[GRASS-user] detrending time series maps

[Veronica Andreo]

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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