Molodensky Transform with PROJ

Question

I'm trying to transform some geodetic coordinates from the Ellipsoid (Hayford) to WGS84. I downloaded QGIS and I'm trying to use PROJ, with the OSGeo4W shell. I'm trying first to run the example given in the documentation. Example

proj +proj=molodensky +a=6378160 +rf=298.25 +da=-23 +df=-8.120449e-8 +dx=-134 +dy=-48 +dz=149 +abridged but I get the follow error get the error:

<proj>: can't initialize operations that produce angular output coordinates program abnormally terminated

Also, why are the parameters +a(semi-major axis) and +rf needed and what does +rf stands for?

Answer 1

Molodensky is a datum transformation, but you are using the proj application, which is used to projections.

You can perform datum transformations with cct application instead (https://proj.org/apps/cct.html#description):

cct +proj=molodensky +a=6378160 +rf=298.25 +da=-23 +df=-8.120449e-8  +dx=-134 +dy=-48 +dz=149 +abridged
0 0 0
 -0.0004311898    0.0013475059     -111.0000           inf

+a and +rf parameters define the source ellipsoid and +da, +df, +dx, +dy, +dz parameters define the transformation.

In the example, the transformation is using South American 1969 ellipsoid as source, but I don't know the target ellipsoid and datum for the transformation used.

I hope you know what parameters to use, the same that you know that this type of transformation is what you want to do.