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

[maning sambale]

On 6/18/08, Glynn Clements <glynn at gclements.plus.com> wrote:
>
> 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.
Excellent!  I hope it gets fixed.  I am glad reporting such problems
(though at times trivial) helps improve the GRASS code.

cheers,

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


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