l.h.hartmann at uva.nl
Tue Jan 27 06:45:56 EST 2009
Thanks Duncan, that is an excellent suggestion. There is a book about
the geoid in the Netherlands
(http://www.ncg.knaw.nl/eng/publications/Green/34DeMin.html) and I'll
try to contact the author.
Even so, my question remains: how can different ellipsoids produce
different lat-lon coordinates? Different datums certainly do: the same
point in the Netherlands has a different lat-lon value for the WGS84
datum than for the older Bessel ellipsoid. I would really appreciate if
someone explained how those lat-lon values are derived for a specific
ellipsoid.
Jan
Duncan Agnew wrote:
> Jan:
>
> I would say that it is better not to have a different ellipsoid
> change the ellipsoidal coordinates (which is what lat/long are).
>
> It may also be that you are looking at the difference betweeen
> astronomical latitude and geodetic. The latter is what you get if the
> reference for the local vertical is the normal to the ellipsoid; the
> former, easier to do, assumes the local vertical is the direction of the
> plumb line. The difference is the deflection of the vertical, which
> depends
> on the slope of the geoid: I don't know what this is for the
> Netherlands.
> It is quite possible that the 1850 values are astronomical: you'd need
> an
> expert on local geodetic history to find out
>
> Duncan Agnew
>
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