# [Proj] How to convert a sphere to ellipsoid with correct datum?

peifer peifer at gmx.eu
Thu Sep 2 07:41:09 EST 2010

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OvV_HN wrote:
>
>
> In my opinion it would be simpler or even better to do a conversion from
> lat-lon to cartesian XYZ starting with the old ellipsoid and to convert
> back
> from cartesian to lat-lon with the parameters for the new ellipsoid.
> Regular
> datum transformation routines do that, but a Helmert transform is done in
> between (in cartesian space). So inactivate the Helmert transformation by
> giving dx=dy=dz=0, rx=ry=rz=0, ds=1.0.
>
> This should also work for converting spherical coordinates to ellipsoidal
> or
> the other way around, but then the geodetic/cartesian conversion routines
> must have a support for the sphere and I don't believe the PROJ routines
> have.
>
> Oscar van Vlijmen
>
>

Oscar,

in practical terms of cs2cs usage (and leaving out the question about the
spherical case), doesn't this simply mean that one has to add +towgs84=0,0,0
on both sides, as I suggested earlier?

Or +towgs84=0,0,0,0,0,0,0  or +towgs84=0,0,0,0,0,0,1. It doesn't seem to
make much of a difference.

Hermann

cs2cs +proj=latlong +ellps=bessel  +to +proj=latlong +ellps=WGS84
10 60
10dE    60dN 0.000
^C

cs2cs +proj=latlong +ellps=bessel +towgs84=0,0,0  +to +proj=latlong
+ellps=WGS84 +towgs84=0,0,0
10 60
10dE    60d0'1.864"N -690.016
^C

cs2cs  +proj=latlong +ellps=bessel +towgs84=0,0,0,0,0,0,0  +to +proj=latlong
+ellps=WGS84 +towgs84=0,0,0,0,0,0,0
10 60
10dE    60d0'1.864"N -690.016
^C

cs2cs +proj=latlong +ellps=bessel +towgs84=0,0,0,0,0,0,1  +to +proj=latlong
+ellps=WGS84 +towgs84=0,0,0,0,0,0,1
10 60
10dE    60d0'1.864"N -690.016
^C

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