[GRASS-user] supervised image classification using textural aspects

[Jonathan Greenberg]

Wout:

    This is more of a general response, rather than a "how to" for 
GRASS.  At 1m, you are absolutely going to need to include some level of 
spatial processing (texture being the "brute force" way of getting at 
these sorts of things).  At that resolution, trees become multi-pixel 
objects, and there will be more spectral variation within a tree crown 
than between any two trees.  Which textures to try are an issue you will 
need to resolve by experimentation -- variance is often an important 
factor, average less so.  The window size choice is extremely important, 
because each window size is picking up different pieces of information.  
For instance, for large trees at 1m, a 3x3 window is going to be picking 
up within-crown variation, so you will get high values near the 
sunlit-to-shadow transition, and near the crown edges, but low variation 
within the shadow or within the sunlit portions of the tree.  Your 
window should be larger than a tree crown if you expect to get fairly 
similar values within the tree crown (which is critical if you want to 
approach this in a pixel based approach).  I don't recommend pixel-based 
approaches for macro-pixel objects, however.

    When working with "hyperspatial" remote sensing data, keep in mind 
you are classifying "trees" as unique landscape objects (polygons, 
really), not the less well defined "forest" -- as such, you should try 
to employ object-based approaches.  You can google scholar "tree crown 
remote sensing" to get some ideas on how people approach this problem.  
Keep an eye out for papers by Lefsky, Pouliot, Popescu, Wulder, and 
Leckie, amongst others.  If you want to understand how to scale from 
tree crown objects to a "forest" I'll tout one of my papers:

http://casil.ucdavis.edu/docman/view.php/52/141/greenbergetal2006b.pdf

    It would be cool to implement some of these algorithms in GRASS, but 
to my knowledge there is no package (you'd have to write one) -- in 
fact, very few remote sensing packages have even the beginnings of these 
capabilities, although some of the authors I mention above may be 
willing to share their code. 

--j

Wout Bijkerk wrote:
> Hello everybody,
>
> I am trying to perform a supervised classification of false color
> images. The resolution of the bands (IR,R,G) is 1 meter. Additionally I
> can use a DEM as input ( hor. res. = 5m), but apparently null-values
> within the training areas are causing some problems (see
> http://lists.osgeo.org/pipermail/grass-user/2008-June/045261.html) so I
> am not using the DEM for the moment. I intend to use the combined
> radiometric and geometric modules i.gensigset and i.smap.
>
> Looking at the images, I wonder if including textural features within
> the images would be usefull: a forest canopy has a far coarser texture
> than a grassland. Also in the Grassbook this is mentioned, and for the
> supervised classification of saltmarshes in Germany, textural features
> are also used (see i.e.
> http://www.nature-consult.de/images/downl/Agit_2008_nature-consult.pdf,
> but in German), but this is not further explained.
>
> This brings me to the following questions:
>
> 1) Is it usefull to make a raster with textural image features  as an
> extra input for i.gensigset / i.smap? The i.gensigset / i.smap procedure
> is partly based on geometry and therefor on texture as well so what does
> a texturemap add?
> 2) if it is usefull, which textural feature is then aproppriate? I have
> been experimenting and until now simply variance seems to make the
> difference between forest and shrubland compared to grassland, and
> reed-vegetation. This was using a windowsize of 5, meaning 5x5 m.
>
> Did anyone have any experience with this?
>
> Regards,
>
> Wout
>
>  
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>   

-- 
Jonathan A. Greenberg, PhD
Postdoctoral Scholar
Center for Spatial Technologies and Remote Sensing (CSTARS)
University of California, Davis
One Shields Avenue
The Barn, Room 250N
Davis, CA 95616
Cell: 415-794-5043
AIM: jgrn307, MSN: jgrn307 at hotmail.com, Gchat: jgrn307