[GRASS-stats] Re: [GRASS-user] Testing i.pca ~ prcomp(), m.eigensystem ~ princomp()

[Hamish]

Markus Neteler wrote:
> > Remember that you have to rescale these data first:
> > 
> > https://lpdaac.usgs.gov/lpdaac/products/modis_products_table
> > -> MOD09Q1      Terra 
>     Surface Reflectance Bands 1–2 
>     Tile      250m 
>     8 Day
> >     https://lpdaac.usgs.gov/lpdaac/products/modis_products_table/surface_reflectance/8_day_l3_global_250m/v5/terra
> >    -> layers
> > 
> > ... MULTIPLY BY SCALE FACTOR ...

Nikos wrote:
> Yep, I know that. But why is it wrong to just use them as they are?


FWIW, the MODIS/Aqua Chlorophyll-a product requires more than simple
linear rescaling:
  r.mapcalc "${map}.chlor_A = 10^(($Slope * $map) + $Intercept)"

http://grass.osgeo.org/wiki/MODIS#Processing_2


so you may need to take care if working directly with raw 0-65535 data.


Hamish