[GRASS-stats] Re: [GRASS-user] Testing i.pca ~ prcomp(),
m.eigensystem ~ princomp()
Nikos Alexandris
nikos.alexandris at felis.uni-freiburg.de
Tue Mar 31 11:43:32 EDT 2009
Nikos:
> >>> Hi Agus. The difference is:
> >>> Using SPOT (range up to max. 255):
Markus M:
> 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).
Right. Sorry for not being precise.
> 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.
The thing is by multiplying by 0.0001 thing are worse concerning the
*eigenvalues* (the eigenvectors are the same):
# using the MODIS bands as they are
r.info -r mod_b2
min=0
max=5504
# use of i.pca gives
r.info -h pca_mod_b267.1
[...]
Data Description:
generated by i.pca
Comments:
Eigen values, (vectors), and [percent importance]:
PC1 6307563.04 (-0.6353,-0.6485,-0.4192)[98.71%]
PC2 78023.63 (-0.7124, 0.2828, 0.6422)[1.22%]
PC3 4504.60 (-0.2979, 0.7067,-0.6417)[0.07%]
# using MODIS bands multiplied by 0.0001
r.info -r mod_b2_x
min=0
max=0.5504
# using i.pca gives
r.info -h pca.mod_x.1
[...]
Data Description:
generated by i.pca
Comments:
Eigen values, (vectors), and [percent importance]:
PC1 0.06 (-0.6353,-0.6485,-0.4192)[98.71%]
PC2 0.00 (-0.7124, 0.2828, 0.6422)[1.22%]
PC3 0.00 (-0.2979, 0.7067,-0.6417)[0.07%]
More information about the grass-user
mailing list