Appendix: Standards

To the largest degree possible, the SVS Image Server will adhere to the guiding principle that open standards should be employed to access and deliver content to the user community. One goal of the project is to promote the appropriate standards within the NASA community, both to facilitate reuse of our materials and to make distributing our material easier. Initial relevant standards and their rationales are listed below.

Link: http://www.opengis.org/specs/?page=specs

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.

The 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.

The 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

Link: http://www.fgdc.gov/standards/projects/FGDC-standards-projects/metadata/base-metadata/index_html

Rationale: Image content on the SVS Image Server cannot stand by itself - it must be accompanied by ancillary data explaining the content, provenance, and audience of the images. This data is called metadata, and WMS provides a method for associating metadata with individual layers. One specific metadata format in the WMS specification is the Content Standard for Digital Geospatial Metadata (CSDGM) from the Federal Geographic Data Committee. This standard is the officially sanctioned method for describing geospatial data resources by United States Federal Agencies and seems flexible enough to describe SVS Image Server resources in some detail.

Issues: The CSDGM does not formally include fields directly related to remote-sensing instruments or educational content standards. The CSDGM is extensible, but private extensions in these areas may require an undesirable amount of internal support and documentation.

Status: Adopted by the SVS Image Server

Link: http://www.fgdc.gov/standards/projects/FGDC-standards-projects/csdgm_rs_ex

Rationale: This standard is a formal extension to the CSDGM that provides additional metadata elements for such items as instrument description, data processing level, and dataset hierarchies.

Issues: This standard is not widely adopted yet. However, these extensions provide few mandatory elements and some of the elements could be very useful, so there is little risk or overhead in adopting this standard once the core CSDGM is adopted.

Status: Adopted by the SVS Image Server