Geometry Boost List: Re: [geometry] Changes in WKB (read) behaviour proposal (WKT Part)

Subject: Re: [geometry] Changes in WKB (read) behaviour proposal (WKT Part)
From: Barend Gehrels (barend_at_[hidden])
Date: 2013-11-04 18:14:16

Hi Mateusz,

Mateusz Loskot wrote On 4-11-2013 23:05:
> On 4 November 2013 21:11, Barend Gehrels <barend_at_[hidden]> wrote:
>> Mats Taraldsvik wrote On 4-11-2013 21:31:
>>>>> The parser should support 3D as long as both the expected point
>>>>> dimensions
>>>>> and the wkb point dimensions match. I have tested this for 3D/Z. M is
>>>>> not supported by WKT yet, apparently, so I could not test this easily.
>>>> M is there too, see OGC specification in version 1.2.0.
>>>
>>> I meant boost::geometry's WKT reading. At least, I got an error when I
>>> tried POINTM(1 2 3), but that might be the wrong syntax.
>>
>> POINT  M ( 1 2)
>> is OK and according to the spec.
>> http://en.wikipedia.org/wiki/Well-known_text
> Barend,
>
> Are you sure this is a valid POINT M?

Oh, thanks for the catch. No it is indeed not officially valid, I stated 
that wrong. But we parse it. We drop the M anyway (currently). We cannot 
handle it because our Point Concept does not contain model-values. 
Something additional would be necessary to be able to assign it to the 
point in question (strategy? policy? assigner?), which can do with it 
what it wants. An easy option would be assigning the M to the third 
dimension (if any), and that is probably currently the case (I have to 
check).

A similar reasoning would apply for WKB.

> The BNF include in the OGC SFS spec is clear the "third value" for M
> is required:
>
> <point z>  ::= <x> <y> <z>
> <point m>  ::= <x> <y> <m>
> <point zm> ::= <x> <y> <z> <m>

There are not unit tests for them all, we should check and extend the tests.

>> We also have additions: we use WKT for the unit-test too, extensively.
>> Therefore we also added BOX and SEGMENT. We also support e.g. 5D (e.g. for
>> distance centroid) and therefore our wkt's can have 2,3,4,5 or more
>> dimensions
> Right, but IMHO those WKTs for tests are sort of Boost.Geometry Extended WKT :)
>

Agreed. We should doc the exact behaviour as it is not 100% clear 
everywhere.

Regards, Barend