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

[Markus Neteler]

On Sat, Jun 21, 2008 at 11:16 PM, Glynn Clements
<glynn at gclements.plus.com> wrote:
>
> Markus Neteler wrote:
>
>> > I'm just not familiar with the specifics of i.cluster (e.g. whether it's finding
>> > clusters in geographic space or colour space, what the output
>> > signifies, etc).
>>
>> It takes several maps and builds up a multi-dimensional feature space
>> from the pixel (spectral, ...) values. Those are then clustered.
>
> Right. So the geographic position of each cell is ignored; a group of
> rasters is just a "set" of points in "colour space" (i.e. each
> coordinate is the intensity of a particular band)?

Yes.
The i.gensiset/i.smap approach is different and also includes the spatial
position (hence it performs usually better). See also my other mail.

>> >> What's needed since we cannot change the format
>> >
>> > No? If the format was changed to %g (i.e. like %f for "typical"
>> > values, like %e for particularly large or small values), it would
>> > remain compatible with anything using scanf("%f"), atof(), strtod()
>> > etc, as those functions all understand exponential notation.
>>
>> Ah, sounds good.
...
>    printf("%g\n", 0.000100001);
> =>      0.000100001
>
>    printf("%g\n", 0.000099999);
> =>      9.9999e-05
>
> So the default precision should be fine unless you actually get images
> with intensity values accurate to 1PPM or better.

Sounds very promising. If you could indicate where to change, I can
run again my test examples.

Markus