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:
+ the format of the image
+ the width and height of the image in pixels
+ the geographic region of the earth covered by the image
+ the style of the image
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.
Go backward to "Accessing the Table of Contents"
Go forward to "Asking for an Animation"
|