BES - Modules - The NetCDF Handler

Kinds of files the handler will serve

There are several versions of the netCDF software for reading and writing data and using those different versions, it's possible to make several different kinds of data files. For the most part, netCDF strives to maintain compatibility so that any older file can be read using any newer version of the library. To ensure that the netCDF handler can read (almost) any valid netCDF data file, you should make sure to use the latest version of the netCDF library when you build or install the handler.

However, as of netCDF 4, there are some new data model components in netCDF that are hard to represent in DAP2 (hence the 'almost' in the preceding paragraph). If the handler, as of version 3.10.x, is linked with netCDF 4.1.x or later, you will be able to read any netCDF file that fits the 'classic' model of netCDF (as defined by documentation) which essentially means any file that uses only data types present in the netCDF 3.x API. In addition, these files can employ both internal compression and chunking.

The new data types present in the netCDF data model present more of a challenge. However, as of version 3.10.x, the Hyrax data handler will serve most of the new cardinal types and the more commonly used 'user defined types'.

Mappings between NetCDF version 4 data model and DAP2 data types

All of the cardinal types in the netCDF 4 data model map directly to types in DAP2 except for the following:

NC_BYTE

There is no 'signed byte' type in DAP2 so these map to an unsigned byte.

NC_CHAR

There is no 'character' type in DAP2 so these map to DAP Strings of length one. Arrays of N characters in netCDF map to arrays of N-1 Strings in DAP

NC_INT64, NC_UINT64

DAP2 does not support 64-bit integers (this will be added soon to the next version of the protocol).

netCDF type name	netCDF type	DAP2 type name	DAP2 type	Notes
NC_BYTE	8-bit signed integer	dods_byte	8-bit unsigned integer	The DAP2 type is unsigned
NC_CHAR	8-bit unsigned integer	dods_str	variable length character string	treated as character data; arrays are treated specially (see text)
NC_SHORT	16-bit signed integer	dods_int16	16-bit signed integer
NC_USHORT	16-bit unsigned integer	dods_uint16	16-bit unsigned integer
NC_INT	32-bit signed integer	dods_int32	32-bit signed integer
NC_UINT	32-bit unsigned integer	dods_uint32	32-bit unsigned integer
NC_INT64	64-bit signed integer	None		Not supported
NC_UINT64	64-bit unsigned integer	None		Not supported
NC_FLOAT	32-bit floating point	dods_float32	32-bit floating point
NC_DOUBLE	64-bit floating point	dods_float64	64-bit floating point
NC_STRING	variable length character string	dods_str	variable length character string	In DAP2 it's impossible to distinguish this from an array of NC_CHAR