Re: Fwd: Re: 20040322: Any idea why gribtocdl does a "floating exception" on this file?

• To: Richard Signell <signell@xxxxxxxxxxxxxxxxx>
• Subject: Re: Fwd: Re: 20040322: Any idea why gribtocdl does a "floating exception" on this file?
• From: Robb Kambic <rkambic@xxxxxxxxxxxxxxxx>
• Date: Wed, 31 Mar 2004 10:11:44 -0700 (MST)

On Tue, 30 Mar 2004, Richard Signell wrote:

Robb Kambic wrote:
>
> Rich,
>
> I have the formulas needed but the results don't seem correct to me. The
> pole is lat/lon  (-32.5, 10).  So ( -24, -5 ) -> ( -32.5, 10 ) and
> (-7,063, 9.563 ) -> ( -15.563, 24.563 ) Since the rotation angle is 0,
> there is no rotation.    The lat/lon increment is .065535004
>
> The problem is the calculated corner from the starting point of
> ( -32.5, 10 ) ends up at ( -17.23, 27.76 ) instead of the given one
> ( -15.563, 24.563 ).  It's a couple of degrees off.  I don't think it's a
> rounding err, so I must be missing something.
>


I think I see your problem.   In the rotated pole coordinate, the grid
is uniformly spaced in lat/lon, but in the rotated pole coordinate,
it *is not*.   You therefore need 2D arrays to hold the true lon/lat
values, and you need to compute each of these values using the formula.

Rich,

OK, I duplicated the code and the results still are not correct. ie

should be lon= 24.563 and lat =  -15.563

straight increment, had error in first calculation
lon[ 234 ] = 25.269655932, lat[ 272 ] = -14.7400139159998

results from duplicated rtll
almd=44.4907691654722, aphd=-43.1948333734209

How about you send the lat lon values thru MatLab and see what values you
get for the ending coordinates.

Robb..

In other words, you need to loop thru each grid point(e.g.):

for j=1:272
  lat=lat(j);
  for i=1:234
    lon=lon(i)
    [lon_true(i,j),lat_true(i,j)]=rtll(pole_lon,pole_lat,lon(i),lat(i));
  end
end

and lon_true, lat_true will be 2D arrays:

lon_true(lat,lon)
lat_true(lat,lon)


> On Mon, 29 Mar 2004, Richard Signell wrote:
>
> > Robb,
> >
> > If it's helpful, here is my MATLAB function for deriving true lon,lat
> > from the rotated pole lon,lat.
> >
> > function [almd,aphd]=rtll(tlm0d,tph0d,tlmd,tphd);
> > %-------------------------------------------------------------------------
> > % RTLL transforms rotated coordinates (tlmd,tphd) into
> > % ordinary geographic coordinates (almd,aphd). i/o decimal degrees (DD)
> > % tlm0d, tph0d: lon e lat of the center of rotation ("North Pole") in
> > DD.
> > %
> > dtr=pi/180.;
> >
> > ctph0=cos(tph0d*dtr);
> > stph0=sin(tph0d*dtr);
> >
> > stph=sin(tphd*dtr);
> > ctph=cos(tphd*dtr);
> > ctlm=cos(tlmd*dtr);
> > stlm=sin(tlmd*dtr);
> >
> > aph=asin(stph0.*ctph.*ctlm+ctph0.*stph);
> > cph=cos(aph);
> >
> > almd=tlm0d+asin(stlm.*ctph./cph)/dtr;
> > aphd=aph/dtr;
> > % end
> >
> >
> > -Rich
> >
> >

--
Dr. Richard P. Signell                 |  signell@xxxxxxxxxxxxxxxxx
NATO/SACLANT Undersea Research Centre  |    Tel: (+39) 0187 527 381
Viale San Bartolomeo 400               |    Fax: (+39) 0187 527 331
19138 La Spezia, ITALY  --> From USA/CANADA, use: APO AE 09613-5000


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==============================================================================
Robb Kambic                                Unidata Program Center
Software Engineer III                      Univ. Corp for Atmospheric Research
rkambic@xxxxxxxxxxxxxxxx                   WWW: http://www.unidata.ucar.edu/
==============================================================================