Michael,
On Sun, May 10, 2015 at 2:35 PM, Michael Powell <mwpowellhtx@xxxxxxxxx>
wrote:
> It's been a few years since I've looked at netCDF at all, so this is a
> refresher. Also, this has no doubt been covered a million times, but I
> am confused about the nature of the data block.
>
You are inquiring specifically about netcdf-3 format (classic and 64-bit
offset), not netcdf-4 (HDF5), right?
Unless you have some very good reason for attempting to access netcdf-3
format directly, please use the netcdf libraries (C, fortran, etc.) to read
and write. Rough knowledge of the low-level format is helpful for
understanding performance issues.
> If I understand the header/data parts correctly (which, I may not),
> non-record can be 'infinite'?
Not infinite, rather about 8 exabytes. But that is probably what you meant.
This is shown in a good summary table in the older netcdf documentation:
http://www.unidata.ucar.edu/software/netcdf/docs/netcdf/Large-File-Support.html
> Rather, the non-record, fixed-length
> parts should occur prior to the record data?
Yes. I think the following overview is a little less confusing than in the
BNF spec. Think of all netcdf-3 files as having five possible parts. All
parts except the header are optional. The two pads are a minor detail,
don't worry about them:
* Header
* Header pad
* Fixed length variables
* Data pad
* Record data
> How does one tell what is record versus non-record?
>
Many applications don't bother to look. Basic read access works the same
either way, except for performance issues with large files.
If you care, the manual method is to look for a dimension labeled
"unlimited" in ncdump -h output. Any variables using this dimension are
record variables. All others are fixed length variables.
There are inquiry functions which can do this under program control.
> I think this is as a function of the var nelems, dimid, etc, but it's
> not especially clear from the BNF grammar, or associated verbiage. For
> example, what is meant by 'interleaved'? As a cross section of the
> specified variables?
Only record variables are interleaved. The sub-arrays ("cross sections")
for record subscript #0 of all record variables are combined together at
the start of the variable length section. The combined block for subscript
#0 is often referred to casually as one "record". The combined block for
subscript #1 is the next record. And so on.
Suppose you have precip(time, lat, lon) and time(time), and time(time).
Then the first record is precip(0,*,*) and time(0) together, and so on.
Also taking into account spiked variable array dimensions?
>
What do you mean by "spiked"?
> I'm sure I'm barely scratching the surface here... Insights are welcome.
>
> Thank you...
>
> Regards,
>
> Michael
>
--Dave
