[Gdal-dev] HDF EOS: cannot retrieve band scale and offset

[Emilio Mayorga]

Andrey,

While I wasn't the one who asked this question, thanks for the
solution! I had pretty much the same need -- extract scale and offset
("Slope" and "Intercept", single values) from SeaWiFS Level 3m
products, but from a geotiff file I created from the original hdfeos
file using gdal_translate. Your solution works like a charm.

Cheers,

-Emilio


> On Thu, Oct 11, 2007 at 09:34:45AM -0700, Jan Cermak wrote:
> > From a MODIS MYD021km file I am trying to read band data including
> > offset and scaling factors. Band data works just fine, but I can't get
> > offset and scale. Without offset and scale the data is completely
> > useless, so this seems to be a major issue to me.
> >
> > Here is some sample code:
> >
> > dsobj =
> > gdal.Open('HDF4_EOS:EOS_SWATH:"myfile.hdf":MODIS_SWATH_Type_L1B:EV_1KM_RefSB')
> > bandobj = dsobj.GetRasterBand(1)
> > scale = bandobj.GetScale()
> > offset = bandobj.GetOffset()
> >
> > scale and offset variables are empty.
> >
> > I am using yesterday's stable branch nightly.
> > A sample file is at
> > ftp://ladsweb.nascom.nasa.gov/allData/5/MYD021KM/2006/220/MYD021KM.A2006220.1330.005.2007140045620.hdf
> >
> > For this file and data set, HDFView yields:
> > radiance_scales =
> > 0.0072957743,0.0051752976,0.0037723905,0.0029522364,0.0023176796,0.001162374,5.6182046E-4,0.0011558639,4.1845915E-4,9.5770456E-4,6.215421E-4,0.006355153,0.009465027,0.0069601014,0.0027906199
> > radiance_offsets =
> > 316.9722,316.9722,316.9722,316.9722,316.9722,316.9722,316.9722,316.9722,316.9722,316.9722,316.9722,316.9722,316.9722,316.9722,316.9722
>
> Hi Jan,
>
> Finally I have solved this problem. These data now fetched and added to
> the list of metadata, so you can use it in you code:
>
> $ gdalinfo HDF4_EOS:EOS_SWATH:"MYD021KM.A2006220.1330.005.2007140045620.hdf":MODIS_SWATH_Type_L1B:EV_1KM_RefSB
>
> Driver: HDF4Image/HDF4 Dataset
> Files: MYD021KM.A2006220.1330.005.2007140045620.hdf
> Size is 1354, 2030
>
> .....
>
>   radiance_scales=0.007295774, 0.005175298, 0.003772391, 0.002952236, 0.00231768, 0.001162374, 0.0005618205, 0.001155864, 0.0004184592, 0.0009577046, 0.0006215421, 0.006355153, 0.009465027, 0.006960101, 0.00279062
>   radiance_offsets=316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722
>   radiance_units=Watts/m^2/micrometer/steradian
>   reflectance_scales=1.348291e-05, 8.769159e-06, 6.162684e-06, 5.057901e-06, 3.954835e-06, 2.42613e-06, 1.172643e-06, 2.478778e-06, 8.973959e-07, 2.389224e-06, 2.062774e-06, 2.196517e-05, 3.504297e-05, 2.574762e-05, 2.468974e-05
>   reflectance_offsets=316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722
>   reflectance_units=none
>   corrected_counts_scales=0.126194, 0.126194, 0.126194, 0.126194, 0.126194, 0.126194, 0.126194, 0.126194, 0.126194, 0.126194, 0.126194, 0.126194, 0.126194, 0.126194, 0.126194
>   corrected_counts_offsets=316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722, 316.9722
>   corrected_counts_units=counts
>
> It is not possible to use our Scale/Offset methods, because driver
> doesn't know what metadata record should be used for any given product,
> but you can easily parse it yourself:
>
> >>> ds = gdal.Open('HDF4_EOS:EOS_SWATH:MYD021KM.A2006220.1330.005.2007140045620.hdf:MODIS_SWATH_Type_L1B:EV_1KM_RefSB')
> >>> md = ds.GetMetadata()
> >>> scales = map(float,md['radiance_scales'].split(','))
> >>> offsets = map(float,md['radiance_offsets'].split(','))
> >>> print "%f/%f" % (scales[0],offsets[0])
> 0.007296/316.972200
>
> Best regards,
> Andrey