[GRASS-user] Problem creating signature file with i.gensig in

[Moritz Lennert]

On 02/07/14 16:01, lbianchi wrote:
> Hi, this is my first message to the list. I'm using Grass 6.4.3 under
> Linuxmint 16.
>
> I'm trying to make a supervised classification.
>
> After digitizing the training areas, I transformed them from vector to
> raster. Then I've done the following:
>
> /|g.region
> rast=train_area_2013_03_05_raster at PERMANENT,2013_03_05_toar4 at PERMANENT|/

I would just use the image as your reference for the region, not the 
training areas.

Also, what was the region setting when you converted your training areas 
to raster ?

Try first setting your region to the satellite image, i.e.

g.region rast=2013_03_05_toar4 at PERMANENT

Then v.to.rast of the training areas, then i.gensig.

You can also use i.class in 6.4.3 (with x-monitor), or g.gui.iclass in 
the wxgui in grass7 to check your classes.

> Here: http://lists.osgeo.org/pipermail/grass-user/2010-April/055821.html
>
>   there's another similar question, but the only answer says: "I suspect
> that either the training areas are suboptimal or are too small."  What
> does suboptimal means? What is an optimal training area? How big should
> they be?
>
> I have 30 areas, 10 for each class. They aren't too big because I have a
> small site to clasify.

Maybe they are too small ?

>
> What does |"/Signature X not invertible/"| means? Is this the real problem?

Yes. And it's linked to the notion of "suboptimal" training areas.

Training areas should be big enough to contain a significant number of 
pixels (i.e. do not use training areas that are only 5 pixels), but 
small enough to be homogeneous.

Moritz