SVS Image Server

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"

Table of Contents

     SVS Image Server Home Page

     Tutorial Introduction
     Accessing the Table of Contents
     Asking for an Image
     Asking for an Animation
     Asking for an Image Sequence
     Design Considerations - Fixed Size Images
     Design Considerations - Projections
     Styles and Legends
     Metadata - Abstracts and Keywords
     Metadata - Attributions and FGDC Metadata


     Available Animations List