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Working with OGC Data

QGIS supports WMS and WFS as data sources.

What is OGC Data

The Open Geospatial Consortium (OGC), is an international organization with more than 300 commercial, governmental, nonprofit and research organizations worldwide. Its members develop and implement standards for geospatial content and services, GIS data processing and exchange.

Describing a basic data model for geographic features an increasing number of specifications are developed to serve specific needs for interoperable location and geospatial technology, including GIS. Further information can be found under http://www.opengeospatial.org/.

Important OGC specifications are:

  • WMS — Web Map Service
  • WFS — Web Feature Service
  • WCS — Web Coverage Service
  • CAT — Web Catalog Service
  • SFS — Simple Features for SQL
  • GML — Geography Markup Language

OGC services are increasingly being used to exchange geospatial data between different GIS implementations and data stores. QGIS can now deal with three of the above specifications, being SFS (through support of the PostgreSQL / PostGIS data provider, see Section PostGIS Layers), WFS and WMS as a client.

WMS Client

Overview of WMS Support

QGIS currently can act as a WMS client that understands WMS 1.1, 1.1.1 and 1.3 servers. It has particularly been tested against publicly accessible servers such as DEMIS.

WMS servers act upon requests by the client (e.g. QGIS) for a raster map with a given extent, set of layers, symbolization style, and transparency. The WMS server then consults its local data sources, rasterizes the map, and sends it back to the client in a raster format. For QGIS this would typically be JPEG or PNG.

WMS is generically a REST (Representational State Transfer) service rather than a fully-blown Web Service. As such, you can actually take the URLs generated by QGIS and use them in a web browser to retrieve the same images that QGIS uses internally. This can be useful for troubleshooting, as there are several brands of WMS servers in the market and they all have their own interpretation of the WMS standard.

WMS layers can be added quite simply, as long as you know the URL to access the WMS server, you have a serviceable connection to that server, and the server understands HTTP as the data transport mechanism.

Selecting WMS Servers

The first time you use the WMS feature, there are no servers defined.

Begin by clicking the mActionAddWmsLayer Add WMS layer button inside the toolbar, or through the Layer ‣ Add WMS Layer... menu.

The dialog Add Layer(s) from a Server for adding layers from the WMS server appears. You can add some servers to play with by clicking the [Add default servers] button. This will add two WMS demo servers for you to use, the WMS servers of the DM Solutions Group and Lizardtech. To define a new WMS server in the tab Layers, select the [New] button. Then enter the parameters to connect to your desired WMS server, as listed in table_OGC_1:

Name A name for this connection. This name will be used in the Server Connections drop-down box so that you can distinguish it from other WMS Servers.
URL URL of the server providing the data. This must be a resolvable host name; the same format as you would use to open a telnet connection or ping a host.
Username Username to access a secured WMS-server. This parameter is optional.
Password Password for a basic authenticated WMS-server. This parameter is optional.
Ignore GetMap URI checkbox Ignore GetMap URI reported in capabilities, use given URI from URL-field above.
Ignore GetFeatureInfo URI checkbox Ignore GetFeatureInfo URI reported in capabilities , use given URI from URL-field above

Table OGC 1: WMS Connection Parameters

If you need to set up a proxy-server to be able to receive WMS-services from the internet, you can add your proxy-server in the options. Choose menu Settings ‣ Options and click on the tab Network & Proxy. There you can add your proxy-settings and enable them by setting the checkbox Use proxy for web access. Make sure that you select the correct proxy type from the Proxy type selectstring dropdown menu.

Once the new WMS Server connection has been created, it will be preserved for future QGIS sessions.

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On WMS Server URLs

Be sure, when entering in the WMS server URL, that you have the base URL. For example, you shouldn’t have fragments such as request=GetCapabilities or version=1.0.0 in your URL.

Loading WMS Layers

Once you have successfully filled in your parameters you can select the [Connect] button to retrieve the capabilities of the selected server. This includes the Image encoding, Layers, Layer Styles and Projections. Since this is a network operation, the speed of the response depends on the quality of your network connection to the WMS server. While downloading data from the WMS server, the download progress is visualized in the left bottom of the WMS dialog.

Your screen should now look a bit like figure_OGR_1, which shows the response provided by the DM Solutions Group WMS server.

Figure OGR 1:

../../../_images/connection_wms.png

Dialog for adding a WMS server, showing its available layers nix

Image Encoding

The Image encoding section now lists the formats that are supported by both the client and server. Choose one depending on your image accuracy requirements.

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Image Encoding

You will typically find that a WMS server offers you the choice of JPEG or PNG image encoding. JPEG is a lossy compression format, whereas PNG faithfully reproduces the raw raster data.

Use JPEG if you expect the WMS data to be photographic in nature and/or you don’t mind some loss in picture quality. This trade-off typically reduces by 5 times the data transfer requirement compared to PNG.

Use PNG if you want precise representations of the original data, and you don’t mind the increased data transfer requirements.

Options

The Options field provides a text field where you can add a Layer name for the WMS-layer. This name will be presented in the legend after loading the layer.

Below the layer name you can define Tile size, if you want to set tile sizes (eg. 256x256) to split up the WMS request into multiple requests.

The Feature limit for GetFeatureInfo defines what features from the server to query.

If you select a WMS from the list a field with the default projection, provided by the mapserver, appears. If the [Change...] button is active, you can click on it and change the default projection of the WMS to another CRS, provided by the WMS server.

Layer Order

The tab Layer Order lists the selected layers available from the current connected WMS server. You may notice that some layers are expandable, this means that the layer can be displayed in a choice of image styles.

You can select several layers at once, but only one image style per layer. When several layers are selected, they will be combined at the WMS Server and transmitted to QGIS in one go.

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WMS Layer Ordering

WMS layers rendered by a server are overlaid in the order listed in the Layers section, from top to bottom of the list. If you want to change the overlay order, you can use the tab Layer Order.

Transparency

In this version of QGIS, the Global transparency setting from the Layer Properties is hard-coded to be always on, where available.

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WMS Layer Transparency

The availability of WMS image transparency depends on the image encoding used: PNG and GIF support transparency, whilst JPEG leaves it unsupported.

Coordinate Reference System

A Coordinate Reference System (CRS) is the OGC terminology for a QGIS Projection.

Each WMS Layer can be presented in multiple CRSs, depending on the capability of the WMS server.

To choose a CRS, select [Change...] and a dialog similar to Figure Projection 3 in Working with Projections will appear. The main difference with the WMS version of the screen is that only those CRSs supported by the WMS Server will be shown.

Tilesets

When using WMS-C (Cached WMS) Services like http://labs.metacarta.com/wms-c/Basic.py you are able to browse through the tab Tilesets given by the server. Additional information like tile size, formats and supported CRS are listed in this table. In combination with this feature you can use the tile scale slider from the View ‣ Tile scale slider, which gives you the available scales from the tileserver with nice slider docked in.

Using the Identify Tool

Once you have added a WMS server, and if any layer from a WMS server is queryable, you can then use the mActionIdentify Identify tool to select a pixel on the map canvas. A query is made to the WMS server for each selection made. The results of the query are returned in plain text. The formatting of this text is dependent on the particular WMS server used.

Viewing Properties

Once you have added a WMS server, you can view its properties by right-clicking on it in the legend, and selecting Properties.

Metadata Tab

The tab Metadata displays a wealth of information about the WMS server, generally collected from the Capabilities statement returned from that server. Many definitions can be cleaned by reading the WMS standards (see OPEN-GEOSPATIAL-CONSORTIUM Literature and Web References), but here are a few handy definitions:

  • Server Properties
    • WMS Version — The WMS version supported by the server.
    • Image Formats — The list of MIME-types the server can respond with when drawing the map. QGIS supports whatever formats the underlying Qt libraries were built with, which is typically at least image/png and image/jpeg.
    • Identity Formats — The list of MIME-types the server can respond with when you use the Identify tool. Currently QGIS supports the text-plain type.
  • Layer Properties
    • Selected — Whether or not this layer was selected when its server was added to this project.
    • Visible — Whether or not this layer is selected as visible in the legend. (Not yet used in this version of QGIS.)
    • Can Identify — Whether or not this layer will return any results when the Identify tool is used on it.
    • Can be Transparent — Whether or not this layer can be rendered with transparency. This version of QGIS will always use transparency if this is Yes and the image encoding supports transparency
    • Can Zoom In — Whether or not this layer can be zoomed in by the server. This version of QGIS assumes all WMS layers have this set to Yes. Deficient layers may be rendered strangely.
    • Cascade Count — WMS servers can act as a proxy to other WMS servers to get the raster data for a layer. This entry shows how many times the request for this layer is forwarded to peer WMS servers for a result.
    • Fixed Width, Fixed Height — Whether or not this layer has fixed source pixel dimensions. This version of QGIS assumes all WMS layers have this set to nothing. Deficient layers may be rendered strangely.
    • WGS 84 Bounding Box — The bounding box of the layer, in WGS 84 coordinates. Some WMS servers do not set this correctly (e.g. UTM coordinates are used instead). If this is the case, then the initial view of this layer may be rendered with a very ‘zoomed-out’ appearance by QGIS. The WMS webmaster should be informed of this error, which they may know as the WMS XML elements LatLonBoundingBox, EX_GeographicBoundingBox or the CRS:84 BoundingBox.
    • Available in CRS — The projections that this layer can be rendered in by the WMS server. These are listed in the WMS-native format.
    • Available in style — The image styles that this layer can be rendered in by the WMS server.

WMS Client Limitations

Not all possible WMS Client functionality had been included in this version of QGIS. Some of the more notable exceptions follow.

Editing WMS Layer Settings

Once you’ve completed the mActionAddWmsLayer Add WMS layer procedure, there is no ability to change the settings. A workaround is to delete the layer completely and start again.

WMS Servers Requiring Authentication

Currently public accessible and secured WMS-services are supported. The secured WMS-servers can be accessed by public authentication. You can add the (optional) credentials when you add a WMS-server. See section Selecting WMS Servers for details.

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Accessing secured OGC-layers

If you need to access secured layers with other secured methods than basic authentication, you could use InteProxy as a transparent proxy, which does support several authentication methods. More information can be found at the InteProxy manual found on the website http://inteproxy.wald.intevation.org.

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QGIS WMS Mapserver

From Version 1.7.0 QGIS has its own implementation of a WMS 1.3.0 Mapserver. Read more about this at chapter QGIS Server.

WFS and WFS-T Client

In QGIS, a WFS layer behaves pretty much like any other vector layer. You can identify and select features and view the attribute table. Since QGIS 1.6 editing (WFS-T) is also supported.

In general adding a WFS layer is very similar to the procedure used with WMS. The difference is there are no default servers defined, so we have to add our own.

Loading a WFS Layer

As an example we use the DM Solutions WFS server and display a layer. The URL is: http://www2.dmsolutions.ca/cgi-bin/mswfs_gmap

  1. Click on the wfs Add WFS Layer tool on the Layers toolbar, the dialog Add WFS Layer from a Server appears
  2. Click on [New]
  3. Enter ‘DM Solutions’ as name
  4. Enter the URL (see above)
  5. Click [OK]
  6. Choose ‘DM Solutions’ from the Server Connections selectstring combobox
  7. Click [Connect]
  8. Wait for the list of layers to be populated
  9. Select the Parks layer in the list
  10. Click [Apply] to add the layer to the map

Note that proxy-settings you have set in your preferences are also recognized.

Figure OGR 3:

../../../_images/connection_wfs.png

Adding a WFS layer nix

You’ll notice the download progress is visualized in the left bottom of the QGIS main window. Once the layer is loaded, you can identify and select a province or two and view the attribute table.

Only WFS 1.0.0 is supported. At this point there have not been many tests against WFS versions implemented in other WFS-servers. If you encounter problems with any other WFS-server, please do not hesitate to contact the development team. Please refer to Section Mailing lists for further information about the mailinglists.

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Finding WFS Servers

You can find additional WFS servers by using Google or your favorite search engine. There are a number of lists with public URLs, some of them maintained and some not.