Re: HDF_SD_ADDDATA problem

adfraser@[EMAIL PROTECTED]
 wrote:
> Thanks, Liam (nice book, by the way).
> 
> I haven't yet investigated this, but I'd hope that the IDL routines
> are smart enough to decompress before opening the files! Actually I'm
> pretty sure they do - I can read the SDS data fine using IDL. But
> you're right, writing may be a different story. I'll check it out.

I believe that they decompress the data sets in memory; but the original 
data is still in the file in compressed format, and that's what causes 
the problem. The only way you can write to a compressed SDS is if it 
defined as having an unlimited dimension, and the write only appends 
data at the end, rather than replacing data within the SDS.

> So LAADS MODIS HDFs are HDF4, not HDF5? That's interesting.

The MODIS project standardized on HDF4 before HDF5 ever came out. HDF4 
wasn't even stable yet; there's traces of bizarre workaround in some of 
our code for  bugs that HDF4 no longer has.

When HDF5 did come out, it was decided that with a planned instrument 
lifetime of 5 years, it wasn't worthwhile going to the trouble of 
switching MODIS products to HDF5. The MODIS instruments launched later 
than originally planned :-( but have lasted longer than planned :-), so 
the decision looks a little less reasonable now than it did at the time. 
Hindsight is 20/20.