[QGIS-Developer] QGIS server issue: bounding box issues
Andreas Neumann
a.neumann at carto.net
Sun May 12 23:51:33 PDT 2019
Hi Jonathan,
Thanks for your feedback.
Seems to me that bounding boxes are a big mess. Yet they are used to
hide data away outside of the bounding box. That behaviour seems really
dangerous to me. People rely on the WMS that they show "all" data, but
if so many bounding boxes are out of date in the wild this means that
clients can't rely on them really. And neither can servers (and filter
away data).
Also, it seems to me that OGC specifications doesn't handle edge cases
well, such as layers with only a single point.
Given the fact that many layer bounding boxes are just plain wrong, I
wonder if WMS clients should use bounding boxes at all, they seem to be
really, really unreliable. Or they should only use the top level
bounding box of the whole service. Many open questions ...
Andreas
On 2019-05-13 02:27, Jonathan Moules wrote:
> Hi list,
>
> Unless GeoServer has changed it of late, the way they do BBOX definition is:
>
> * Layer BBOXes are defined at layer creation time.
>
> * Layer BBOXes are entered manually, though there is a button to automatically calculate it from the data extent which automatically fills in the manual boxes - the values can then be manually tweaked as desired.
>
> * Layer BBOXes are not automatically calculated at use-time.
>
> ----
>
> It looks like GeoServer also turns a single point into a BBOX of a single point: https://gis.stackexchange.com/questions/113166/the-request-bounding-box-has-zero-area
>
> ----
>
> De-factor treatment of bounding boxes: Layers do often have BBOXes that do not actually represent the data.
>
> In fact, of the 1.2million WMS, WFS, WCS, WMTS layers in my database, nearly 55,000 don't even have BBOXes (or have not-valid-wgs84 coordinates)!
>
> There's no easy way to check how many of the rest are declaring the correct BBOX, but experience suggests a lot don't.
>
> ----
>
> De jure usage: I've just taken a quick glance at the standards (WMS 1.3, WFS 2, WCS 2) and they standards themselves don't seem to address the issue of keeping the bboxes contemporary at all or even exactly what they're for. The closest I could find as to specifying the exact purpose of the bboxes is in the WFS 2 spec:
>
> "The ows:WGS84BoundingBox element may be used to indicate the edges of an enclosing rectangle in decimal
> degrees of latitude and longitude in WGS84. Its purpose is to facilitate geographic searches by indicating where
> instances of the particular feature type exist. Since multiple ows:WGS84BoundingBox elements may be
> specified, a WFS may indicate where various clusters of data exist. This knowledge aids client applications by
> letting them know where they should query in order to have a high probability of finding feature data."
>
> And this is mildly telling from the WMS 1.3 spec:
>
> "There is no provision for describing disjoint bounding boxes. For example, consider a dataset which covers two
> areas separated by some distance. The server cannot provide two separate bounding boxes in the same Layer using the
> same CRS to separately describe those areas. To handle this type of situation, the server may either define a single larger
> bounding box which encloses both areas, or may define two separate Layers that each have distinct Name and BoundingBox
> values."
>
> So it doesn't look like handling changing extents is something the spec writers have specified.
>
> And I can assure you, many servers don't have valid BBOXes defined. In fact
>
> Cheers,
>
> Jonathan
>
> On 2019-05-09 15:37, Alessandro Pasotti wrote:
>
> On Thu, May 9, 2019 at 4:16 PM Eric Lemoine <eric.lemoine at oslandia.com> wrote: On Thu, 9 May 2019 11:28:00 +0200
> Andreas Neumann <a.neumann at carto.net> wrote:
>
>> Hi QGIS (server) devs,
>
> Hi Andreas
>
>>
>> We came across issues around calculating bounding boxes in QGIS
>> server.
>>
>> 1. Layers with only one point feature:
>>
>> If a layer contains only one single point feature, QGIS server
>> calculates a bounding box where the minx equals maxx and miny equals
>> maxy, so resulting in a bounding box without a width and height.
>> Sounds logical to QGIS server developers,
>
> Yes. The BBOX of a point has minx=maxx and miny=maxy. Even PostGIS says
> so :)
>
>> but combined with the fact
>> that QGIS server doesn't take into account rendered symbol sizes
>> (another issue we have, see issue nr 2), it means that no WMS client
>> will ever see this one single symbol rendered, which can't be the
>> solution here ...
>
> If the GetMap request's BBOX param is set to the layer extent (the
> BBOX with no dimension here) then it makes sense that there's nothing
> rendered in the resulting image. If the GetMap request's BBOX param is
> set to a BBOX that contains the layer extent then the point should be
> rendered in the resulting image.
>
> So to me this is a client issue, not a QGIS Server issue.
>
>> 2. Layer bounding boxes do not take into account rendered symbol
>> sizes:
>>
>> Please have a look at
>> http://www.carto.net/neumann/temp/qgis_server_bounding_box_issue.png
>> - The green rectangle and the green arrows are not part of the QGIS
>> server rendering, but they are added as an annotation to the rendered
>> QGIS server graphics, to highlight the issues.
>
> What software do you use on the client side? Does the green
> rectangle correspond to the BBOX requested by the client? And does the
> requested BBOX equal the layer extent in this case? Or does it contain
> the layer extent?
>
> I may be wrong but I understand that the requested BBOX (the green
> rectangle) is the layer extent. And in that case it makes sense that
> the symbols are cut for points that are closed to the boundaries. Again
> it's a client issue.
>
>> Here we have the issue that QGIS server only uses the "raw" geometry
>> of point symbols without taking into account rendered symbol sizes.
>> Now, I do understand that calculating symbol sizes is scale dependent
>> and there is no single solution to that, but again, I think the
>> current behavior of QGIS server (simply cutting off symbols at layer
>> bounding boxes) is not a good and nice behavior. At least, I think
>> the author of the WMS service should have a chance to define an extra
>> margin to be added to the bounding boxes of the raw geometries of the
>> point layer, either as a "per project" or "per layer" QGIS server
>> configuration.
>>
>> @Andrea: I wonder what Geoserver does in such cases?
>>
>> Any thoughts how to solve these issues? The current behavior of QGIS
>> server is not satisfactory to me, for both cases.
>
> I'd like to better understand the issues that you're seeing but from
> what I currently understand the behavior of QGIS Server is correct.
> Happy to be proven otherwise :)
>
> Cheers,
> Hi Èric,
>
> I agree with you that QGIS Server does the right thing here, I think that the main question is:
>
> 1. is the WMS GetCapabilities layer's BoundingBox meant to be the features BBOX or can it be larger than that?
> 2. if the latter is true, we need a way to tell QGIS Server that he needs to advertise a BoundingBox in GetCapabilities which is not the layer's BBOX stored in the QGIS project but it's a different (probably larger) one.
>
> all the rest will follow, because the client will get a larger BBOX from GetCapabilities and it will request a larger image that has enough buffer for the symbols.
>
> Note that I checked mapserver and it behaves by default exactly like QGIS Server does (I didn't check the single point but the symbols are cut-off at the layer's bbox in general), except that mapserver allows you to override the layer extent per-layer.
>
> IMO the fix is in the client, either by allowing to override the layer extent advertised by the server and to store it in the project itself (this may require some work in the server side too in order to handle the override) or by setting an option in the WMS provider that will always request the canvas extent.
>
> Cheers
>
> --
> Alessandro Pasotti
> w3: www.itopen.it [1]
>
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