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Re: M3IOConvention

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  • Subject: Re: M3IOConvention
  • Date: Mon, 25 Feb 2013 14:51:46 -0700
Hi Robert:

It would be simple to modify M3IOConvention.java to look for this
information, and pass the radius to the projection:

      new LambertAzimuthalEqualArea(lat0, lon0, 0.0, 0.0, 6370000.0);
I notice that the sample file i have does not have that info in it. I am cc'ing Todd Plessel, the original author, to see if he has any comments or wants to add this feature. 

Regards,
John


On 1/28/2013 1:22 PM, Schmunk, Robert B. (GISS-611.0)[TRINNOVIM, LLC] wrote:


John,

A Panoply user had a problem with a small shift in the placement of a
projected lon-lat grid in a netCDF dataset. The gridding uses the
Lambert conformal conic grid, and the dataset was written using the
M3IO convention. I got into the netCDF-Java source code for
ucar.nc2.dataset.conv.M3IOConvention.java to see if I could get a
better grip of what was going on and found that that class is looking
for a grid type specified by the dataset's GDTYP global attribute.
Further definition of the grid is determined by checking the XCENT,
YCENT, P_ALP, and P_BET attributes.

The problem that is arising is that there is apparently no way to
pass along the value of the Earth radius used by the projected grid.
M3IOConvention.java is not looking for the value and is defaulting to
6,371.229 km, but the grid in the problem dataset is based on a
radius of 6370 km. (A very small diference, but the effect is visible
in a plot a couple egrees on a side.) The dataset (and perhaps the
M3IO format) do not include a single specific attribute for the Earth
radius, but does include info about it in a FILEDESC global
attribute, a very long string which includes "… EARTH RADIUS ASSUMED
IN MCIP:  6370000.000 m …".

Is it possible that in a coming update to the NJ libraries that
M3IOConvention.java could be coded to look for this radius
information in the FILEDESC attribute? Or do you have any suggestions
on how one might otherwise deal with this non-default radius? The
user who contacted me about this apparently has hundreds of similar
files, so we're looking for a consistent gridding/plotting solution.


rbs


-- Robert B. Schmunk Webmaster / Senior Systems Programmer NASA
Goddard Institute for Space Studies 2880 Broadway, New York, NY
10025