Practical experiences with OGC services

By Michal Sredl, Karel Charvat, Jachym Cepicky, Czech centre for Science and Society

INSPIRE Conference, Istanbul, June 23. 2012

CCSS role

• To build URM enviroGRIDS portal• To support partners with deployment of services

CCSS role

System overview

Web Client

HSLayers

(JavaScript)

- OpenLayers

- Ext

Server proxy

Proxy4ows(Python)

- Mapserver / MapScript- Gdal / Ogr- OwsLib

OWS Servers

MapServer GeoServer InterGraph ArcGis

WMS++

WMSWMS-TWFS, FES WCS...

Technology behind the CCSS Geoportal

Mission

Challanges & Results

Display huge WFS data in web client → Proxy4ows Filter Encoding (FES) → OpenLayers, OwsLib, MapServer patches WFS Axis order issue → MapServer, OwsLib patches

→ some issues still remain

Proxy4ows

Transforms WFS and WCS to WMS

For WMS, it transforms the coordinate systems

http://proxy4ows.org/

Is presented in a standalone presentation – Tuesday 14:20

Filter Encoding

OpenLayers – <Filter_Capabilities> not parsed

Part of a WFS 1.1.0 GetCapabilities response

Filter Encoding

OpenLayers – <Filter_Capabilities> not parsed

Desired result

Patch proposed- Comparison filters & related stuff- WFS 1.0.0 and 1.1.0

• "wfs_filter" '<ogc:PropertyIsEqualTo xmlns:ogc="http://www.opengis.net/ogc"> <ogc:PropertyName>STUPEN</ogc:PropertyName> <ogc:Literal>3</ogc:Literal> </ogc:PropertyIsEqualTo>'

MapServer – Filter produced as invalid XML

Filter Encoding

FILTER=<ogc:Filter> <ogc:PropertyIsEqualTo+xmlns:ogc="http://www.opengis.net/ogc"> <ogc:PropertyName>STUPEN</ogc:PropertyName> <ogc:Literal>3</ogc:Literal> </ogc:PropertyIsEqualTo></ogc:Filter>

Mapfile

Mapserver code

Outgoing filter

• "wfs_filter" '<ogc:Filter xmlns:ogc="http://www.opengis.net/ogc"> – <ogc:PropertyIsEqualTo>– <ogc:PropertyName>STUPEN</ogc:PropertyName> <ogc:Literal>3</ogc:Literal>

</ogc:PropertyIsEqualTo> </ogc:Filter>'

MapServer – Filter produced as invalid XML

Filter Encoding

FILTER=<ogc:Filter+xmlns:ogc="http://www.opengis.net/ogc"> <ogc:PropertyIsEqualTo> <ogc:PropertyName>STUPEN</ogc:PropertyName> <ogc:Literal>3</ogc:Literal> </ogc:PropertyIsEqualTo></ogc:Filter>

Mapfile

One-char patch

Outgoing filter

WFS Axis Order

WFS 1.0.0 Axis order clearly defined in standard (East, North) <wfs:LatLongBoundingBox> causes confusion sometimes

WFS 1.1.0 As widely accepted, axis order should follow the definition of the particular CRS However, not a strict requirement in the standard Implementations of standards differ, heaps of confusion Axis order depends not only on the CRS, but also on the notation used

WFS Axis Order

Axis order depends on the SRS used

Check http://www.epsg-registry.org/ for definitions

BBOX url parameter Sometimes you need to negotiate with the service providers to

follow the definition, as otherwise no data may be returned

Data themselves MapServer did not understand, that the axis order depends on the

SRS used, data was rotated by 90° MapServer patch provided

WFS 1.1.0

WFS Axis Order

Axis order depends on the notation of the SRS as well:

EPSG:EPSG_code

→ WFS 1.0.0 style – fixed East, North

urn:EPSG:geographicCRC:epsg_code

→ Follow-up the definition

http://www.opengis.net/gml/srs/epsg.xml#EPSG_code

→ We don't know

WFS 1.1.0

WFS Axis Order

More details here:

http://les-ejk.cz/2012/03/mapserver-as-wfs-1-1-0-client-not-suitable/

MapServer as WFS 1.1.0 Client

Patches available

OpenLayers – <Filter_Capabilities> not parsed

http://trac.osgeo.org/openlayers/ticket/3652(Patch proposed)

MapServer – Filter produced as invalid XML

http://trac.osgeo.org/mapserver/ticket/4171 (Expected in 6.2)

MapServer – WFS Axis order

http://trac.osgeo.org/mapserver/ticket/4228(Expected in 6.2)

OwsLib patches already deployed

FES - Filter Encoding SpecificationFiltering WFS Layers in enviroGRIDS Geoportal

Filter Encoding Standard enables you to create XML filter expressions and is incorporated in the WFS standard.

In Geoportal the FES is used with the help of Proxy4ows to filter the WFS layers.

How to use it

- Add a WFS Layer

- In the layer switcher, right-click on the layer name and select “Filter”

How to use it

How to use it

- Filter window appears with comparison filter

- Select the property name, comparison operator and type in the desired value

- Apply the filter

How to use it

How does it work

How does it work

WFS layers are displayed as WMS in the HSLayers Web Client (see Proxy4ows for details).

1. WFS GetCapabilities request is sent directly to the WFS server. Capabilities document is parsed and filter capabilities are saved.

2. WFS DescribeFeatureTypes request is sent directly to the WFS server. Reply is parsed and properties of the feature type are saved.

3. User creates the filter in the GUI, saved information from steps 1-2. is used.

How does it work

4. User-created filter is written to FES and new WMS request is sent to Proxy4ows, accompanied with the filter in an additional parameter.

5. Proxy4ows takes the filter, creates new WFS request involving the filter, and sends it to the WFS server. Server replies with filtered layer.

6. Proxy4ows transforms the returned WFS layer to WMS and sends it to the client.

Next Steps

In terms of standards, FES 1.0.0 and FES 1.1.0 is supported. In the future, FES 2.0.0 can be added.

In terms of filter types, only the Comparison filter is currently supported. In the future, more filters types can be added. That means the logical filter enabling to combine various expressions, spatial filter, expression editor, filtering based on feature ids, support for server-side functions.