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%] _______________________________________________ grass-stats mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-stats
