The hrepack tool is a command line tool from The HDF Group that copies an input HDF4 file to an output HDF4 file reproducing all the high level objects and optionally rewriting the objects with compression/uncompression and/or chunking. Many Hierarchical Data Format (HDF) files stored at the NSIDC DAAC are internally compressed using hrepack. This includes the MODIS Version 5 (V005) snow and sea ... ice data products stored on the Data Pool and distributed through subscriptions. This compression should be transparent to all user applications that read data from HDF files using HDF library calls since the HDF library performs uncompression automatically when reading compressed data. However, in some cases it is necessary to generate uncompressed HDF files either because a user-written program reads data from an HDF file directly or because the overhead involved in uncompressing the data on-the-fly is deemed too high for the particular application. Note that both IDL and ENVI appear to handle internally compressed HDF files properly. Please visit the The HDF Group hrepack Web site for documentation pertaining to the hrepack tool.
[Summary provided by NSIDC.]