[GRASS-user] low radiance values after i.atcor and i.topo.corr

[Glynn Clements]

maning sambale wrote:

> >> After implementing gain and bias correction and topographic
> >> correction, I get very low values for landsat data data
> >> for example range of values of band1:
> >>
> >> GRASS 6.3.1svn (p112r056):~ > r.info lsat7_2000.toar.1 -r
> >> min=0.030303
> >> max=0.372067
> > ...
> >> Running these range of values to an i.cluster reports very low
> >> separability:
> >
> > As far as I see it is related to the fact that i.cluster reads in as
> > integer (CELL)
> > through G_get_c_raster_row():
> >  http://download.osgeo.org/grass/grass6_progman/gis_2get__row_8c.html
> > ...
> >> I assume this is due to the very low range of values (0.0xxx) of the
> >> input images.  Running r.rescale.eq to "stretch" the range of values
> >> doesn't also help.
> >
> > You may try to multiply the values with 100 or 1000 using r.mapcalc.
> >
> > Please let us know how it works.
> 
> Thanks!  Multiplying by 1000 with r.mapcalc gives better results.  Any
> chance adding floating points (FCELL) to i.cluster?

I had a brief look at the code[1], and cannot see any obvious reason
why the values would need to be integers, so I'm assuming that it's
just a legacy of the days before FP support was added.

[1] Most of the code is actually in the lib/imagery/c_*.c files. That
should either be made part of i.cluster (nothing else uses it), or at
least split into a separate library.

-- 
Glynn Clements <glynn at gclements.plus.com>