[Proj] ECEF

[Frank Warmerdam]

On Sat, 26 Feb 2005 16:26:57 -0600, Noel Zinn <ndzinn at houston.rr.com> wrote:
> I've read the documentation plus recent threads on this list serve that
> discuss datum shifts (of the 7-parameter variety), which, apparently, Proj.4
> does not support.  My question is whether Proj.4 supports Earth-Centered
> Earth-Fixed (ECEF) geocentric Cartesian coordinates or not (i.e, lat/lon/hgt
> => ECEF X/Y/Z).  Seems it would be simple to build that functionality into
> the Proj.4 code, but I gather that the documentation's references to
> "cartesian coordinates" means Eastings/Northings in projected space, and not
> ECEF.  Non-support for geocentric datum shifting seems to support this
> tentative conclusion.  Can someone confirm for me? 

Noel,

As others have mentioned there is a distinction between 
what I refer to as PROJ.4 (the project hosted as 
www.remotesensing.org/proj) and libproj4 (Gerald's 
new library focused on just projections).  

The PROJ.4 (rso) package does include support for
a few kinds of datum shifting:
 o Shifting with a grid shift file in either PROJ.4's internal 
   format (used for continental US files) or the NTv1 or
   NTv2 formats (used in Canada, Australia and perhaps a
   few other places). 
 o 3 parameter datum shifts (x,y,z cartesian shift).  
 o 7 parameter datum shifts (offset, rotation and scaling). 
 
It also has support for a geocentric coordinate system 
(using proj=geocent).  However, my understanding is
that there are two types of geocentric coordinates.  One
from the center of the ellipsoid, and one from a height 
perpendicular to the ellipsoid.  I'm not sure, off hand, 
which it is supported by PROJ.4 but can investigate if 
needed. 

The libproj4 library, as noted, focuses just on the projections
support and does not attempt to address datum shifting, 
or geocentric issues to the best of my knowledge. 

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent