20010829: grddisp contouring

>From: address@hidden
>Organization: University of Virginia
>Keywords: 200108291955.f7TJtZ129681 McIDAS GRDDISP TE

Mark,

>I am a graduate student at the University of Virginia.  I am rather 
>new to Mcidas.  I am trying to display absolute vorticity, which I 
>have succeeded in doing.  My question involves contouring or color 
>filling certain values.  Is there a way to contour or color fill only 
>values greater than a certain threshold.

Yes.

>I have also looked into 
>using the dash keyword to dash certain threshold values but have been 
>unsuccessful in any of these attempts.  Any direction would be greatly
>appreciated.  Thanks for your time.

As far as specifying which contour levels you want to display, you have
pretty good control.  What you want to do is define the list of contour
levels as the value of a McIDAS string, and specify it as the value
for the CINT= keyword.  Here is an example.

Suppose you have a grid of temperatures and you want to plot the -2, 0, 6, 10,
17 isotherms only.  First you define a string to hold these values:

TE TCINT "-2 0 6 10 17

Then use TCINT values for the CINT= keyword:

GRDDISP group/descriptor PAR=T ... CINT=TCINT

GRDDISP will see that the value of CINT= is a string name; get the
list of contour lines to be plotted as defined by the string values;
and then only plot those.

For vorticity, you will have to specify floating point numbers, or you
will need to scale the data (POWer=) and then specify scaled contour
levels in the string definition.

There is no direct way to do color filled contours in McIDAS.  You can,
however, convert a GRID into an image and then display that.  I provide
a McBASI script, CFILLCLP.MCB, in the McIDAS distribution that will do
this for you easily.  Here is an example of visualizing a 12 hour forecast
surface temperature grid from the 12 Z ETA model run from today clipped
at land/water boundaries:

CFILLCLP RTGRIDS/ETA T SFC #Y 12 12 LAMB MYDATA/IMAGES.1234 Y Y

For this to work, you will need to have the AREA file AREA9000 on
your system.  This is standard in McIDAS-X 7.8, but was optional in
7.7.

Since the GRID gets converted into an image, there is no direct way
to colorize specific data value regimes.  You will have to interrogate
the display (IMGPROBE) to find out which brightness levels correspond
to the grid values you are interested in.  After doing that, you can
use the EU command to colorize your display in the way you want.

Please let me know if you have questions.

Tom Yoksas