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[IDV #FGA-644757]: IDV - sigma-levels over Antarctica - part 2

Hi Mark-

I'm sorry for the delay in responding to this.  I've been trying to 
get my head around the projection information and how we could support
these sigma levels in a non-regular grid.

> > The projection defines the region of coverage.  The number of gridpoints
> > will determine the resolution.  So presumably when they went from 30km
> > to 20km, there were more grid points in the x and y direction.
> >
> > The projection should define the region of coverage.
> >
> 
> I must be missing something then.  The projection attributes which I am
> using do not define the region of coverage, but instead cover the
> characteristics of the projection.
> 
> My projection attributes:
> char polar_stereographic ;
> polar_stereographic:grid_mapping_name = "polar_sterographic" ;
> polar_stereographic:straight_vertical_longitude_from_pole = 180.f ;
> polar_stereographic:latitude_of_projection_origin = -90.f ;
> polar_stereographic:scale_factor_at_projection_origin = 1.f ;
> polar_stereographic:false_easting = 0.f ;
> polar_stereographic:false_northing = 0.f ;

the x and y coordinates need to be in kilometers on the projection
plane for this to work.  Sorry for the confusion. 

> I selected these attributes from the CF 1.0 instructions:
> http://www.cgd.ucar.edu/cms/eaton/cf-metadata/CF-1.0.html
> Polar stereographic
> grid_mapping_name = polar_stereographic
> Map parameters:
> * straight_vertical_longitude_from_pole
> * latitude_of_projection_origin - Either +90. or -90.
> * Either standard_parallel or scale_factor_at_projection_origin
> * false_easting
> * false_northing
> Map coordinates: The x (abscissa) and y (ordinate) rectangular
> coordinates are identified by the standard_name attribute values
> projection_x_coordinate and projection_y_coordinate respectively.
> 
> I went ahead and removed all lat lon references.  This included the lat
> lon variables and associated attributes, as well as the coordinates
> attribute (lat lon) for each variable.  These changes are included in
> the file:  20050601.nc
> The result was a step backwards as when attempting to open in the Data
> Chooser, I get the error:
> No gridded data found for:
> /vardar/data4/seefeldm/amps/20050601.nc
> Are you sure this is a gridded file?

Okay, my bad.

> I tried one more time time with the coordinates attribute specifying xc
> and yc for each variable, the rectangular coordinates.  These changes
> are included in the file:  20050801.nc
> I receive the same error as without the coordinate attributes set:
> No gridded data found for:
> /vardar/data4/seefeldm/amps/20050801.nc
> Are you sure this is a gridded file?
> 
> These nc files can be retrieve from:
> http://foehn.colorado.edu/antarctica/amps/nc/
> -The file 20050701.nc is the original file I created to work on this
> problem.

There's a typo in this file that keeps the IDV from picking up
the projection corretly.  In your projection variable:

grid_mapping_name should be :stereo" not "stero", so projection not found

  char polar_stereographic;
    :grid_mapping_name = "polar_sterographic";

Then, if you can get the xc and yc to be the values of the grid in
kilometers on the projection instead of grid row/column, you should be in 
better shape.  If i just change the unit of these to km, I was able to
get something to plot on sigma coords (although not in the right place).
You might be able to just add scale_factor and add_offset attributes
to the xc/yc variables.

If you need more help with that, let me know.

Don


> -I have no idea as to why the file sizes are so drastically different.
> Other than than the defining of the attributes, and the removal of the
> lat lon fields, all other code used in creating the netcdf file is the same.
> 
> Mark
> 
> 


Ticket Details
===================
Ticket ID: FGA-644757
Department: Support IDV
Priority: Critical
Status: Open