Asking for an Animation

An animation is a sequence of images, with each image usually depicting a different time and with the time "step" between images being uniform for the whole sequence. If a named layer in the Capabilities XML for the SVS Image Server contains a <Dimension> parameter and the parameter has multiple allowed values, then the layer represents an animation. To request an animation from the African Fires layer, note that the following lines were contained in the Capabilities XML for the African Fires layer:

  <Dimension name="time" units="ISO8601" default="2002-12-31">
    2002-01-01/2002-12-31/P1D
  </Dimension>

This line indicates that every date in 2002 is available at one day intervals (P1D), so you can ask for an image for a specific date by using the TIME parameter along with any 2002 date in the WMS request, e.g.,

http://svs.gsfc.nasa.gov/cgi-bin/wms?SERVICE=WMS
&VERSION=1.3.0&REQUEST=GetMap&LAYERS=2890_17402&FORMAT=image/png
&WIDTH=1024&HEIGHT=1024&CRS=CRS:84&BBOX=-22,-39,58,41
&STYLES=composite&TIME=2002-01-01

Since no time was indicated in the original request, the image for the default time (2002-12-31) was returned. The WMS documentation contains an explanation of the date and date range formats.

The obvious way, therefore, to get an animation is to ask for each image in the sequence by issuing a separate WMS request for each time in the animation. The client can then display each image as it is received or save the images for later playback.

It is not necessary to request an image for every time in the <Dimension> list. However, most animations on the SVS Image Server are designed to be played using every time in the list, and all of the descriptions and metadata assume that all the images are shown and shown in temporal order. Note that some animations have uneven timesteps between images, usually because the data for the animation is not available at uniform time intervals.

At this time, there is no way to request multiple images in a single file (such as MPEG or animated GIF) or with a single WMS request. In fact, the Capabilities XML explicitly prohibits asking for multiple times in a single WMS request unless the layer has the definition multipleValues=1 in the <Dimension> parameter. This capability should come in a future revision of the server.

Note: In WMS version 1.1.1, the <Dimension> parameter functionality is contained in two parameters, <Dimension> and <Extent>. If using version 1.1.1, a layer with multiple values in the Extent parameter is an animation.

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