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

Markus Metz markus.metz.giswork at googlemail.com
Tue Mar 31 11:26:56 EDT 2009

Nikos Alexandris wrote:
> Nikos:
>>> Hi Agus. The difference is:
>>> Using SPOT (range up to max. 255):
That implied to me that the max possible value of SPOT was used, not the 
max observed (often the max observed is also the max possible). In my 
experience, when comparing imagery or derived products from different 
sensors, the least errors occur if everything is rescaled to the same 
range, e.g. 0 - 255 for surface reflectance or -1 - 1 for NDVI, whatever 
is convenient or appropriate.
Note that the scale factor for MODIS is 0.0001 which would give you a 
new range of -0.01 - 1.6 (I hope I got my math right), not 0 - 255 as 
for e.g. SPOT or Landsat.

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