[gdal-dev] New OGC standard about WKT for projections

Andre Vautour andre.vautour at caris.com
Mon May 11 07:50:41 PDT 2015


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 <jukka.rahkonen at maanmittauslaitos.fi>:
>
>> 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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