MapServer, OpenLayers and the WFS Maze¶

date:: 2011-06-10 18:07
author:: admin
category:: mapserver, OpenLayers, web development
tags:: projections, srs, wfs
slug:: mapserver-openlayers-and-the-wfs-maze
status:: published

First a little background on what I was trying to achieve. I am developing a GIS that has both a web and desktop component. It’s built using SQL Server 2008, MapServer, OpenLayers, and using MapInfo and QGIS for desktop connections.

On the web system I have an OpenLayers map, with an OpenStreetMap background layer. As with nearly all the online tiling services, these are projected in EPSG:900913 (the Web Mercator / Google projection).

I wanted to display a road network on top of these as a WFS (Web Feature Service). The source data is defined in the Irish National Grid projection (EPSG:29902).

At the same time I also wanted people to be able to connect to the WFS via a desktop GIS client using the Irish National Grid projection.

Multiple Spatial Reference Systems (SRSs)¶

The Rules for Handling SRS in MapServer WFS are not the easiest to work with. The first paragraph of the rules imply that multiple projections are not available for a single WFS layer available through MapServer:

Contrary to WMS, the OGC WFS specification doesn’t allow a layer (feature type) to be advertised in more than one SRS. Also, there is no default SRS that applies to all layers by default in the OGC WFS spec. However, it is possible to have every layer in a WFS server advertised in a different SRS.

However there is now support for multiple SRSs - added in version 6.0 of MapServer as outlined in this ticket. You can check how a WFS layer is “advertised” to a client using a URL similar to the following:

http://geographika.azurewebsites.net/mapserver/mapserv.exe?map=C:/PathToMapFile/MyMap.map&service=WFS&request=GetCapabilities&version=1.1.0

Each of the layers should have some XML which lists the DefaultSRS, and if you have set it up one or more OtherSRS tags:

<FeatureType>
    <Name>Test</Name>
    <Title>Test</Title>
    <DefaultSRS>urn:ogc:def:crs:EPSG::900913</DefaultSRS>
    <OtherSRS>urn:ogc:def:crs:EPSG::29902</OtherSRS>

To get to this point however is not the simplest of journeys..

How to Set Up Multiple SRSs in MapServer¶

The SRS rules explain how MapServer advertises the available projections for each of your WFS layers:

If a top-level map SRS is defined* then this SRS is used and applies to all layers (feature types) in this WFS. In this case the SRS of individual layers is simply ignored even if it’s set.

If there is no top-level map SRS defined* then each layer is advertised in its own SRS in the capabilities.

By “SRS is defined”, we mean either the presence of a PROJECTION object defined using an EPSG code, or of a “wfs_srs” metadata at this level.

I recommend you read the further explanations in the official documentation, as there are a lot of other rules on how priorities and conflicts between different configuration settings work. From the documentation:

Confusing? As a rule of thumb, simply set the wfs_srs at the map level (in web metadata) and never set the wfs_srs metadata at the layer level and things will work fine for most cases.

However this only applies if you don’t want to use multiple projections. This cut-down example of a map file may help to illustrate what these rules actually look like in code:

A few additional notes from spending a number of hours struggling with these:

The DefaultSRS is always the first item in the wfs_srs list. **The order these are listed is therefore critical. **
If no metadata is available then the SRS is taken from the Map Projection. In this case there are no OtherSRS values.
I’d recommend not sharing the SRS definitions between WFS and WMS (you can do this with the ows_srs metadata setting) as the order makes no real difference to a WMS service, but is critical for a WFS service.
You need to set the PROJECTION object for each WFS layer with multiple projections. Even if the layer is in the same projection as the MAP projection. This was probably the hardest issue to track down. If you don’t set it you end up with the nasty situation of the client BBOX in one projection searching for features in the other projection. It would be nice if MapServer just threw an error at this point. Instead you’ll likely get a message such as:

<gml:boundedBy>
<gml:Null>missing</gml:Null>
</gml:boundedBy>

The WFS Client - OpenLayers¶

Once you’ve got you DefaultSRS and OtherSRSs set up correctly in MapServer and checked them with the GetCapabilities request, you can to try to connect to them with a real WFS client. The MapInfo desktop WFS involved entering the server name, and selecting layers from a list. The OpenLayers connection was more difficult.

To add a WFS to OpenLayers, you create a vector layer with the WFS protocol. Make sure the WFS version is set to 1.1.0. Why? If you use WFS 1.0.0 the srsName is not supported, so you can’t use OpenLayers to decide in which projection you want your features returned.

protocol: new OpenLayers.Protocol.WFS({
             featureType: “Test”,
             featurePrefix: "ms",
             srsName: "EPSG:900913",
             version: "1.1.0",
             url: url
          })

If you set the srsName in the WFS protocol your WFS requests will include now include the srsName (it can also pick this up from the map, but I find it easier to be explicit about projections). If you check your requests in FireBug or Fiddler you can see the XML that is posted to MapServer when you switch on a vector layer:

<wfs:Query typeName="Test" srsName="EPSG:900913">

A full OpenLayers request looks similar to this:

However there is one more piece of the puzzle left - as adding the srsName value to the WFS protocol has very little effect! Features will be correctly selected, but they will be returned in the DefaultSRS, in this example EPSG:29902, so they will never appear in OpenLayers. Whether this is an issue with MapServer or the WFS specification I’m not sure.

What you need to do is to pass the srsName to the URL of your WFS server. E.g.

http://geographika.azurewebsites.net/mapserver/?map=C:/PathToMapFile/mymap.map&SERVICE=WFS&VERSION=1.1.0&srsName=EPSG:900913

The POST request remains the same, but setting the SRS in the URL forces MapServer to reproject the features. The WFS protocol should therefore look something like this:

protocol: new OpenLayers.Protocol.WFS({
             featureType: “Test”,
             featurePrefix: "ms",
             srsName: "EPSG:900913",
             version: "1.1.0",
             url: url + "&srsName=EPSG:900913",
          })

If you enter a SRS which is not listed for your WFS layer you’ll get the following message:

msWFSGetFeature(): WFS server error. Invalid GetFeature Request:Invalid SRS.
Please check the capabilities and reformulate your request.

If all has gone well you’ll have a MAP file containing WFS layers that can be requested by clients in a variety of projections - very useful when some people are connecting through a web system based on Web Mercator, and others through a desktop using other datasets projected to national grids.