Specification of the HDF data file format

See the main Data format specification page
for some general notes on the data formats.

Simply said, an HDF file has the following structure (note that the vertical column field or cloud cover fraction may not be available):

• File header, with the file attributes
  these specify the product name, data, version, units, etc.
• Data sets:
  • SO₂ slant column field
  • Error in the slant column field
    from the slant column retrieval
  • SO₂ vertical column field for different plume heights
  • Error in the vertical column fields
    from the slant column error
  • Cloud cover fraction

 
There are several tools around on the web for viewing and treating HDF files, notably at the NCSA HDF Home Page. Software for viewing and reading HDF files is available via this page, where you can find a binary distributions for several operating systems (UNIX, Linux, Windows).
The HDF file can also be read with many plot programs, such as IDL (example page) and MatLab, as well as with the BEAT toolbox.
 

Name of the HDF files

File header

 
Remarks regarding some of the attributes in the file header:

• Product
  This attribute gives so to say the title of the product. In the example it says that the file has SO₂ slant column data for one day.
  • For 3-day composites " -- 3-day composite" is added.
  • For monthly averages " -- monthly average" is added.
  • ... etc.
  If the file contains slant column as well as vertical column densities, the main title is "SO₂ column densities [DU]".

• Data_version
  The coding for this is described in the section on the Data and Services version history.

• Product_status
  This can be "preliminary" or "archive".

• SO₂_field_date_1 & SO₂_field_date_2
  These two attributes give the period for which the data applies in whole days (UTC). In the example above this is one day; for longer time periods they give the first and last day.

• Data_begin & Data_end
  These two give the date and time of the very first and the very last measurement included in making the gridded data.
  Measurements of SCIAMACHY come in principle as one file per orbit. The data included is from all data files which have a start time in the given period. The last orbit of a given day can, of course, continue into the next day. In that case "Data_end" shows the beginning of the next day, as in the above example.

• Cloud_fraction
  If no cloud cover fraction is included, the value of this attribute is "None included" and there is no 'ccf' data set.

• Longitude & latitude ranges and steps
  These are given in degrees.
  See further down on how to (re)construct the longitude-latitude grid of the data sets.

• Number_of_VCD_sets
  This is the number of different plume heights used to compute vertical column densities (VCDs) and thus gives the number of VCD data sets; which plume heights were used is connected to the version number of the data product.

• Product_code
  This is for internal use, to ease post-processing.

• Data sets
  The above example is for a file which contains the slant column density (SCD), three data sets for the vertical column densities (VCD) and the cloud cover fraction (CCF).
  The error values given in the HDF files for the SCD and VCD fields are an average over the error values given in the orbit data files; they include no "standard deviation" of the averaging itself.

Grid

The data sets in the HDF file are given in the form of a two-dimensional array with each grid cell having a specific longitude-latitude grid. This grid can be (re)constructed using the information given in the header of the HDF file: gridlon(ilon)=lonmin+(ilon-1)*dlon gridlat(ilat)=latmin+(ilat-1)*dlat Note that the coordinates of the grid refer to the centre of the grid cells, as indicated by the filled circles in the graph on the right.

An example Fortran code for this can be found in the so2hdfgrid.f program. It generates the grid and gives a few lines of output as example:

 ilon ilat   gridlon   gridlat
    1    1  -179.8750  -89.8750
 1440  720   179.8750   89.8750
   38  562  -170.6250   50.3750
  722  362     0.3750    0.3750
 1222   62   125.3750  -74.6250