Using TimeManager for WMS-T layers
This is a guest post by Karolina Alexiou (aka carolinux), Anita’s collaborator on the Time Manager plugin.
As of version 2.1.5, TimeManager provides some support for stepping through WMS-T layers, a format about which Anita has written in the past. From the official definition, the OpenGIS® Web Map Service Interface Standard (WMS) provides a simple HTTP interface for requesting geo-registered map images from one or more distributed geospatial databases. A WMS request defines the geographic layer(s) and area of interest to be processed. The response to the request is one or more geo-registered map images (returned as JPEG, PNG, etc) that can be displayed in a browser application. QGIS can display those images as a raster layer. The WMS-T standard allows the user of the service to set a time boundary in addition to a geographical boundary with their HTTP request.
We are going to add the following url as the web map provider service: http://mesonet.agron.iastate.edu/cgi-bin/wms/nexrad/n0r-t.cgi
From QGIS, go to Layer>Add Layer>Add WMS/WMST Layer and add a new server and connect to it. For the service we have chosen, we only need to specify a name and the url.
Select the top level layer, in our case named nexrad_base_reflect and click Add. Now you have added the layer to your QGIS project.
To add it to TimeManager as well, add it as a raster with the settings from the screenshot below. Start time and end time have the values 2005-08-29:03:10:00Z and 2005-08-30:03:10:00Z respectively, which is a period which overlaps with hurricane Katrina. Now, the WMS-T standard uses a handful of different time formats, and at this time, the plugin requires you to know this format and input the start and end values in this format. If there’s interest to sponsor this feature, in the future we may get the format directly from the web service description. The web service description is an XML document (see here for an example) which, among other information, contains a section that defines the format, default time and granularity of the time dimension.
If we set the time step to 2 hours and click play, we will see that TimeManager renders each interval by querying the web map service for it, as you can see in this short video.
Querying the web service and waiting for the response takes some time. So, the plugin requires some patience for looking at this particular layer format in interactive mode. If we export the frames, however, we can get a nice result. This is an animation showing hurricane Katrina progressing over a 30 minute interval.
If you want to sponsor further development of the Time Manager plugin, you can arrange a session with me – Karolina Alexiou – via Codementor.
