[GRASS-user] v.to.rast, r.to.vect, "stable" transformation

[Johannes Radinger]

Hi again,

> On 26/01/12 10:29, Johannes Radinger wrote:
> > Hello,
> >
> > as it seems that there was something wrong with my post yesterday I send
> it
> > again (if everything was ok already yesterday, sorry for double
> posting):
> >
> >
> > For a certain application I need to transform a river from the
> > raster format into the vector format and vice versa.
> >
> > I'd like to know how good does it work to tranform a line
> > feature from vector into raster and back again into
> > a vector? Is there any "stable" line which doesn't
> > change anymore after x-times transforming back and
> > forwards? I haven't had time to test it but I thought about:
> >
> > v.to.rast
> > r.thin
> > r.to.vect
> >
> > this should result in a "stable" line or does it
> > need several iterations?
> 
> I think the best way is to try, but some remarks here:
> 
> - I don't think that r.thin avec v.to.rast will change anything. 
> v.to.rast should already limit the raster version of the line to a 
> widthof one pixel.
> 
> - The main issue here will probably be the resolution.
> 
> - To test, just try running v.to.rast/r.to.vect several times in a loop 
> and then compare the end result to the original river...
>

I tried it to run v.to.rast/r.thin/r.to.vect... I am not sure but it seems
that already the first run creates a "stable" vector as the the vector is just the connection of the midpoints of the cells (so only a limited number of vector directions: horizontal, vertical, diagonal). So far as I think this must be a stable vector....

The r.thin process is an important step (as recommended also in the manual) and should not be skipped at all! And so far as I think now it isn't an issue of the resolution as it is always the procedure of reconnecting the cell-midpoints...

Maybe I just see something wrong..anyway i thought about testing the stability in a loop, but I got stucked in the correct map calc operation.
What is the correct (boolean) operator to get:
1) NULL + NULL = NULL
2) not NULL + not NULL = NULL
3) not NULL + NULL = 1
4) NULL + not NULL = 1

this is just to find out if the two rasters are congruent (of the same shape).

Maybe anyone has some suggestions...

best regards,

Johannes




> Moritz
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