Image classification

From GRASS-Wiki
Revision as of 08:29, 28 October 2009 by Neteler (talk | contribs) (new tutorial)

Jump to: navigation, search

Image classification

Classification methods in GRASS

supervised 1
supervised 2
radiometric & geometric
Preprocessing i.cluster i.class (monitor digitizing) i.gensig (using training maps) i.gensigset (using training maps)
Computation i.maxlik i.maxlik i.maxlik i.smap

Interactive setup

  • i.class - Generates spectral signatures for an image by allowing the user to outline regions of interest.
The resulting signature file can be used as input for i.maxlik or as a seed signature file for i.cluster.


  • i.cluster - Generates spectral signatures for land cover types in an image using a clustering algorithm.
The resulting signature file is used as input for i.maxlik, to generate an unsupervised image classification.

Unsupervised classification

  • i.maxlik - Classifies the cell spectral reflectances in imagery data.
Classification is based on the spectral signature information generated by either i.cluster, i.class, or i.gensig.

Supervised classification

  • i.smap - Performs contextual (image segmentation) image classification using sequential maximum a posteriori (SMAP) estimation.

Small classification tutorial

Required: GRASS >= 6.4.0

We are using Landsat satellite data prepared for the North Carolina sampling site. It is a multispectral dataset where each channel is stored in a separate map.

We are using the North Carolina location here.

Warming up with Landsat TM

 # look what maps are there
 g.list rast
 # set to one of the color channels and print current region settings
 g.region rast=lsat7_2002_10 -p
 # look at some Landsat map (1: blue, 2: green, 3: red, 4: near infrared etc.)
 d.mon x0
 d.rast lsat7_2002_10
 d.rast lsat7_2002_20
 d.rast lsat7_2002_30 
 d.rast lsat7_2002_40

We can easily query the spectrum for individual points:

 d.what.rast lsat7_2002_10,lsat7_2002_20,lsat7_2002_30,lsat7_2002_40

Histogram of a channel:

 d.histogram lsat7_2002_10

Create RGB view on the fly, true and false color:

 d.rgb b=lsat7_2002_10 g=lsat7_2002_20 r=lsat7_2002_30
 d.rgb b=lsat7_2002_70 g=lsat7_2002_20 r=lsat7_2002_30
 d.rgb b=lsat7_2002_70 g=lsat7_2002_50 r=lsat7_2002_30
 d.rgb b=lsat7_2002_70 g=lsat7_2002_50 r=lsat7_2002_10

Check thermal channel (encoded to fit into 0.255 data range):

  2. Look up LMIN/LMAX: "Table 11.2 ETM+ Spectral Radiance Range watts/(meter squared * ster * µm)"
  3. we won't go into details here, r.mapcalc would be used to convert to Kelvoin/degree Celsius.
 # look at encoded thermal channel
 d.rast lsat7_2002_61

NDVI as example for map algebra:

 r.mapcalc "ndvi = 1.0 * (lsat7_2002_40 - lsat7_2002_30)/(lsat7_2002_40 + lsat7_2002_30)"
 d.rast.leg ndvi
 r.colors ndvi color=ndvi
 d.rast.leg ndvi

Show selected NDVI "classes":

 d.rast ndvi val=0.3-1.0
 d.rast ndvi val=-1.0--0.1
 d.rast ndvi val=0.0-0.2
 d.rast ndvi val=0.0-0.3

Look at panchromatic channel, compare resolution:

 g.region rast=lsat7_2002_80 -p
 d.erase -f
 d.rast lsat7_2002_80
 d.rast lsat7_2002_30
 d.rast lsat7_2002_80

Image Classification

Set region setting to red image:

 g.region rast=lsat7_2002_30 -p

Create a group group=lsat7_2002 subgroup=lsat7_2002 \

Unsupervised classification

Generate unsupervised statistics

 i.cluster  group=lsat7_2002 subgroup=lsat7_2002 sigfile=lsat7_2002_sig classes=10 reportfile=lsat7_2002.txt
 # look at report file
 gedit lsat7_2002.txt 

Assign pixels to classes, check quality of assignment:

 i.maxlik group=lsat7_2002 subgroup=lsat7_2002 sigfile=lsat7_2002_sig class=lsat7_2002_class reject=lsat7_2002_reject
 d.rast.leg lsat7_2002_class
 d.mon x1
 d.rast.leg lsat7_2002_reject
 d.mon x2
 d.histogram lsat7_2002_reject

Play with different cluster separations:

 i.cluster group=lsat7_2002 subgroup=lsat7_2002 sigfile=lsat7_2002_sig classes=10 reportfile=lsat7_2002.txt separation=1.5
 i.maxlik group=lsat7_2002 subgroup=lsat7_2002 sigfile=lsat7_2002_sig class=lsat7_2002_class2 reject=lsat7_2002_reject2
 d.mon x4
 d.rast.leg lsat7_2002_class2
 d.rast.leg lsat7_2002_reject2
 d.rast.leg lsat7_2002_reject
 d.rast.leg lsat7_2002_reject2
 d.rast.leg lsat7_2002_reject
 d.rast.leg lsat7_2002_reject2
 d.rast.leg lsat7_2002_class2

Compare to RGB:

 d.rgb b=lsat7_2002_70 g=lsat7_2002_50 r=lsat7_2002_10
 d.rast lsat7_2002_class2 cat=1 -o
 d.rgb b=lsat7_2002_70 g=lsat7_2002_50 r=lsat7_2002_10
 d.rast lsat7_2002_class cat=1 -o
 d.rgb b=lsat7_2002_70 g=lsat7_2002_50 r=lsat7_2002_10
 d.rast lsat7_2002_class cat=1 -o

We do a 3rd attempt with more classes

 i.cluster group=lsat7_2002 subgroup=lsat7_2002 sigfile=lsat7_2002_sig classes=15 reportfile=lsat7_2002.txt 
 i.maxlik group=lsat7_2002 subgroup=lsat7_2002 sigfile=lsat7_2002_sig class=lsat7_2002_class3 reject=lsat7_2002_reject3
 d.rgb b=lsat7_2002_70 g=lsat7_2002_50 r=lsat7_2002_10
 d.rast lsat7_2002_class3 cat=1 -o

Supervised classification

Rlaunch GUI for digitizer:

 g.gui wxpython

 # ... digitize training areas of water, asphalt, forest, uncovered soil; polygons/centroids with attribute
 # advantage: we know what the classes are

Assign also numerical ID for each class in new column lsat7_training
 v.db.addcol lsat7_training col="id integer" lsat7_training
 v.db.update lsat7_training col=id where="name = 'water'" val=1
 v.db.update lsat7_training col=id where="name = 'forest'" val=2
 v.db.update lsat7_training col=id where="name = 'asphalt'" val=3
 v.db.update lsat7_training col=id where="name = 'soil'" val=4 lsat7_training

Transform vector training map to raster model: in=lsat7_training out=lsat7_training use=attr col=id labelcol=name --o
 d.mon x0
 d.rast.leg lsat7_training 

Generate statistics from training areas

 i.gensigset group=lsat7_2002 subgroup=lsat7_2002 sig=lsat7_2002_smap training=lsat7_training

Perform supervised classification

 i.smap group=lsat7_2002 subgroup=lsat7_2002 sig=lsat7_2002_smap  out=smap
 d.rast.leg smap
 d.rgb b=lsat7_2002_10 g=lsat7_2002_20 r=lsat7_2002_30

Vectorize result: smap out=smap feat=area
 d.rgb b=lsat7_2002_10 g=lsat7_2002_20 r=lsat7_2002_30
 d.vect smap  type=boundary col=red

That's it :)

Further reading classification with GRASS