WFS, a protocol for vector geospatial data on the Internet

Qgis use of cadaster WFS

WFS stands for Web Feature Service, and is a protocol for retrieving and transmitting vector geospatial data over the Internet.

A WFS service will return data in vector format, such as GML (Geography Markup Language) or JSON (JavaScript Object Notation) format, which can be interpreted and used by GIS software and web applications.

From theory, as written above, to practice, obtaining the desired data, there is a difficult XML documentation file involved. Let’s see how to understand it.

So forget for a while a “simple” webgis (the one in the picture) and be ready to dive into a verbose text file.

screenshot of a web gis application
Screenshot

Let’s use the Basque Country (a region of Spain) cadastral data service as an example.

Here is the file describing the service: https://apli.bizkaia.eus/apps/Danok/INSPIRE/inspireBizkaiaDownloadServiceWFSMetadata.xml

As we already knew, the xml is terribly verbose. Here is the contact section:

<gmd:contact>
<gmd:CI_ResponsibleParty>
<gmd:organisationName>
<gco:CharacterString>
Bizkaiko Foru Aldundia – Diputación Foral de Bizkaia
</gco:CharacterString>
</gmd:organisationName>
<gmd:contactInfo>
<gmd:CI_Contact>
<gmd:address>
<gmd:CI_Address>
<gmd:electronicMailAddress>
<gco:CharacterString>programak@bizkaia.eus</gco:CharacterString>
</gmd:electronicMailAddress>
</gmd:CI_Address>
</gmd:address>
</gmd:CI_Contact>
</gmd:contactInfo>
<gmd:role>
<gmd:CI_RoleCode codeList=”http://standards.iso.org/iso/19139/resources/gmxCodelists.xml#CI_RoleCode” codeListValue=”pointOfContact”>PointOfContact</gmd:CI_RoleCode>
</gmd:role>
</gmd:CI_ResponsibleParty>
</gmd:contact>

Very well structured but it could be summarized as:

Contact:
Point of contact:
Bizkaiko Foru Aldundia – Diputación Foral de Bizkaia: progr…@bi…eus

 

The advice, therefore, is to open, from time to time, the part of the xml structure that interests us and close all the other parts (to maintain a slightly compact view)

xml tree

Coming to the data contained (or rather obtainable), first look for the <gmd:transferOptions> section

It is worth saying that the “geographical capabilities” of a WFS (synthesized in geocapabilities) are the things that a WFS knows how to do, including, for cadastre cases, providing the polygon of a cadastral parcel

The “geographic capabilities” of a WFS service (geocapabilities) include all the functionality that the service is able to provide in relation to geographic data. These capabilities can include various operations such as:

Retrieving geographic data (for example, through GetFeature requests).
Querying geographic data based on parameters such as spatial extent, data attributes, etc.
Filtering geographic data based on specific criteria.
Geographic data transactions, such as adding, modifying, or deleting entities.
Geographic metadata information management.

 

First we find the base URL, in the srv:connectPoint section, then gmd:CI_OnlineResource, then gmd:linkage and finally gmd:URL: here it is, finally the URL on which to build our query:
https://geo.bizkaia.eus/arcgisserverinspire/rest/services/Catastro/Annex1/MapServer/exts/InspireFeatureDownload/service?SERVICE=WFS&REQUEST=GetCapabilities&VERSION=2.0.0.

And then, the other features?

Well, the advice is to open the layer in a viewer, such as QGIS, and obtain the call made from there.

 

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