[postgis-users] 3D Datum shifts from postgis

[Stephen Woodbridge]

Frank Warmerdam wrote:
> Paul Ramsey wrote:
>> Well, I am pretty sure PostGIS only does transform() in 2d, no matter 
>> what. But I also suspect that Proj4 only has a 2d internal model, 
>> which means at the library level nothing may be available either.
>>
>> Frank, can you confirm what proj4 provides in terms of 3d shifts?
> 
> Paul,
> 
> The PROJ.4 pj_transform() function attempts to transform "z" as well
> as x and y.  Note that datum shifts performed with grid shift files
> normally have no z shift.  But 3/7 parameter transformations also affect
> Z, and if the shift is done strictly ellipsoid to ellipsoid then Z shifts
> are also possible.
> 
> I think it would be desirable to connect up this Z shifting in 
> pj_transform()
> to the Z coordinate of postgis geometries.

I assume this would only be done if the postgis data was defined as 3/4D 
geometry, and for 2D geoms the Z would be ignored. BTW, I think this 
would be a good feature, even though I do not have any use for it at 
this time.

-Steve W.