[CITE-Forum] reopnened: BBOX with zero extent

Arnulf Christl arnulf.christl at metaspatial.net
Tue Apr 9 18:10:06 EDT 2013

Hash: SHA1

right on, I completely agree. But this still does not really help to
pragmatically solve the issue when this thing is going to happen. And
if we can think it up it will happen, so why not address it right away?

A point is a point and that means it will have a zero sized BBOX. It
is no use to forbid this because there will be points and it is their
perfectly natural right to have a zero sized BBOX. :-)

My point is that we should simply add a note letting people know that
this thing can happen (and therefore will happen, as I have just
recently proven). We are even inviting this to happen with a layer
that has just one single point in the CITE dataset. There is simply no
"right" or "correct" way to deal with it given the current standard
and test process.

Plus - in my personal opinion - it does make sense to disallow things
that are perfectly natural, like a zero sized BBOX for a point. But I
don't really care enough about it to carry it through as a CR for the
WMS spec, I guess there are more important things to worry about.


On 04/09/2013 10:20 PM, goerke at lat-lon.de wrote:
> 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
> 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
>>>>> [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 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
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