Hi,
while calculating the NDVI I found following r.mapcalc
behaviour:
r.mapcalc "ndvi=float((lsat.4 - lsat.3)/(lsat.4+lsat.3))"
r.info -r ndvi
min=0.000000
max=0.000000
-> not desired
r.mapcalc "ndvi=float(1.0* (lsat.4 - lsat.3)/(lsat.4+lsat.3))"
r.info -r ndvi
min=-0.473684
max=0.672727
-> looks fine, but multiplication with 1. required
Is there a chance to change r.mapcalc to produce float
when using float()?
Thanks
Markus
Markus Neteler wrote:
while calculating the NDVI I found following r.mapcalc
behaviour:r.mapcalc "ndvi=float((lsat.4 - lsat.3)/(lsat.4+lsat.3))"
r.info -r ndvi
min=0.000000
max=0.000000-> not desired
r.mapcalc "ndvi=float(1.0* (lsat.4 - lsat.3)/(lsat.4+lsat.3))"
r.info -r ndvi
min=-0.473684
max=0.672727-> looks fine, but multiplication with 1. required
Is there a chance to change r.mapcalc to produce float
when using float()?
float() does what it is supposed to, i.e. it converts its argument to
a float. In your fist example, the argument to float() is always
(integer) zero. Nothing which could be done to the float() function
would make it able to recover lost data.
If you want floating-point division, at least one of the arguments has
to be a floating-point value. Multiplying one of them by 1.0 will
produce a floating-point result, as will using float():
r.mapcalc "ndvi=float(lsat.4 - lsat.3) / (lsat.4+lsat.3)"
--
Glynn Clements <glynn.clements@virgin.net>