WFS Client

Author:Jean-François Doyon Contact:jdoyon at nrcan.gc.ca Author:Jeff McKenna Contact:jmckenna at gatewaygeomatics.com Revision:$Revision: 8371 $ Date:$Date: 2008-12-31 12:44:04 -0800 (Wed, 31 Dec 2008) $ Last Updated:2006/05/02

Contents

• WFS Client
  • Introduction
  • Setting up a WFS-client Mapfile
  • TODO / Known Limitations

Introduction

MapServer can retrieve and display data from a WFS server. The following document explains how to display data from a WFS server using MapServer.

A WFS ( Web Feature Service ) publishes feature-level geospatial data to the web. This means that it is possible to use this data as a data source to render a map. In effect, this is not unlike having a shapefile accessible over the web, only it’s not a shapefile, it’s XML-Encoded geospatial data (GML to be exact), including both geometry AND attribute information.

WFS-Related Information

Although in-depth understanding of WFS and GML is neither necessary nor required in order to implement a MapServer application that reads remote WFS data, it is recommended to at least get aquainted with the concepts and basic functionality of both. Here are the official references (including a newly added OGC workshop with MapServer):

• OGC Web Feature Service Implementation Specification.
• Geography Markup Language Implementation Specification.
• MapServer OGC Web Services Workshop package.

Software Requirements

In order to enable MapServer to serve WFS, it MUST be compiled against certain libraries:

• PROJ.4: The reprojection library. Version 4.4.3 or greater is required.
• GDAL/OGR: I/O support librairies. Version 1.1.8 or greater is required.
• LibCURL: Used to help MapServer act as an HTTP client. Version 7.10 or greater is required.

Please see the MapServer UNIX Compilation and Installation HOWTO for detailed instructions on compiling mapserver with support for these librairies and features. For Windows users, look on the MapServer website to see if there are any binaries available that meet these requirements.

Setting up a WFS-client Mapfile

Storing Temporary Files

You must set the IMAGEPATH parameter in your mapfile since MapServer uses this directory to store temporary files downloaded from the remote WFS server.

MAP
  ...
  WEB
    IMAGEPATH "/tmp/ms_tmp/"
    IMAGEURL ...
  END
  ...
END

WFS Layer

A WFS layer is a regular mapfile layer, which can use CLASS objects, with expressions, etc.

As of MapServer 4.4, the suggested method to define a WFS Client layer is through the CONNECTION parameter and the layer’s METADATA. The necessary mapfile parameters are defined below:

• CONNECTIONTYPE: must be “wfs”

• CONNECTION: The URL to the WFS Server. e.g. http://www2.dmsolutions.ca/cgi-bin/mswfs_gmap? The path to the mapfile on the WFS server is required if it was required in the GetCapabilities request e.g. you would have to specify the MAP parameter in the CONNECTION for the following server: http://map.ns.ec.gc.ca/MapServer/mapserv.exe?MAP=/mapserver/services/envdat/config.map &SERVICE=WFS&VERSION=1.0.0&REQUEST=GetCapabilities

• METADATA: The LAYER’s must contain a METADATA object with the following parameters:

  • wfs_connectiontimeout (optional): The maximum time to wait for a remote WFS layer to load, set in seconds (default is 30 seconds). This metadata can be added at the layer level so that it affects only that layer, or it can be added at the map level (in the web object) so that it affects all of the layers. Note that wfs_connectiontimeout at the layer level has priority over the map level.

  • wfs_filter: This can be included to include a filter encoding parameter in the getFeature request (see the Filter Encoding Howto for more information on filtering). The content of the wfs_filter is a valid filter encoding element.

    ...
    METADATA
      "wfs_filter"   "<PropertyIsGreaterThan><PropertyName>POP_RANGE</PropertyName><Literal>4
                      </Literal></PropertyIsGreaterThan>"
    END
    ...
    

  • wfs_latlongboundingbox (optional): The bounding box of this layer in geographic coordinates in the format “lon_min lat_min lon_max lat_max”. If it is set then MapServer will request the layer only when the map view overlaps that bounding box. You normally get this from the server’s capabilities output.

  • wfs_maxfeatures (optional): Limit the number of GML features to return.

  • wfs_request_method (optional): Can be set to “GET” to do a Get request to WFS servers that do not support Post requests. The default method in MapServer is Post.

    ...
    METADATA
      "wfs_filter"   "GET"
    END
    ...
    

  • wfs_typename (required): the <Name> of the layer found in the GetCapabilities. An example GetCapabilities request is: http://www2.dmsolutions.ca/cgi-bin/mswfs_gmap?SERVICE=WFS&VERSION=1.0.0&REQUEST=getcapabilities

  • wfs_version (required): WFS version, currently “1.0.0”

Note

Each of the above metadata can also be referred to as ‘ows_*’ instead of ‘wfs_*’. MapServer tries the ‘wfs_*’ metadata first, and if not found it tries the corresponding ‘ows_*’ name. Using this reduces the amount of duplication in mapfiles that support multiple OGC interfaces since “ows_*” metadata can be used almost everywhere for common metadata items shared by multiple OGC interfaces.

Example WFS Layer

LAYER
  NAME "park"
  TYPE POLYGON
  STATUS ON
  CONNECTIONTYPE WFS
  CONNECTION "http://www2.dmsolutions.ca/cgi-bin/mswfs_gmap?"
  METADATA
    "wfs_typename"          "park"
    "wfs_version"           "1.0.0"
    "wfs_request_method"    "GET"
    "wfs_connectiontimeout" "60"
    "wfs_maxfeatures"       "1"
  END
  PROJECTION
    "init=epsg:42304"
  END
  CLASS
    NAME "Parks"
    STYLE
      COLOR 200 255 0
      OUTLINECOLOR 120 120 120
    END
  END
END # Layer

Connection - deprecated

As of MapServer v4.4 the method of specifying all of the connection information in the CONNECTION parameter has beendeprecated. The preferred method is mentioned above. If the metadata is not provided, VERSION, SERVICE, and TYPENAME will be fetched from the CONNECTION, as shown below

CONNECTION    "http://www2.dmsolutions.ca/cgi-bin/mswfs_gmap?SERVICE=WFS&VERSION=1.0.0&TYPENAME=park"

TODO / Known Limitations

1. Temporary WFS (gml) files are written to the IMAGEPATH directory, but this could become a security concern since it makes the raw GML data downloadable by someone who could guess the gml filename. We should consider having a “wfs_cache_dir” metadata that, if it is set will define a directory where temp files should be written. The default would still be to use the value of IMAGEPATH if “wfs_tmpdir” is not set.
2. Xerces is an annoying dependency for GML support in OGR. There have been discussions about modifying the OGR GML driver to use in internal XML parser by default. This would save us from the Xerces dependency. There is no formal plan for when this would happen yet.