7.4.6. GetFeature operation¶
The GetFeature operation lets a client query CityGML features from the 3D City Database. The Simple WFS conformance class only mandates WFS server implementations to support GetFeature queries that use the predefined stored query GetFeatureById as query expression and filter criteria. For this reason, the current version of the 3D City Database WFS handles GetFeatureById queries but no ad-hoc queries. The GetFeature support will be extended in future releases.
A valid GetFeature operation is shown below. The gml:id of the city object that shall be returned by the WFS is passed as parameter to the GetFeatureById stored query.
<?xml version="1.0" encoding="UTF-8"?>
<wfs:GetFeature service="WFS" version="2.0.0"
xmlns:wfs="http://www.opengis.net/wfs/2.0">
<wfs:StoredQuery id="http://www.opengis.net/def/query/OGC-WFS/0/GetFeatureById">
<wfs:Parameter name="id">ID_0815</wfs:Parameter>
</wfs:StoredQuery>
</wfs:GetFeature>
The WFS will answer the above request with either the CityGML city
object(s) whose gml:id value matches ID_0815 or with an exception report
in case no matching city object was found in the 3D City Database.
A single GetFeature operation can also be used to request more than one feature.
<?xml version="1.0" encoding="UTF-8"?>
<wfs:GetFeature service="WFS" version="2.0.0"
xmlns:wfs="http://www.opengis.net/wfs/2.0">
<wfs:StoredQuery id="http://www.opengis.net/def/query/OGC-WFS/0/GetFeatureById">
<wfs:Parameter name="id">first gml:id</wfs:Parameter>
</wfs:StoredQuery>
<wfs:StoredQuery id="http://www.opengis.net/def/query/OGC-WFS/0/GetFeatureById">
<wfs:Parameter name="id">second gml:id</wfs:Parameter>
</wfs:StoredQuery>
</wfs:GetFeature>
If a GetFeature request results in more than one city objects or
consists of more than one stored query, the response will be wrapped by
one or more <wfs:FeatureCollection> elements. Please refer to the WFS
2.0 specification for details on the encoding of the response document.
The GetFeature operation can be influenced by the following XML attributes.
XML attribute
|
O / M
|
Default value
|
Description
|
service
|
M
|
WFS (fixed)
|
The service attribute indicates the
service type. The value “WFS” is fixed.
|
version
|
M
|
2.0.x
|
The version of the WFS Interface
Standard to be used in the
communication.
|
handle
|
O
|
The handle parameter allows a client to
associate a mnemonic name with the
request that will be used in exception
reports.
|
|
outputFormat
|
O
|
application/gml+xml;
version=3.1
|
Controls the encoding of the response.
Per default, the WFS uses CityGML /
GML 3.1.1. The outputFormat attribute
may also take the value
“application/json”, in which case the
response is encoded in CityJSON.
|
count
|
O
|
unlimited
|
The maximum number of features to be
returned by the WFS service.
|
resultType
|
O
|
results
|
If the value of the resultType parameter
is set to “results” the server generates a
response document containing features
that satisfy the operation. If set to “hits”
the server generates an empty
response document indicating the count
of the total number of features that the
operation would return.
|
A KVP-encoded GetFeature request is shown below.
http[s]://[host][:port]/[context_path]/wfs?
SERVICE=WFS&
VERSION=2.0.2&
REQUEST=GetFeature&
STOREDQUERY_ID=http%3A%2F%2Fwww.opengis.net%2Fdef%2Fquery%2FOGC-WFS%2F0%2FGetFeatureById&
ID=ID_0815
Note that the last parameter ID in the above request is not a WFS parameter but instead is required by the invoked stored query.
The supported KVP parameters are listed in the following table.
KVP parameter
|
O / M
|
Default value
|
Description
|
SERVICE
|
M
|
WFS (fixed)
|
see above
|
VERSION
|
M
|
2.0.x
|
see above
|
NAMESPACES
|
O
|
Used to specify namespaces and their
prefixes. The format shall be
xmlns(prefix,escaped_url).
|
|
OUTPUTFORMAT
|
O
|
application/gml+xml;
version=3.1
|
see above
|
COUNT
|
O
|
unlimited
|
see above
|
RESULTTYPE
|
O
|
results
|
see above
|
STOREDQUERY_ID
|
M
|
The identifier of the stored query to
invoke.
|
|
storedquery_parameter
=value
|
O
|
Each parameter of the stored query
shall be encoded in KVP as key-value
pair.
|