Determine 'native' size of wms layer based on GetCapabilities response

Question

I'm trying to understand the relation between a <LatLongBoundingBox> and <BoundingBox> in a GetCapabilities WMS response, given that the SRS of the bounding box is EPSG:4326.

Here's an example of the WMS that I'm trying to query:

<Layer>
<!-- snip -->
<LatLonBoundingBox maxx="37.968" maxy="70.144" minx="-4.646" miny="48.833"/>
<BoundingBox SRS="EPSG:4326" maxx="0.662672" maxy="1.224240" minx="-0.081091" miny="0.852295"/>
<!-- other BoundingBoxes omitted -->
</Layer>

My questions related to this response:

Why are the BoundingBox values different from the LatLonBoundingBox values an how is this useful? It looks like a scaling hint of some sorts.
Is it possible to determine the 'native' resolution of the layer (in pixels) using these values?

nmtoken · Accepted Answer · 2014-04-22T10:29:07.813

There does seem to be something very odd going on there. In a WMS 1.1.1 response such as the one you have given (because it has LatLonBoundingBox) they should exactly match for example here's the output for a UK wide WMS:

<LatLonBoundingBox minx="-8.6476" miny="49.8639" maxx="1.76943" maxy="60.8622" />
<BoundingBox SRS="EPSG:27700"
            minx="-77493.4" miny="-4039.86" maxx="670977" maxy="1.23824e+006" />
<BoundingBox SRS="EPSG:29902"
            minx="153489" miny="-154594" maxx="901891" maxy="1.10946e+006" />
<BoundingBox SRS="EPSG:3034"
            minx="2.71888e+006" miny="2.60232e+006" maxx="3.56176e+006" maxy="3.88362e+006" />
<BoundingBox SRS="EPSG:3413"
            minx="1.91073e+006" miny="-3.64649e+006" maxx="3.29885e+006" maxy="-2.20789e+006" />
<BoundingBox SRS="EPSG:3857"
            minx="-962646" miny="6.42274e+006" maxx="196972" maxy="8.59425e+006" />
<BoundingBox SRS="EPSG:4258"
            minx="-8.6476" miny="49.8639" maxx="1.76943" maxy="60.8622" />
<BoundingBox SRS="EPSG:4326"
            minx="-8.6476" miny="49.8639" maxx="1.76943" maxy="60.8622" />
<BoundingBox SRS="EPSG:900913"
            minx="-8.6476" miny="49.8639" maxx="1.76943" maxy="60.8622" />

There is something else odd in your example too, the order of the bounds for a LatLonBoundingBox MUST be minx="" miny="" maxx="" maxy="" but you have it the other way around.

<!-- The LatLonBoundingBox attributes indicate the edges of the enclosing
rectangle in latitude/longitude decimal degrees (as in SRS EPSG:4326 [WGS1984
lat/lon]). -->
<!ELEMENT LatLonBoundingBox EMPTY>
<!ATTLIST LatLonBoundingBox 
    minx CDATA #REQUIRED
    miny CDATA #REQUIRED
    maxx CDATA #REQUIRED
    maxy CDATA #REQUIRED>

The response from a WMS 1.3.0 GetCapabilities request (for the same area above) is as below, note we no longer have a LatLonBoundingBox element.

<EX_GeographicBoundingBox>
<westBoundLongitude>-8.6476</westBoundLongitude>
<eastBoundLongitude>1.76943</eastBoundLongitude>
<southBoundLatitude>49.8639</southBoundLatitude>
<northBoundLatitude>60.8622</northBoundLatitude>
</EX_GeographicBoundingBox>
<BoundingBox CRS="EPSG:27700" minx="-77493.4" miny="-4039.86" maxx="670977" maxy="1.23824e+006"/>
<BoundingBox CRS="EPSG:29902" minx="153489" miny="-154594" maxx="901891" maxy="1.10946e+006"/>
<BoundingBox CRS="EPSG:3034" minx="2.60232e+006" miny="2.71888e+006" maxx="3.88362e+006" maxy="3.56176e+006"/>
<BoundingBox CRS="EPSG:3413" minx="1.91073e+006" miny="-3.64649e+006" maxx="3.29885e+006" maxy="-2.20789e+006"/>
<BoundingBox CRS="EPSG:3857" minx="-962646" miny="6.42274e+006" maxx="196972" maxy="8.59425e+006"/>
<BoundingBox CRS="EPSG:4258" minx="49.8639" miny="-8.6476" maxx="60.8622" maxy="1.76943"/>
<BoundingBox CRS="EPSG:4326" minx="49.8639" miny="-8.6476" maxx="60.8622" maxy="1.76943"/>
<BoundingBox CRS="EPSG:900913" minx="-8.6476" miny="49.8639" maxx="1.76943" maxy="60.8622"/>

It looks like your GetCapabilities request response is in error here, and that is the root of your problem.

As a hunch I would guess that this GetCapabilities response has been hand crafted, and whoever has created it has made some mistakes...

Hm, yeah I'm assuming that the response is wrong somehow then. Odd thing is that the BBOX parameter to get a tile does use the given coords in BoundingBox. From your answer I also take that if the response is correct, there's still no way to get the optimal or native resolution for the given layer right? — botteaap, Apr 22 '14 at 15:35
There is no way to determine the native coordinate system from a GetCapabilities response; even if only one CRS/SRS is declared, there is no way of knowing whether the data is in that system or being reprojected by the service. — nmtoken, Apr 23 '14 at 08:34

Determine 'native' size of wms layer based on GetCapabilities response

1 Answers1