[GRASS-user] get the shifted minimum from a raster series

[Frank David]

Hi,

Thanks for your reply. I can have numerous raster (several hundreds). I 
want to find a minimum but with a tolerance, it's why I want to be able 
to shift the value. In fact I want to sort the value of my all input 
raster for each x,y cell and get the second or third, or any position in 
the sorted list, to build my output raster. My application is to get the 
area not visible from a road (I did 1600 viewshed raster from the roads 
every 250m). I want a tolerance in order that one point of view (top of 
hill) does not reduce too much the hidden area. To do so, a fixed number 
of my point of view (20%) does not reduce the hidden area. The result is 
the area is hidden from 80% of the road.

I already wrote a python script that read all rasters for each x,y cell 
and make a array(), sort the array values, and build a raster. It works 
on reasonable number of cell, but seems to fails with large number of 
cells. I'm not a programmer !

I wondered if this kind of treatment exists already in Grass.

If somebody wants to hemps me to develop this, I could be nice. But I 
think it could be an improvement of r.series function.

Sorry for my bad english, I hope you have understand my wishes !

Cheers

Frank

Le 07/02/2019 à 12:04, Veronica Andreo a écrit :
> Hi Ken,
>
> There isn't any build-in function for that in grass, afaik. Depending 
> on how long is your series, you could shift the map list you use in 
> each run and then use r.univar on each output from r.series to get the 
> minimum and by comparison, get the second minimum of the series of 
> maps (a scalar).
>
> However, if what you need is a map of second minimums per pixel, I 
> believe that might require some programming for a new function... Do 
> you know any other software which has this function? Maybe, if there's 
> such thing in Python for example, it could be recycled or used with 
> grass maps in a script... Dunno, just thinking out loud
>
> best,
> Vero
>
> El jue., 7 feb. 2019 07:16, Ken Mankoff <mankoff at gmail.com 
> <mailto:mankoff at gmail.com>> escribió:
>
>     Hi Frank,
>
>     On 2019-02-07 at 08:27 +0100, Frank David <frank.david at geophom.fr
>     <mailto:frank.david at geophom.fr>>
>     wrote...
>     > I try to find how to get the shifted minimum value of a series of
>     > raster map. I want to shift by one, or more, the "minimum" value
>     found
>     > on each cell to get the "almost minimum" of the series. Is there an
>     > available function/method to do that ?
>
>     I don't think I fully understand what you want. Are you working
>     with strds? You mention "series". If I wanted to get the
>     second-minimum of a single raster, I would use r.stats with the
>     "-1" flag, sort, and search for the 2nd (or 2nd-to-last).
>
>       -k.
>
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