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

[OvV_HN]

From: peifer <peifer <at> gmx.eu>
Subject: Re: How to convert a sphere to ellipsoid with correct datum?
Date: 2010-09-02 12:41:09 GMT

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

.......

REPLY:

This is indeed what you get with a simple conversion from lat-lon on Bessel 
1841 to geocentric and then from geocentric to lat-lon on WGS84. If this 
conversion makes any sense, one has to judge carefully. Frank is very right 
about that!

BTW: in my earlier mail I suggested a scaling parameter ds=1.0.
This should be 0. What I meant was a scaling of one, but usually a number in 
ppm is asked, which will be added to 1.0 internally by the conversion 
routine.

Exactly HOW cs2cs works, I don't know. Read the manual or ask Frank.


Oscar van Vlijmen