Ubuntu Manpage: i.pca - Principal components analysis (PCA) for image processing.

name
keywords
synopsis
description
notes
example
see also
authors
source code

NAME

       i.pca  - Principal components analysis (PCA) for image processing.

KEYWORDS

       imagery, transformation, PCA, principal components analysis

SYNOPSIS

       i.pca
       i.pca --help
       i.pca  [-nf] input=name[,name,...] output=basename  [rescale=min,max]   [percent=integer]   [--overwrite]
       [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:
       -n
           Normalize (center and scale) input maps
           Default: center only

       -f
           Output will be filtered input bands
           Apply inverse PCA after PCA

       --overwrite
           Allow output files to overwrite existing files

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       input=name[,name,...] [required]
           Name of two or more input raster maps or imagery group

       output=basename [required]
           Name for output basename raster map(s)
           A numerical suffix will be added for each component map

       rescale=min,max
           Rescaling range for output maps
           For no rescaling use 0,0
           Default: 0,255

       percent=integer
           Cumulative percent importance for filtering
           Options: 50-99
           Default: 99

DESCRIPTION

i.pca is an image processing program based on the algorithm provided by Vali (1990), that processes n (n
>= 2) input raster map layers and produces n output raster map layers containing the principal components
of the input data in decreasing order of variance ("contrast"). The output raster map layers are
assigned names with .1, .2, ... .n suffixes. The numbers used as suffix correspond to percent importance
with .1 being the scores of the principal component with the highest importance.

The current geographic region definition and MASK settings are respected when reading the input raster
map layers. When the rescale option is used, the output files are rescaled to fit the min,max range.

The order of the input bands does not matter for the output maps (PC scores), but does matter for the
vectors (loadings), since each loading refers to a specific input band.

If the output is not rescaled (rescale=0,0, the output raster maps will be of type DCELL, otherwise the
output raster maps will be of type CELL.

By default, the values of the input raster maps are centered for each map separately with x - mean. With
-n, the input raster maps are normalized for each map separately with (x - mean) / stddev. Normalizing
is highly recommended when the input raster maps have different units, e.g. represent different
environmental parameters.

The -f flag, together with the percent option, can be used to remove noise from input bands. Input bands
will be recalculated from a subset of the principal components (inverse PCA). The subset is selected by
using only the most important (highest eigenvalue) principal components which explain together percent
percent variance observed in the input bands.

NOTES

       Richards  (1986) gives a good example of the application of principal components analysis (PCA) to a time
       series of LANDSAT images of a burned region in Australia.

       Eigenvalue and eigenvector information is stored in the output maps’ history files. View with r.info.

EXAMPLE

       PCA calculation using Landsat7 imagery in the North Carolina sample dataset:
       g.region raster=lsat7_2002_10 -p
       i.pca in=lsat7_2002_10,lsat7_2002_20,lsat7_2002_30,lsat7_2002_40,lsat7_2002_50,lsat7_2002_70 \
           out=lsat7_2002_pca
       r.info -h lsat7_2002_pca.1
          Eigen values, (vectors), and [percent importance]:
          PC1   4334.35 ( 0.2824, 0.3342, 0.5092,-0.0087, 0.5264, 0.5217) [83.04%]
          PC2    588.31 ( 0.2541, 0.1885, 0.2923,-0.7428,-0.5110,-0.0403) [11.27%]
          PC3    239.22 ( 0.3801, 0.3819, 0.2681, 0.6238,-0.4000,-0.2980) [ 4.58%]
          PC4     32.85 ( 0.1752,-0.0191,-0.4053, 0.1593,-0.4435, 0.7632) [ 0.63%]
          PC5     20.73 (-0.6170,-0.2514, 0.6059, 0.1734,-0.3235, 0.2330) [ 0.40%]
          PC6      4.08 (-0.5475, 0.8021,-0.2282,-0.0607,-0.0208, 0.0252) [ 0.08%]
       d.mon wx0
       d.rast lsat7_2002_pca.1
       # ...
       d.rast lsat7_2002_pca.6
       In this example, the first two PCAs (PCA1 and PCA2) already explain 94.31% of the  variance  in  the  six
       input channels.

       Resulting PCA maps calculated from the Landsat7 imagery (NC, USA)

AUTHORS

       David Satnik, GIS Laboratory

       Major modifications for GRASS 4.1 were made by
       Olga Waupotitsch and Michael Shapiro, U.S.Army Construction Engineering Research Laboratory

       Rewritten for GRASS 6.x and major modifications by
       Brad Douglas

       Last changed: $Date: 2015-09-14 19:06:11 +0200 (Mon, 14 Sep 2015) $

SOURCE CODE

       Available at: i.pca source code (history)

       Main index | Imagery index | Topics index | Keywords index | Graphical index | Full index

       © 2003-2018 GRASS Development Team, GRASS GIS 7.4.0 Reference Manual