Re: [idvusers] executing fog products in IDV

Hi Caitlin --

If your surface temperature values are on grids, then you can use the
"simple difference" to subtract the two.  First, use the Choosers to
select the data sources (grid and image), then in the Field Selector,
choose "Formula" and drill down in Misc to the difference formula.
When you pick that, select an output display (Color shaded plan view),
then when you click "display", the IDV will prompt you to identify the
two parameters to subtract.   (NOTE:  if your gridded data is really
3D, then you will need to use the Formula "slice at leve" or "2D
slice" when naming the gridded data; the IDV will then ask you to
select the grid -- the reason is that the image is 2D and the grid
must be 2D as well......)

The domain of the first data source will define the domain of the
result -- the code will resample the second data source to the domain
of the first one and then do the subtraction.   If you select the
image as the first data source, then the grid (2nd data source) will
be sampled onto the image domain -- this might take a while to

If you want to find out only where the difference is < 4K, you can use
the built-in mask() function to create your own formula.  Something
like this:

mask( (a - b) , "<", 4.0)

When you display this, you will be prompted for "a" and "b".  Again,
just select the two data sources.  The display will then contain only
0s and 1s -- and you'll need to create a color table range

Hope that gets you sure to check the Users Guide on
formulas for more details, if you haven't had a chance already...


On Wed, Mar 25, 2009 at 4:56 PM, Caitlin Hart <chart2@xxxxxxxxxxxxx> wrote:
> Hello,
> I am an undergraduate and very new to IDV.
> Is it possible to create composite images out of McIDAS AREA files and 
> surface plots (GEMPAK or netCDF files)?
> specifically, I am trying to evaluate a fog product and one of the criteria 
> is that surface temperature - longwave (ch4 on GOES) brightness temperature < 
> 4K.
> I have AREA files of satellite data and GEMPAK/netCDF files of surface 
> temperature, but I don't know what to tell IDV in a formula to get a 
> composite of the two.
> Thank you,
> Caitlin Hart
> SUNY Brockport Meteorology
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Tom Whittaker
University of Wisconsin-Madison
Space Science & Engineering Center (SSEC)
Cooperative Institute for Meteorological Satellite Studies (CIMSS)
1225 W. Dayton Street
Madison, WI  53706  USA
ph: +1 608 262 2759