IDL's HDF_SD_ routines can accept two different types of ID numbers. Documentation for these routines in the IDL Reference Guide refers to these ID numbers as the SD_ID and SDS_ID arguments.
The SD_ID is the Scientific Dataset interface ID. There is only one SD_ID per HDF file. For each actual dataset used, you will also need an SDS_ID , which is the ID for the particular dataset.
Some routines, such as HDF_SD_ATTRFIND accept either an SD_ID or an SDS_ID . In these cases, the documentation refers to the ID as an S_ID , meaning that either type of ID is accepted.
Both the old DFSD and the new MFSD HDF interfaces use the same tags. Therefore, you must be careful when reading or writing SDs. Use only the appropriate "HDF_SD_" or "HDF_DFSD_" routines. The wrong SD interface may seem to work, but may return bad results.
HDF and IDL support many different data types. Many of the HDF routines allow you to perform a data type conversion "on the fly" by setting keywords such as FLOAT. When the data type desired is not explicitly specified, IDL uses the conversions shown in the following tables. Note that single-precision floating-point is the default data type and that the complex data type is not supported.
When writing IDL data to an HDF file, IDL data types are converted to the HDF data types shown in See Type Conversions when Writing IDL Data to an HDF File :
When reading data from an HDF file, HDF data types are converted to the IDL data types shown in See Type Conversions when Reading HDF Data into IDL :
HDF type codes for the supported HDF data types are shown in See HDF Data Type Codes :
Common HDF Tag Numbers lists common HDF tag numbers and their meanings.