Re: HDF SDS data attributes

["Liam Gumley" <Liam.Gumley(at)ssec.wisc.edu>]

William B. Clodius <wclodius@lanl.gov> wrote in message
39C6A8DC.6C0F3BA1@lanl.gov">news:39C6A8DC.6C0F3BA1@lanl.gov...
> My team has been using HDF files to store data from a multi-spectral
> imaging sensor. The images are typically stored as band sequential
> arrays in SDS data sets. Ideally we would like to attach various
> information to the SDS data sets such as band names, x, y coordinate
> mappings, band wavelengths, etc. Some of thsi information can be
> readilly implemented using IDL's HDF interface  but the dimension
> specific information such as band name and band wavelength doe not seem
> to be easilly implemented in IDL. The C and Fortran interfaces to HDF
> appear to let you define attributes associated with specific dimensions
> of an SDS, but not the IDL interface. Does anyone know of a workaround?

Let me see if I understand your problem. You have a dimension named
'Band_Number', which is used for image SDS arrays. However you'd also like
to record the actual band numbers which correspond to the 'Band_Number'
dimension.

In the HDF world, you can create an SDS known as a 'dimension scale', which
assigns values to a dimension (in the netCDF world, this type of variable is
known as a coordinate variable). Dimension scales encode the 'values' of a
dimension, and attributes may be added as well (e.g. 'units'. For example,
if you stored atmospheric profiles using a 'Pressure_Level' dimension, you
might want to also store the pressure values at each level. In this case you
would create a dimension scale named 'Pressure_Level' which contained the
pressure values, e.g.

float Pressure(Pressure) ;
    Pressure:long_name = "Pressure Levels for Atmospheric Profiles" ;
    Pressure:units = "hPa" ;

Here's an example from IDL 5.3 for Windows. The trick when creating the
dimension scale is to use the desired dimension name as the SDS name:

;-----
PRO TESTHDF

;- Create the file
hdfid = hdf_sd_start('test.hdf', /create)

;- Create the profile SDS
varid = hdf_sd_create(hdfid, 'Profile', [10], /float)
dimid = hdf_sd_dimgetid(varid, 0)
hdf_sd_dimset, dimid, name='Pressure'
hdf_sd_adddata, varid, findgen(10)
hdf_sd_endaccess, varid

;- Check for coordinate variable
index = hdf_sd_nametoindex(hdfid, 'Profile')
varid = hdf_sd_select(hdfid, index)
print, hdf_sd_iscoordvar(varid)
hdf_sd_endaccess, varid

;- Create the pressure coordinate variable (aka dimension scale)
varid = hdf_sd_create(hdfid, 'Pressure', [10], /float)
dimid = hdf_sd_dimgetid(varid, 0)
hdf_sd_dimset, dimid, name='Pressure'
hdf_sd_adddata, varid, (findgen(10) + 1.0) * 100.0
hdf_sd_endaccess, varid

;- Check for coordinate variable
index = hdf_sd_nametoindex(hdfid, 'Pressure')
varid = hdf_sd_select(hdfid, index)
print, hdf_sd_iscoordvar(varid)
hdf_sd_endaccess, varid

;- Close the file
hdf_sd_end, hdfid

END
;-----

You can add attributes to the dimension scale as desired. When this
procedure is executed, it correctly identifies the second variable
'Pressure' as a coordinate variable (aka dimension scale).

IDL> testhdf
           0
           1

Here are the contents of the file as seen by the HDF version of ncdump:

netcdf test {
dimensions:
 Pressure = 10 ;

variables:
 float Profile(Pressure) ;
 float Pressure(Pressure) ;

data:

 Profile = 0 , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9  ;

 Pressure = 100 , 200 , 300 , 400 , 500 , 600 , 700 , 800 , 900 , 1000  ;
}

For more information, see the HDF documentation at
http://hdf.ncsa.uiuc.edu/UG41r3_html/SDS_SD.fm7.html#40381

Cheers,
Liam.
http://cimss.ssec.wisc.edu/~gumley