20030701:GEMPAK NIDS display

>From: Unidata User Support <address@hidden>
>Organization: Unidata Program Center/UCAR
>Keywords:


Daryl,


Following up on our converastion last week regarding obtaining a lat/lon
mapping of a NIDS product:

 GRDAREA  = 35.8;-101.4;48.0;-87.1
 PROJ     = mer
 KXKY     = 200;200
 CPYFIL   =  
 GFUNC    = n0r
 RADTIM   = 030701/1700
 RADDUR   = 30
 RADFRQ   =  
 STNFIL   = nexrad.tbl
 RADMODE  = pc
 SATFIL   = test.gray
 COMPRESS = no
 GEMPAK-NEX2GINI>

The above produces a GINI file called test.gray. The GINI header will
be 533 bytes, so the remaining 40000 bytes (200x200) are the pixel
values of a lat/lon raster with corners as specified. This was what
I believe you were looking for.

There are several ways to deal with the raster. The easiest is to just treat
the values a gray scale (byte) values which can be display using tools
such as imagemagick: display -size 200x200+533 -depth 8 test.gray

Another method would be to convert the raw indicies to RGB values using
the GEMPAK color table (eg osf_ref16.tbl), such as :


/* Note, names hardcoded here as an example */
#include <stdio.h>
#include <string.h>

int main()
{
int i, ncol;
FILE *fp;
char line[256];
int ir, ig, ib, ix;

unsigned char carr[40000];

unsigned char red[256];
unsigned char green[256];
unsigned char blue[256];

/* open color table */
if((fp = fopen("/home/gempak/NAWIPS/gempak/tables/luts/osf_ref16.tbl","r")) == 
NULL)
   {
   printf("color table file not found\n");
   exit(-1);
   }

/* read color table values */
ncol = 0;
while(fgets(line,255,fp) != NULL)
   {
   if(line[0] == '!') continue;
   sscanf(line+23,"%d %d %d",&ir, &ig, &ib);
   red[ncol] = ir;
   green[ncol] = ig;
   blue[ncol] = ib;
   ncol++;
   }
fclose(fp);

/* open image file, seek to start of raster, load image */
fp = fopen("test.gray", "r");
fseek(fp,533, SEEK_SET);
fread (carr, 1, 40000, fp);
fclose(fp);

/* output RGB values */
fp = fopen("test.rgb", "w");
for(i=0;i<40000;i++)
   {
   ix =  ( (float)(ncol-1) / 255.0 ) * (float)carr[i];
   fwrite(&red[ix], 1, 1, fp);
   fwrite(&green[ix], 1, 1, fp);
   fwrite(&blue[ix], 1, 1, fp);
   }
fclose(fp);
}


Now the image can be displayed (or converted to the preferred format) with
display -geometry 900x900 -size 200x200 -depth 8 test.rgb
or
convert -size 200x200 -depth 8 test.rgb test.gif

So, this is all readily done using simple tools. But, I would believe that 
the real solution would be to create a "nex2gini" that would write the
GEOtiff directly (since it knows the projection and values). If this
is what you need, I can create that tool as a one step process.

As a dataset for a server, if you were to use nex2gini to create a single 
composite mosaic for the entire country, then selecting the appropriate
region from the larger image for the display product would be straight forward.

Steve Chiswell