The way I see it, the new spec does clear up some big holes like:
1. Being able to specify an unambiguous operation/projection method (by
identifier) for projections instead of loosely relying on the
projection's name (9.3.3)
2. Knowing the units of projection parameters (9.3.4)
On 2015-05-09 12:38, Even Rouault wrote:
Quoting Jukka Rahkonen <[email protected]>:
Hi,
There seems to be a new OGC (and ISO) standard "Geographic information -
Well-known text representation of coordinate reference systems"
http://docs.opengeospatial.org/is/12-063r5/12-063r5.html
On a quick look, one thing that struck me is that there will no longer be any
direct equivalent of TOWGS84.
The differences between the older WKT spec and the one currently
implemented by GDAL are explained in Annex C. For TOWGS84 specifically,
see C 3.3.3.
The transformation is not part of the coordinate reference system in
both the EPSG registry and the ISO 19111 specification. What was weird
with the older WKT spec is that you could have an identified CRS that
had TOWGS84 specified, even though that definition is technically not
part of the identified object. That being said, for a lot of practical
applications, the transformation is essential, so it'll be interesting
to see how it all plays out.
18.1 Bound CRS
The definition of a CRS is not dependent upon any relationship to an
independent CRS. However in an implementation that merges datasets
referenced to differing CRSs, it is sometimes useful to associate the
definition of the transformation that has been used with the CRS
definition.
AFAIU, the BOUNDCRS concept which can embrace the
concept of TOWGS84 through a Coordinate Frame method is to be used when the
transform to the TARGETCRS has already been done and document what was the
SOURCRS and the transformation applied.
Even, I'm not sure if I completely understood that last part. It's
entirely possible that what follows is what you were trying to say. If
that is the case, fell free to just ignore me. :)
The way I understand the BOUNDCRS concept is that it binds a
transformation to a given CRS. That is, even though the transformation
is not part of the CRS, you can use that construct to associate the
transformation with a CRS (as it was previously done with TOWGS84). So,
for a TOWGS84 equivalence, you'd have a BOUNDCRS where the source is the
CRS being defined, the target would be WGS 84, and the operation method
would be a 7 parameter geocentric transformations with the rotation
convention (position vector/coordinate frame) explicitly defined.
I for one certainly hope it gets adopted by the industry, as I find
there is too much ambiguity in the old WKT specification to truly
achieve interoperability,
André
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