[Liblas-devel] ECEF?

[Ben Discoe]

On Sat, Mar 5, 2011 at 9:35 AM, Frank Warmerdam <warmerdam at pobox.com> wrote:
>> OK, my first question: how does one store points in ECEF coordinates?
>> In our case, the data is gathered and processed in ECEF, served as ECEF,
>> and rendered on the client in ECEF.
>
> ECEF (also referred to as geocentric) coordinates have a well defined
> description in WKT, and I would suggest we use that.

Great!  I will give that a try.

> Spatialreference.org offers this example:
> GEOCCS["WGS 84 (geocentric)",
>    DATUM["World Geodetic System 1984",
>        SPHEROID["WGS 84",6378137.0,298.257223563,
>            AUTHORITY["EPSG","7030"]],
>        AUTHORITY["EPSG","6326"]],
>    PRIMEM["Greenwich",0.0,
>        AUTHORITY["EPSG","8901"]],
>    UNIT["m",1.0],
>    AXIS["Geocentric X",OTHER],
>    AXIS["Geocentric Y",EAST],
>    AXIS["Geocentric Z",NORTH],
>    AUTHORITY["EPSG","4328"]]
>
> I also skimmed OGC document 01-009 (Coordinate Transformation Services)
> and is seems to use the above definition though I am not at all clear
> on why it is meaningful to include the DATUM.

Me neither; there's no earth shape relevant, hence no DATUM.  One
could also wonder about the purpose of the PRIMEM, although perhaps
it's to clarify where the X axis points.  Also mysterious are the axis
names ("OTHER, EAST, NORTH") where the only unambiguously "correct"
axis name is NORTH for Z; Y is most definitely not EAST.

> GDAL/OGR [..] does not have any specific support for GEOCCS but I'd be
> interested in adding it.  PROJ.4 does support geocentric (ECEF) coordinates.

I suspect it will be closely related to the vertical datum support
already in the works for OGR/Proj.  The math to convert ECEF to/from
an ellipsoid/spheroid elevation is fairly trivial, but not
particularly useful; most real elevation data wants to be relative to
"sea level", e.g. a real geoid, and ECEF to/from a real geoid is way
more involved, from what i understand.

> I will add doing so to my TODO list and try to address it shortly.

Thanks!

-Ben