Design Considerations - Fixed Size Images
Having stepped through one specific example for using the SVS Image Server,
it is worthwhile to discuss some of the design considerations for the server and its
use of the Web Map Service interface.
WMS was originally created to provide a standard method for GIS-style applications
to access maps from remote data servers. For example, a user running a GIS client
on a PC needed a protocol that could perform the following kind of query to a
remote data server:
"I have a 500x500 pixel window looking at a Mercator projection of Africa. Send me
an image of elevation that will fit exactly into that window."
WMS satisfies this requirement very well, but was originally rooted in the idea that
the client needed the server to resize and reproject images based on the client's
current view of some geographic region. The SVS Image Server has somewhat different
design considerations, i.e.,
+ the primary requested
items are animations illustrating Earth Science phenomena, and
+ the client will reproject
the requested images as needed, primarily by using the images as texture maps within a 3D application.
Since the content on the SVS Image Server is not created on the fly,
these considerations imply that the primary use of the WMS capability will be
to access tens or hundreds of pre-existing images at a time. If the server allowed
resizing or reprojection of all these images, the performance of the server could be
severely affected.
In addition, the animations themselves have been carefully designed to illustrate
some phenomenon occuring in some particular region of the Earth. If the user can
request lower resolution images than in the original animation or images reprojected
to a different geographic region, the point of the animation could be compromised or
distorted. For example, small fires would disappear from the African Fire images at a
lower resolution and requesting the animation over South America would make no sense.
Higher-than-original resolution image requests would be inefficient, since the server
could only scale existing images up with no real gain in information.
These considerations have led to the restriction that layers on the SVS Image
Server will not, as a rule, allow resizing or reprojection. Clients must pay strict attention
to the width and height in the <Layer> parameter and the geographic region
specification in the layer's <BoundingBox> parameter. A GetMap query to
the server must use these values exactly in order to get the layer's images. If
different sizes of a specific animation are available, they will probably be available
as separate named layers. The image size currently appears in the name of the layer
so the user can decide which size to request based on network bandwith, client graphics
capability, or other considerations.
Go backward to "Asking for an Image Sequence"
Go forward to "Design Considerations - Projections"
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