| MODIS43: Question on processing albedo lookup table... [message #53963] |
Thu, 10 May 2007 09:17 |
kim20026
Messages: 54
Registered: November 2006
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Member |
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Hi, everyone! I am back to ask a question on albedo processing with
IDL.
I am dealing with MODIS43 for processing albedo. As most of you may
know, however, MODIS43 only provides black sky albedo (BSA) and white
sky albedo (WSA). We need to calculate actual albedo based on these
values.
Recently I downloaded the look up table (sky_lut.dat) and several
source codes from ...
http://www-modis.bu.edu/brdf/userguide/tools.html
However, these files are written in C. Because I am not familiar with
C yet, I got stuck now.
At this point, I have two questions...
1. Is there anyone who has been working on MODIS43-albedo processing
with IDL (not with C)? If there is someone, please let me know.
2. I found the 'spawn' statement dealing with the codes written in
other computer languages (i.e: FORTRAN or C) in IDL environment. But I
don't know how to begin. If I want to run 'tell_skyl.c' code with
'spawn' statement, what can I do?
Harry
--------------------------------
Tell_skyl.c
/**
* Calculate fraction of diffuse skylight through a look-up table
* which was pre-calculated with the 6S code for seven MODIS land
spectral
* bands plus three broadbands (visible, NIR and shortwave)
* [-od] aerosol optical depth at 550nm [0..1]
* [-szn] solar zenith angle [0..89]
*
* by Feng Gao at Boston University on July 2002
* contact fgao@bu.edu
*
**/
#include <stdio.h>
#include <math.h>
typedef struct
{
int aerosol_type;
int bandno;
float solar_zenith;
float optical_depth;
float skyl;
} SKYL;
void read_skyl_table (float skyl_lut[2][10][90][50]);
float get_skyl (SKYL s,float skyl_lut[2][10][90][50]);
void main(int argc, char **argv)
{
int i,band;
float skyl_lut[2][10][90][50];
SKYL s;
if(argc!=5) {
printf("Usage example: %s -od 0.2 -szn 30.0\n", argv[0]);
exit(1);
}
/* parse command line */
for(i=1;i<argc;i++){
if(strcmp(argv[i],"-od")==0)
s.optical_depth=atof(argv[++i]);
else
if(strcmp(argv[i],"-szn")==0)
s.solar_zenith=atof(argv[++i]);
else{
printf("\nWrong option:%s\n",argv[i]);
printf("Usage example: %s -od 0.2 -szn 30.0\n", argv[0]);
exit(1);
}
}
/* read lookup table from input file "skyl_lut.dat" */
read_skyl_table(skyl_lut);
printf("BAND Fraction_of_diffuse\n");
for(band=0; band <10; band++) {
s.bandno=band;
/* assume continental type, but finally need to get it from
BRDF_type
0: continental 1: maritime */
s.aerosol_type=0;
/* get SKYL from SKYL lookup table (it depends on optical depth,
solar zenith angle, aerosol type, and bands) */
s.skyl=get_skyl(s,skyl_lut);
printf(" %2d %5.3f\n",band+1,s.skyl);
}
}
/*******************************************************
read SKYL LUT from pre-generated LUT file (skyl_lut.dat)
2 aerosol types
10 bands (7 MODIS bands + 3 broad bands (VIS, NIR, SW)
90 degrees (0-89 degrees with 1 degree step
50 optical depth ( 0-1 with 0.02 step)
********************************************************/
void read_skyl_table (float skyl_lut[2][10][90][50])
{
char str[200];
FILE *in;
int aerosol,band,szn,od,NBANDS;
NBANDS=10; /* MODIS defaults */
if((in=fopen("skyl_lut.dat","r"))==NULL) {
printf("Can't open SKYL LUT file (skyl_lut.dat) \n");
exit(1);
}
for(aerosol=0;aerosol<2;aerosol++) {
fscanf(in,"%s %s\n",str,str);
for(band=0;band<NBANDS;band++) {
fscanf(in,"%s %s\n",str,str);
for(od=0;od<51;od++)
fscanf(in,"%s ",str);
for(szn=0;szn<90;szn++){
fscanf(in,"%s ",str);
for(od=0;od<50;od++) {
fscanf(in,"%f ",&(skyl_lut[aerosol][band][szn][od]));
}
}
}
}
fclose(in);
}
/* get SKYL value from LUT */
float get_skyl (SKYL s,float skyl_lut[2][10][90][50])
{
int szn,od;
szn=(int)(s.solar_zenith+0.5); /*get solar zenith index */
if(szn==90) szn=89;
od=(int)(s.optical_depth/0.02+0.5); /*get optical depth index */
if(od>=50) od=49;
return skyl_lut[s.aerosol_type][s.bandno][szn][od];
}
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