Nikos: > >> I tested... tested... & tested... (almost all tests below) and I just > >> can't understand WHY i.pca does NOT perform data centering when using > >> MODIS data (raw, rescaled at 0~255 or 0~254). > > > > The attached html file (exported from Tomboy note-taker) is (I suppose) > > easier to read :-)
Roger: > Not easier to read, please simply post things on a website! Hmmm!!? I thought opening this html file in a web-browser will look just fine. Sorry. > In your previous mail which you deleted here, you asked whether the code > of i.pca shows if the input data is centred. Yes, it is centred, the > function calc_mu(), as its name suggests, does this. The mean values are > then passed to the function calculating the covariance matrix: > calc_covariance(), where the means are subtracted from the input values of > bands before accumulating the covariance matrix. Good. That's data centering. > Is is possible that where > you do not see any effect of center=TRUE in prcomp that the means are all > zero? Sure, when I _manually_ center the data using r.mapcalc. But what happens with the untouched modis bands [1] and i.pca delivers different numbers is what makes no sense to me. BTW, the data are available (the same as with the previous problem concerning the NULLs). > You will find reading imagery/i.pca/main.c interesting and not hard. :-). I tried once but there are many programming variables which do not make sense on first sight. And this takes time... :-p Is there any explanation why i.pca on mod_b2, mod_b6 and mod_b7 gives the same results as prcomp(mod_b267, center=FALSE, scale=FALSE) when their mean is _!=0_ ? Thanks, Nikos --- [1] r.univar of # mod_b2 n: 88185 minimum: 0 maximum: 5504 range: 5504 mean: 2686.22 mean of absolute values: 2686.22 standard deviation: 535.612 variance: 286880 variation coefficient: 19.9392 % sum: 236884367 # mod_b6 n: 88202 minimum: 93 maximum: 5488 range: 5395 mean: 2702.4 mean of absolute values: 2702.4 standard deviation: 645.377 variance: 416511 variation coefficient: 23.8816 % sum: 238356766 # mod_b7 n: 88148 minimum: 0 maximum: 4133 range: 4133 mean: 1740.07 mean of absolute values: 1740.07 standard deviation: 536.718 variance: 288067 variation coefficient: 30.8447 % sum: 153383351 _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
