For data that are accessible via the network we define four levels of interoperability at the data level:

• Level 0 - no syntactic or semantic metadata - FTP.
  
  
• Level 1 - rigid syntactic, no semantic metadata - DODS.
  
  
• Level 2 - rigid syntactic, human readable semantic metadata - A subset of DODS data sets.
  
  
• Level 3 - rigid syntactic, consistent semantic metadata; i.e., machine readable - A subset of the DODS Level 2 data sets.