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>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