[Proj] Lat-Lon values under different ellipsoides

[Jan Hartmann]

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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