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NcML Aggregation vs Feature Collections

This guide provides an overview of NcML Aggregation and Feature Collections within NetCDF. Learn how to modify attributes in CDM files without rewriting them, and discover alternatives to ncgen and ncdump. Understand the differences between NcML aggregations and feature collections, including the handling of multidimensional arrays and different feature types. We discuss the benefits of using these tools for creating logical datasets from file collections, and provide insights into their performance and usability challenges. Ideal for users working with GRIB files and seeking efficient data management strategies.

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NcML Aggregation vs Feature Collections

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  1. NcML Aggregation vsFeature Collections

  2. NcML functionality • Modify the objects found in CDM files • Especially Attributes • Don’t have to rewrite the files, esp on the server • Make modest changes “by hand” • Alternative to ncgen, ncdump • CDL in XML • Aggregations • Creating logical datasets out of file collections

  3. NcML Aggregations • union • Logical union of objects inside multiple files • joinNew • Each file contains one slice – add new dimension • joinExisting • Concat existing multidim arrays together • tiled (never released) • Stitch horizontal tiles together • FMRC (deprecated as of 4.2)

  4. NcML Aggregations • Syntactic aggregation • Manipulating multidimensional arrays • Doesn’t know what the data means • High homogeneity requirements • Files must be identical, except for certain things • Difficult for Users to understand or to track • Assemble at runtime • Inefficient for large collections • Caching strategies cause trouble for dynamic datasets

  5. Feature Collections • Feature Types • Coverage: grid, swath, FMRC, ugrid*, image* • Discrete: point, station, profile, etc • Radial • Understands more of the dataset semantics • Mostly about the coordinate systems • Can “do the right thing” without user • Handle more inhomogeneity in the files * Coming soon

  6. Application Scientific Feature Types Datatype Adapter NetCDF-Java/ CDM architecture NetcdfDataset CoordSystem Builder NetcdfFile THREDDS I/O service provider Catalog.xml OPeNDAP NetCDF-3 NIDS NetCDF-4 GRIB NcML HDF5 GINI Nexrad DMSP

  7. What do you need to know? • NcML aggregation works and is stable code • Must know how your collection of files varies • Use it for simple cases • Feature Collections are still work in progress • Much easier to work with • All future development is here • You have our attention • Details tommorrow • If you have GRIB files • You want to use GRIB feature collections (4.3)

  8. Good Luck!

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