Web Map Service
Rationale: The core capability of the SVS Image
Server is the ability for an application running on a user's PC to discover what
information the Image Server has available and then to request that specific
information be delivered to the application. The Web Map Service (WMS)
specification is a protocol for implementing such a capability for
georeferenced images. Using the WMS protocol, the user's application first requests
a list of maps (or layers) the server has available, then follows with a
request for a specific image or sequence of images. For the SVS Image Server,
each layer will be an animation of a specific physical event or process.
Issues: The type of content that the SVS Image
Server will provide is within the scope of WMS, but is not exactly mainstream
for the protocol. Certain issues will need to be worked out, possibly
resulting in future modifications to the standard. Active discussion on these
issues is proceeding with the editor of the WMS specification.
first issue is support for sequences of maps. In order to present our
animations as a coherent entity, it is desirable that a user not be required to
request every image in an animation with a separate WMS request. The WMS
specification permits a single WMS request for a sequence of values for a
parameter, e.g., time. However, if the requested format is an image format,
then such a request may imply that all values should be returned in the same image.
If our desire is to return an individual image for each value, particularly in
a format that does not support multiple images or animations (such as PNG),
then several options might be possible. The request may return 1) a metafile
with pointers to the individual images being requested (no such metafile
standard currently exists), 2) a sequence of images as a multi-part mime
document, or 3) a file that contains all of the images, such as a zip archive.
Each of these options is somewhat nonstandard, either in WMS or mime format,
and a nonstandard response is unlikely to be understood by WMS clients.
second issue unresolved by WMS is related to the first issue. Some animation
sequences available to be served by the SVS Image Server have geographic
boundaries that change during the sequence. One example of this is a sequence
of high-resolution images of Hurricane Isabel centered on the moving eye of the
hurricane. There is currently no standard way to issue a single WMS request
for this sequence without expanding the bounding box of each image to a uniform
bounding box that encompasses all the images, with the resulting overhead due
to increased image size.
Status: Adopted by the SVS Image Server