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Part III. Appendix 2: Data Transport• 32-bit floating point numbers• 64-bit floating point numbers• strings• COMPOUND TYPES• structures• arrays• lists• sequences• gridsplus two datatypes relevant to the Web environment; these are considered to be strings and thus tobe simple datatypes:• URLs• pointers to URLs(There is also the “data set” datatype, which is used to wrap the entire data declaration; one DDScontains one “data set.”)For more details on these data types see section 6.3 of the DAP users’ guide: http://www.unidata.ucar.edu/packages/dods/user/guide-html/guide_58.html.The potential complexity of the data structures expressible in a DDS is suggested by the following:• Structures can contain simple, array, list, sequence, and grid datatypes as well as other structures;• Arrays can contain simple, structure, sequence, grid datatypes;• Sequences can contain simple, array, list, structure, sequence, and grid datatypes;• Grids contain arrays.To date, the DDS has been able to express all formats used by actual servers and clients; the boundsof its expressiveness have hardly been challenged.QUERIES TO A SERVEROPeNDAP is not a mechanism for transferring files as such. Instead, it a mechanism for transferringthe information contained in files by using the DDS, the DAS, and the DataDDS, and thisrequires that the server open and parse the datafile containing the information to be transmitted.Since the server will be processing the contents of the datafile, OPeNDAP was also designed to permitthe client to instruct the server to return only data which fit the conditions in a query from clientto server.186