[McIDAS #OKG-462913]: Color Bar and Units when channel differencing

Hi Greg,

re:
> I'm trying to produce a ch4 - ch2 difference image from GOES for a
> field project.

Sounds like the Fog product...

> I'm able to produce a difference AREA file through
> IMGOPER, but I'm having trouble displaying this product and getting a
> colorbar on the plot with the right ranges and units.
> 
> I first remap each band to the appropriate sector

Excellent.

> The IMGOPER command looks like:
> 
> IMGOPER MYDATA/IMAGES.023 MYDATA/IMAGES.024 MYDATA/IMAGES.025 FORM=ADD
> BAND=4 2 COEF=1 -1

Good, but not complete.  Since the range in the differences can be negative, you
need to add an offset to the values so that they fall in the brightness range
of 0-255.

Please take a look at the Fog product exercise included in the Unidata McIDAS-X
workshop materials:

http://www.unidata.ucar.edu/software/mcidas/current/workshop/toc.html
  15 Applying Lessons Learned
     15.1 IR Channel Differencing
          
http://www.unidata.ucar.edu/software/mcidas/current/workshop/image.differencing.html

Notice how the example not only adds a constant to the differences, but it also 
scales the
differences to have a wider range.

> This works fine and I can display the result easily through IMGDISP.
> However, I only want to show the range from -2 to +6 of the resultant
> field.

Your desired range will change based on the additive constant and scaling 
factors you
decide to use.

> I've tried making my own stretch table but I'm unsure what the
> units are supposed to be.

I would use BRIT.

> When I try to use TEMP and display with this
> Stretch table, I get an error: Units requested not available for this image.

The simple difference you made results in an image with BRITness values only,
the default for IMGOPER.  If you want the result to be in TEMPerature, you
will need to specify the UNIT= keyword on your IMGOPER invocation command line.

> What units should I use for this channel difference?
> How do I get a colorbar that shows the scale from -2 to +6 instead of 0
> to 255?

It depends on whether your -2 - +6 range is BRITness or TEMPerature.

For reference, the GOES calibration for TEMP <-> BRIT is:

/home/mcidas/linux64/workdata% su.k TABLE IRTEMP
BREAKPOINTS STORED IN TABLE : IRTEMP.ST
INPUT       OUTPUT
-----       ------
 163           255
 241.9999      177
 242           176
 330           0
CALIBRATION TYPE  : GVAR
CALIBRATION UNITS : TEMP
BAND NUMBER       : -1
INTERPOLATION TYPE: LIN
su.k: DONE

So, every 2 britness counts is about 1 K.  So, if the difference range you are
interested in is in terms TEMP, you need to highlight BRIT differences in twice
the range, or -4 - +12.  I can not say exactly how to create the stretch without
first knowing how you will scale the difference values.

> Thanks,

No worries.  Sorry if I am not being clear enough.  I am currently in Madison, 
WI
attending the McIDAS Users Group meeting.

Cheers,

Tom
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Ticket Details
===================
Ticket ID: OKG-462913
Department: Support McIDAS
Priority: Normal
Status: Closed