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Ben,The organization of data into collections, both physically and logically, is primarily an implementation issue. Ron is right that the THREDDS catalog can be implemented in ebRIM profile in CSW with data hierarchies be exposed to the clients. Because the goal, and more importantly, the resources, of our project is to make data cataloged in THREDDS be searchable through the CSW GMU hace already implemented, we didn't plan to implement a new CSW server that can fully expose THREDDS catalog including its hierarchies. Our focus was to mapping the THREDDS metedata items to the GMU CSW so that CSW clients can search THREDDS data, through the mapping relationship, in the CSW server. This means that we are focused on searching data based on client provided criteria, rather than display the data products hierarchically, in the CSW. We need to include in our logical data model the parent and child relationships between a data product, either a collection or a direct data set, and its parent (for non-root data, i.e., highest collection) and its child (for non-leaf data, i.e., a direct data set). With such we will be able to know the hierarchy of the catalog. To expose the hierarchy to a client, the server needs to know to which level the client is expecting to view. It seems that in THREDDS, the server will always expose the immediate child nodes of a specific node to a client and the client can then click on one of the child nodes to see the next level nodes. This can be recursively continue until leaf nodes, direct data sets, are exposed. In CSW, our current implementation is to search all direct data sets and return a subset which meet the search criterion/criteria. With in parent/child relationship be added, our CSW can also be implemented to search either immediate child or n-level down for a certain node. However, additional information (parameters/Values) will be needed to specify how a client can let the server know the depth of search and from where the search begins. We need to investigate if such additional parameters/values are compliant with the ISO19115 profile (it should not be a problem in ebRIM because it's flexibly extensible). 19115 does provide description of metadata hierarchical levels but this parameter/value information for specifying levels of exposure is a different issue.
In summary, I agree that the data collection/hierarchy information is useful in many ways. If time and resources permit, we'll certainly explore the this. At this moment, we want to focus on our original plan to complete the mapping of THREDDS to 19115 at data set level.
Regards. Wenli At 19:06 2006-11-19 -0700, Ben Domenico wrote:
Hello again,I think I may need to clarify the situation. The current primary focus of the CSW/THREDDS gateway that GMU is working on is mainly on mapping THREDDS catalog metadata to ISO 19115 so that THREDDS metadata can be made available in an international standard form. It would be a mistake to divert that project from it's high priority objectives.On the other hand, the question of how to provide inventory catalogs of "collections" of datasets, and catalogs of those collections -- as THREDDS does -- keeps coming up in many different settings. It arose in the OGC GALEON interoperability experiment; it came up in discussions at the 3rd Interoperability Workshop on the Automated Harvesting of Data and Metadata last week.So I sent the message to the THREDDS and GALEON email lists in order to get a wider group thinking about the issue which I think is a key to making all these data services work together. For those of you who are not familiar with THREDDS catalogs, an example of a heirarchical set of catalogs is available for a variety of real-time data at:http://motherlode.ucar.edu:8080/thredds/catalog.htmlAs you will note as you drill down through the collections, you can get the underlying xml representation of any of any of these catalogs by replacing the .html with .xml in the URL.From Ron Lake's notes, it sounds like CSW.ebRIM can be used to provide this type of functionality via a standards-based interface. It's important though that, while we consider the long range goals, we also retain realistic expectations of the current project.I hope this clarifies rather than confuses the issue. -- Ben
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