Get an Observation at a location

This use case is to get an observation at a specific location. There is a nice BBOX example in OGC 06-009r6 (8.4.5.3) that I have adapted to our situation

Request

<sos:GetObservation 
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
   xmlns:gml="http://www.opengis.net/gml" 
   xmlns:ogc="http://www.opengis.net/ogc" 
   xmlns:sos="http://www.opengis.net/sos/1.0" 
   xmlns:om="http://www.opengis.net/om/1.0" 
   xsi:schemaLocation="http://www.opengis.net/sos/1.0 http://schemas.opengis.net/sos/1.0.0/sosAll.xsd" 
   service="SOS" version="1.0.0" srsName="EPSG:4326">
   <sos:offering>urn:GIN:offering:groundwaterLevel:1</sos:offering>
   <sos:observedProperty>urn:ogc:def:property:OGC:GroundWaterLevel</sos:observedProperty>
   <sos:featureOfInterest>
      <ogc:BBOX>
         <ogc:PropertyName>sa:position</ogc:PropertyName>
         <gml:Envelope>
            <gml:lowerCorner>-79 44</gml:lowerCorner>
            <gml:upperCorner>-78 45</gml:upperCorner>
         </gml:Envelope>
      </ogc:BBOX>
   </sos:featureOfInterest>
   <sos:responseFormat>text/xml; subtype=&quot;om/1.0.0&quot;</sos:responseFormat>
   <sos:resultModel xmlns:wml="http://www.wron.net.au/waterml2">wml:WaterMonitoringObservation</sos:resultModel>
   <sos:responseMode>inline</sos:responseMode>
</sos:GetObservation>

This asks for all Observation of offering "urn:GIN:offering:groundwaterLevel:1" where the Observed property is urn:ogc:def:property:OGC:GroundWaterLevel where the location of the wml:WaterMonitoringObservation is within -90,45,-89,46.

Interesting note, the srsName is not on the property, but on the root tag (GetObservation)

Result

From the mock up service I implemented on GIN

http://ngwd-bdnes.cits.rncan.gc.ca/service/api_ngwds/en/sosform.html

Issues

  • The BBOX request uses the featureOfInterest to constrains the location, which means that the client must have a prior knowledge of what feature is of interest and what is the schema of that feature. Which is a problem. Right now, I'm ignoring the property.

Implementation in a WMS:GetFeatureInfo context

To query data in a GetFeatureInfo context, we must convert the pixel based request into a real world coordinate.

The simplest way is to create a proxy service that get the GetFeatureInfo, rewrite it in terms of SOS BBOX above and route it to the SOS service.

A GetFeatureInfo query looks like

http://ngwd-bdnes.cits.rncan.gc.ca/service/gin/wms/mediator/px? VERSION=1.1.1 &FORMAT=image%2Fpng &LAYERS=OntarioWaterWell &SERVICE=WMS &REQUEST=GetFeatureInfo &STYLES= &EXCEPTIONS=application%2Fvnd.ogc.se_xml &TRANSPARENT=TRUE &SRS=EPSG%3A4326 &BBOX=-90.151733,48.078798,-89.55857,48.519867 &X=416&Y=224 &INFO_FORMAT=text%2Fhtml &QUERY_LAYERS=OntarioWaterWell &WIDTH=780 &HEIGHT=580

There is basically all the same info than a WMS GetMap request (because the response might want to draw the map along with the data). The important bits are

&BBOX=-90.151733,48.078798,-89.55857,48.519867 (minx,miny,maxx,maxy) &X=416&Y=224 &WIDTH=780 &HEIGHT=580

The real world location can easily be calculated with

  • nx = (x / width) * (maxx - minx) + minx
  • ny = ((height - y) / height) * (maxy - miny) + miny (note that Y axis in pixel is downward)

With this, you can easier rewrite the BBOX request above.

-- EricBoisvert - 25 Nov 2009
Topic revision: r4 - 01 Dec 2009, EricBoisvert
 

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