Re: data transformation questions

Hi Don, thanks for the prompt reply.

I did try the geosubsetting in idv 1.3b2, but it didn't quite do what I needed. My subgrid is actually defined by grid points, or equivalently, kilometers, rather than lat lon as catered for by IDV. My grid has a lambert conformal projection from x,y to lat,lon. So I completed a nice netcdf filter app which takes index ranges for coordinate variables and uses Array.section to dice up the data.

Also, in trying to display my data (wind speed) in IDV as knots rather than ms-1, I don't see a 'Change Display Unit' in the Edit menu of the display control (I tried all three Plan Views with same result). The User's Guide mentions that 'Some displays allow you to change the unit that is used in the display' but doesn't state exactly which ones ?

Thanks again.

BTW if anyone is interested in my netcdf filter app, i can post it.

Stuart Maclean

Don Murray wrote:
Hi Stuart-

Stuart Maclean wrote:

I have a netcdf file of an mm5 model output. One variable is max wind speed, defined over a 2d grid, with values every 3 hours. The nc file has CF conventions in place so IDV can display it.

I would like to perform two transformations on the data in order to display it in idv:

1 create a smaller area. the orig grid is say 100x100 and I want to view just say an inner box of 20x20. I know I can zoom in on the area but the mere presence of the full grid has influence on the contouring/display so I would prefer to clip the data first

With the 1.3B2 release, if you double click on the data source
in the Field Selector, it will bring up the dialog for setting
the properties on that file (or right click and select
Properties).  Under the Geo-Subsetting tab in the dialog, you
can subset by stride (every Nth point) and/or by geographic
location.  This will apply to any displays you create with
that grid.   In the next release, you will be able to do
this at the dataset or at the parameter level (like with
Times).  This will clip the data and just read out what
you have defined.

2 the units from the model are ms-1. i would like to display (and contour) in knots.

Once you have your display loaded, you can use the Display Control's
Edit->Change Display Unit.. menu to set the unit to knots.  If
you want this to be the default, then use the Parameter Defaults
Editor (Edit->Resources->Parameter Defaults menu) to set the
default unit for your variable.

I suspect that the first task cannot be done in IDV. I had hoped that NcML would provide some filtering leverage like this, but I haven't found it.

Try the Geo-subsetting.

The unit conversion can be done in IDV ??


Any help gratefully appreciated

If this doesn't work, let us know.

Don Murray                               UCAR Unidata Program
dmurray@xxxxxxxxxxxxxxxx                        P.O. Box 3000
(303) 497-8628                              Boulder, CO 80307

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