IDRISI->GRASS (negative numbers)

[Michael Shapiro]

>How to get IDRISI files containing negative values to GRASS.

I need a little more information about IDRISI. Are  they  2  byte
data  values? If so you have to work a little harder to get GRASS
to see them as negatives. GRASS stores negative in a  funny  (but
machine  portable)  way.  They  are  alway 4 byte numbers. 2 byte
negatives will look like large positive values to GRASS. Assuming
byte   swapping   is  putting  the  numbers  in  highbyte/lowbyte
ordering, and assuming that "IDRISI" is using  2s  complement  (a
likely  possibility)  then  -1 will look like 1111111111111111 in
IDRISI, to GRASS like  255*256+255  =  65535,  -2  in  idrisi  is
1111111111111110  or  255*256+254 = 65534. The largest positive 2
byte value is 0111111111111111 = 128*256 + 255 = 32767. Thus  you
could  import  the  IDRISI  files (after byte swapping) and run a
mapcalc script to convert large positive  values  back  to  their
correct negatives

  r.mapcalc "grass = if (idrisi < 32768, idrisi, idrisi - 65536)"

This assumes 2 byte data, high/low byte ordering after  swapping,
2s complement negatives, etc. If you have something else then you
need to work out a similar scenario.

-----------------------------------------------------------------
Michael Shapiro                        U.S. Army CERL                  
email:   shapiro at zorro.cecer.army.mil  Environmental Division          
phone:   (217) 352-6511  ext 526       P.O. Box 9005                   
fax:     (217) 373-7222                Champaign, Ill. 61826-9005
-----------------------------------------------------------------