Re: Map image with a sparse array

[William Daffer <whdaffer(at)earthlink.net>]

"Liam E. Gumley" <Liam.Gumley@ssec.wisc.edu> writes:

> whdaffer@my-deja.com wrote:
> 
> >   Can anyone see what I'm doing wrong?
> >
> >   When I tv the mapped image and compare it to the unmapped image, it's
> > clear that the mapped image has included some 'bad' data in the
> > calculations of points within the grid.
> >
> >   To be fair, this possibility is not explicitly excluded in the help
> > file. If I'm reading it correctly, all it says is  that output grid
> > values  which are within the valid mapping limits but which exceed input
> > maxima will be set to the 'bad data' value, not that the input 'bad
> > data' value will be excluded from caluclations.
> >
> >   The effect is not large, but it isn't small either. The person I'm
> > doing the work for noticed it right off.
> >
> >  By the way, I did try your method first, (bytscl, then map_image) and
> > got similar results, but thought that I should make the bytscl be the
> > last step, since it reduces the 'color' resolution from 360 values to
> > about 200. I'll go back and try your method again. Perhaps there's some
> > magic that I'm not seeing.
> 
> William,
> 
> I pulled that section of code from a procedure that projects a 720x360
> global grid with missing data onto various map projections. I cannot see any
> hint that the missing data corrupted the resulting image:
> ftp://origin.ssec.wisc.edu/pub/gumley/IDL/grid_project002.gif
> 
> Cheers,
> Liam.
> 

  Liam;

    It looks like your array is considerably more filled-out than
   mine.

   Go to ftp://catspaw.jpl.nasa.gov/pub/sparse-array

  The three files therein are:

   h_mss_c249-v1.jpg : map_image then bytscl
   h_mss_c249-v2.jpg : bytscl then map_image
   tarray.gif        : The data, array size unchanged, 
                       bytscl'd and tv'd only

   The colored lines in tarray.gif are the data, the grey background
   are the grid elements with no data. 


   I keep meaning to have a close look at map_image, but other
   exigencies have intruded. I'm not at all sure that this isn't a
   problem of my own making.

William


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