You're saying you're not supporting TOWGS84, but how can things work at all without that? If the WKT uses unknown names (i.e. an user-defined projection/datum), the coordinate system cannot be properly set up when the datum shift is ignored. Or am I missing something?

Ok, that was the answer I was looking for. Unfortunately I couldn't find any test data to verify which conversion is now really the right one here.

@mkennedy: I think I can follow you on this one. The original question hasn't been answered yet, though. We can see it in the added examples from Andre well: Why is there a difference between his examples 2 and 3? I would expect them to be equal, because +towgs84=0,0,0,0,0,0,0 should be the default if it is not specified.

Do you have test points to convert from WGS84->Clarke 1880? I guess that would confirm our assumption that the epsg.io implementation is broken (our mine and esri...).

No, that doesn't change anything on the map there. I guess I could implement that shift for my code, but that doesn't answer the primary question.

That's something else (I think I have the methods that do the Geographic 2D offset transform). For now, I only need an answer to the question whether changing the ellipsoid only (no datum transformation) can change latitude and/or longitude.

What method? EPSG 1447? I was talking about a datum change without transformation. A transformation from WGS84 to NAD83 would be such an operation.

Unfortunately, I couldn't find anything reliable. Maybe somebody got some reference data for another coordinate system with the same behavior (0,0,0 shift)? NAD83 could do, for instance. I may be able to find that.

I have official reference coordinates for other systems, where they fit very well. I will be looking for data on this system.

The difference you see is exactly the problem I was observing. Actually, the output of your first line is, according to @mkennedy wrong. And it seems that the error is widely spread.

I've updated my question. Seems I'm getting the same result as you using the 0,0,0 transformation. Question is why epsg.io doesn't. How do I know I have to use the epsg:1447 transformation? Neither the gml nor the wkt definitions I've seen mention it.