[postgis-devel] multisurface definition in bnf-wkt.txt

[Mateusz Łoskot]

Hi,

I see.
The *messy* differences between SQLMM and SFSQL 1.2 confused me.

The SFSQL 1.2 first says:

"""
MultiSurface is an instantiable class in this Standard, and may be
used to represent heterogeneous surfaces
collections of polygons and polyhedral surfaces.
"""

then it also says

"""
A MultiPolygon is a subtype of the non-instantiable type MultiSurface.
"""

and finally, it includes no specification of MultiSurface for WKT.


Whereas in ISO SQLMM,

"""
The ST_MultiSurface type may be instantiable.
"""

then it specifies WKT (which I copied in my first email)

*but*, AFAIU, that WKT does not allow polyhedral surfaces as part of
MultiSurface, does it?

If my understanding is correct, this is quite a significant difference
between the SQLSF adn SQLMM.

M.

On 10 January 2014 19:08, Paul Ramsey <pramsey at cleverelephant.ca> wrote:
> Yes, these are types picked up from drafts. polyhedral surface did
> make SFSQL 1.2. haven't seen tin/triangle in anything (which makes me
> less happy that we have them, since they seem to get little enough
> use, and I'm not really convinced they are good ways to model that
> stuff, but water under the bridge...)
>
> P
>
> On Fri, Jan 10, 2014 at 5:02 AM, Mateusz Łoskot <mateusz at loskot.net> wrote:
>> Hi,
>>
>> The bnf-wkt.txt [1], with last commit saying "Document ISO WKT/WKB (#1451)"
>> specifies:
>>
>> <multisurface text representation> ::=
>>     MULTISURFACE [ <z m> ] <multisurface text> |
>>     <multipolygon text representation> |
>>     <polyhedralsurface text representation> |
>>     <tin text representation>
>>
>> whereas the ISO/IEC 13249-3 (Date: 2009-01-16) specifies:
>>
>> <multisurface text representation> ::=
>>      MULTISURFACE [ <z m> ] <multisurface text>
>>    | <multipolygon text representation>
>>
>> <multisurface text> ::=
>>      <empty set>
>>    | <left paren> <surface text>
>>       { <comma> <surface text> }... <right paren>
>>
>> <surface text> ::=
>>      CURVEPOLYGON <curvepolygon text body>
>>    | <polygon text body>
>>
>>
>> Why bnf-wkt.txt adds polyhedralsurface and tin to
>> multisurface text representation?
>> Is that PostGIS-specific behaviour?
>>
>> [1] https://github.com/postgis/postgis/blob/svn-trunk/doc/bnf-wkt.txt
>>
>> Best regards,
>> --
>> Mateusz  Łoskot, http://mateusz.loskot.net
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-- 
Mateusz  Łoskot, http://mateusz.loskot.net