[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

20010718: HDF to AREA (cont.)

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


>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

>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

     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

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

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


  Miscellaneous McIDAS documents

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


NOTE: All email exchanges with Unidata User Support are recorded in the Unidata inquiry tracking system and then made publicly available through the web. If you do not want to have your interactions made available in this way, you must let us know in each email you send to us.