Asking for an Image

If the user selects the layer with the title "African Fires during 2002 (1024x1024 Animation)", the client constructs a WMS query using the name of the layer, i.e., "2890_17402". This query begins with

http://svs.gsfc.nasa.gov/cgi-bin/wms?SERVICE=WMS
&VERSION=1.3.0&REQUEST=GetMap&LAYERS=2890_17402

In order to complete this request, the query must, at a minimum, include the following additional information about the image being requested:

Without this information in the query, the server will return an error message, or "exception". (Note that the Capabilities XML included

<Exception><Format>XML</Format></Exception>

which indicates that the returned exception will be in XML format.)

At the beginning of the Capabilities XML, the image formats supported by the SVS Image Server were defined in the <GetMap> element by

<GetMap><Format>image/png</Format>...</GetMap>

Since PNG (Portable Network Graphics) is a supported format, the query can request a PNG image, i.e.,

http://svs.gsfc.nasa.gov/cgi-bin/wms?SERVICE=WMS
&VERSION=1.3.0&REQUEST=GetMap&LAYERS=2890_17402&FORMAT=image/png

The first line in the Capabilities XML for the African Fires layer was

<Layer opaque="0" noSubsets="1" fixedWidth="1024" fixedHeight="1024">

The "fixedWidth" and "fixedHeight" parameters indicate that the images from this layer are only available in one size, 1024x1024 pixels. Using these values, the query now becomes

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

The "noSubsets" parameter in that line indicates that the only geographic region that can be requested is the full bounding box of the layer, which is given in the line

<BoundingBox CRS="CRS:84" minx="-22" miny="-39" maxx="58" maxy="41"/>

To use this bounding box in the query, separate parameters must be added for both the coordinate reference system (CRS) and the bounding box (BBOX). Now the query takes this form

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

Finally, a parameter must be added to define the style of the image being returned. The simplest way to request a style is to leave the STYLES parameter blank, which results in a request for the default style for the layer, 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=

Accessing this link should return the correct image from the server.

A better solution is to use an available style parameter from the layer description in the Capabilities XML. In this case, the following two lines define the available styles

<Style><Name>composite</Name>...</Style>
<Style><Name>overlay</Name>...</Style>

A request for the first of these styles would look like this:

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

Note that this image is not the same as the previous one. This one added a background image of Africa, where the first one had a transparent background so it could be laid over any image of Africa. Since the WMS standard does not define which of the available styles is the default, it is better to request a specific style than use the default. A more complete description of styles and their use of this server will be given later in this document.

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