[CITE-Forum] reopnened: BBOX with zero extent (was: Re: Solved)

goerke at lat-lon.de goerke at lat-lon.de
Tue Apr 9 17:20:30 EDT 2013


Hi Arnulf,

I disagree.
I think, WMS SWG was aware of the problem of layers consisting of only 1
point while creating the standard and therefore specified that bounding
boxes must have an extent >0.
My second point is, that following your suggestion makes it impossible to
query the bounding box of the defined layer.
But the strongest argument is, that in the context of WMS services you
never speak about bounding boxes of geometries. It is all about layers.
There is no definition, that the bbox of a layer containing only a single
point has to be a zero area. So the bounding box of a layer is not
restricted to the bounding box of its contents. This makes sense as a
service provider could restrict the bounding box of a layer to only a
smaller extent of its datasource behind.

Following this, the WMS spec is well defined regarding bounding boxes for
layers which contain only a single point.

Best Regards

Sebastian

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> Hi,
> instead I would suggest to modify the standard and add an informative
> section:
>
> 1. "A Bounding Box should not have zero area."
> Reasoning: A point geometry has by definition a zero extent (area),
> anything else would be an arbitrary.
>
> Then add something like this:
>
> Informative note:
> Point geometries have by definition a zero extent and software has to
> make sure this does not lead to division by zero (or whatever else
> tech note might help to clarify the point).
>
> Reasoning: This will make sure that implementers look into the problem
> and make sure their software handles the problem correctly by at least
> producing a sensible error message.
>
> Cheers,
> Arnulf
>
>
> On 04/09/2013 07:05 PM, Richard Martell wrote:
>> Hi all,
>>
>>
>> Clause 6.7.4 in ISO 19128 (WMS 1.3) states that:
>>
>> "A Bounding Box shall not have zero area."
>>
>>
>> This constraints applies to both service metadata and requests. If
>> the current test suite does not verify this for the capabilities
>> document (getcapabilities.xml doesn't appear to), then a new test
>> is warranted.
>>
>>
>> -- Richard
>>
>>
>>> -----Original Message----- From:
>>> cite-forum-bounces+rmartell=galdosinc.com at lists.opengeospatial
>>> .org
>>> [mailto:cite-forum-bounces+rmartell=galdosinc.com at lists.openge
>>> ospatial.org] On Behalf Of Sebastian Goerke Sent: Tuesday, 02
>>> April, 2013 01:02 To: cite-forum at lists.opengeospatial.org
>>> Subject: Re: [CITE-Forum] Solved
>>>
>> Hi Arnulf,
>>
>> the WMS spec says in 7.3.3.6 BBOX:
>>
>> If a request contains an invalid BBOX (e.g. one whose minimum X is
>> greater than or equal to the maximum X, or whose minimum Y is
>> greater than or equal to the maximum Y) the server shall throw a
>> service exception.
>>
>> This implicates, that requesting a bbox with equal ll and ur is
>> impossible for compliant WMS implementations. As a consequence,
>> there is somehow an implicit requirement for WMS servers, not to
>> advertise such bounding boxes as they are not useable. Maybe this
>> is something for a change request.
>>
>> Regards
>>
>> Sebastian
>>
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>
> - --
> Arnulf Christl
> http://metaspatial.net
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