[GRASS-user] detrending time series maps

[Ivan Marchesini]

Hi Veronica

Thank you. It goes in the direction of my idea evn if  my problem is 
exactly trying to take into account the correct gaps between that data

I have another idea.

if it works I will come back here to explain how I did

thank you again

Ivan


On 22/12/23 13:45, Veronica Andreo wrote:
> Hello Ivan,
>
> AFAIU you could use the slope and offset maps from t.rast.series 
> within t.rast.algebra to detrend the values of the maps within the 
> strds, something like "detrended_strds = trend_strds - 
> (trend_strds*map(slope) + map(offset))". Others suggest, to detrend by 
> subtracting the previous value, i.e. that would imply using the 
> temporal algebra with the temporal index, something like 
> "detrended_strds = trend_strds[1] - trend_strds[0]".
>
> I haven't tested any of these, just a couple of ideas ;-) However, I 
> do not know how this might interact with seasonality within data, or 
> irregular gaps.
>
> hth somehow
> Vero
>
> El vie, 22 dic 2023 a las 5:10, Ivan Marchesini via grass-user 
> (<grass-user at lists.osgeo.org>) escribió:
>
>     Dear colleagues
>
>     I would like to the advantage of the t.* modules for detrending a
>     strd.
>
>     In the strd I have earth observation data irregularly sampled (2 or 3
>     times per month), in the period November-February, for 7 years.
>     They are
>     not equally spaced (i.e gaps have different duration)
>
>     A simple t.rast.series analysis (opion=slope,offset) highlights that
>     probably there is a descending trend when considering the maps
>     ordered
>     by id.
>
>     I would like to fit a proper time depending fitting curve for each
>     pixel
>     and then subtract the function from the real data.
>
>     any hints on how I can do this task exploiting the GRASS GIS
>     modules or
>     some simple bash/python scripting?
>
>     thank you
>
>     Ivan
>
>
>
>
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