20010718: HDF to AREA (cont.)

>From: Darren Gallant <address@hidden>
>Organization: UCAR/JOSS
>Keywords: 200107091620.f69GKI102721 McIDAS HDF AREA

Darren,

>Does a Mcidas areafile contain the raw and unscaled lines/sample values or
>the scaled values one would see if an image is displayed?

An AREA can contain calibrated or uncalibrated brightnesses.  If it
contains uncalibrated data, a calibration block needs to be included in
the file header.  Imagery that we receive in the Unidata-Wisconsin
datastream (LDM feed type MCIDAS) is pre-calibrated.  What this means
is that a calibration block does not have to be included, and the
brightness values are directly translatable to temperatures for IR
images.

>For example, I
>have an HDF file which contains 2 byte Channel 2-5 data. The units are
>degrees Celsius. However the data itself contains values far larger than
>typical Earth temps.

The values are most likely the 10-bit values from GOES.  These values
translate into temperatures or any other unit for which there is a
calibration block.

If the data is pre-calibrated, the values stored in the file would
be 1-byte.  The formula used by McIDAS for pre-calibrated GOES images
is:

     T = 418 - B   (B>176 OR B=176)
     T = 330 - (B / 2)    (B<176 OR B=176)

Here B is a 1-byte brightness; it can have values from 0 to 255.

>The HDF file contains a scaling factor and an offset.
>Inorder to recover the IR temps, I would multiply every line/sample value
>by this scaling factor and then add the offset.

OK.  In order to keep the 10-bit data, your AREA files will need to contain
a calibration block.

>Does an areafile store the
>raw values and then Mcidas applies a scaling factor and adds an offset
>when the image is displayed or Does the areafile contain just albedos and
>temperatures? 

It can, yes.  As I noted above, the values can already be calibrated
also.

There are a couple of files in the Unidata McIDAS distribution that
present information on AREA file calibrations by platorm.   The files
are area.doc and area2.doc.  The get installed in the ~mcidas/help
directory.  Their contents can be listed from the Help -> McIDAS Topics
-> AREA_files action from the Unidata MCGUI interface.  The same
information is also available online at:

Unidata McIDAS-X HomePage
http://www.unidata.ucar.edu/packages/mcidas/770/

  DOCS
  http://www.unidata.ucar.edu/packages/mcidas/770/document.html

  Miscellaneous McIDAS documents
  http://www.unidata.ucar.edu/packages/mcidas/miscdocs/misc_docs.html

You will see that there are two links in the last page.  One covers platforms
before GVAR; the other contains GVAR.

Tom