[GRASS-user] t.rast.algebra with strds of different granularity

[Veronica Andreo]

Dear Soeren,

Thanks much for the explanations and tips. Will try to document this
example, as well :)

Best,
Vero

2017-03-02 22:09 GMT+01:00 Sören Gebbert <soerengebbert at googlemail.com>:

> Hi,
>
> 2017-03-02 13:35 GMT+01:00 Veronica Andreo <veroandreo at gmail.com>:
> > Hi list,
> >
> > Just by mistake, I did an operation between two time series (strds) with
> > t.rast.algebra, and only after a while I realized that they have
> different
> > granularity (while I thought they had the same). One of them is daily
> (from
> > 00:00:00 to 00:00:00, 24 hs intervals) and the other is also daily, but
> > intervals are from 10:30 to 22:30 (12 hours). Anyway, the second one is
> > temporally included/contained in the first one. However, when I make a
> > substraction among them, I get an empty strds as result and no error nor
> > warning message.
>
> The default temporal-topological operator is "equal". The expression:
>
> C = A - B
>
> is the short form of:
>
> C = A {-, equal, l} B
>
> Since your timestamps are not equal, the result is an empty STRDS.
> From my standpoint this is the correct result of this expression, no
> need to raise an error or a warning. r.mapcalc for example will not
> warn you, if the result of your expression is an empty raster map.
>
> Using the correct temporal-topological operator is the key in your
> case. Your suggestion below is correct. You need to specify the
> "contains" relationship to compute the desired results.
>
> The following expression:
>
> C = A {-, contains} B
>
> means, compute the difference between all maps from A and B that
> fulfill the "contains" temporal relationship. The temporal topological
> operation will be evaluated between all maps from A and B. If a map
> from A has a contains relationship to one or several maps of B, then
> the difference between the two/many maps will be computed and the
> timestamp of the left side of the statement (timestamps from A) will
> be used for the resulting maps. You can specify if the left or right
> timestamps should be used for the result maps or the intersection or
> union of the involved map timestamps. Be aware that implicit
> aggregation will be performed if more than one map is found to compute
> the difference.
>
> Best regards
> Soeren
>
> >
> > This is how I wrote the command:
> >
> > t.rast.algebra -n basename=lwr_minus_orig expression="abs_lwr_minus_orig
> =
> > abs(LST_Day_2016_lwr1 - LST_Day_2016_orig)"
> >
> > How should it look like for the temporal topology to be considered, then?
> > Something like:
> >
> > expression="abs_lwr_minus_orig = abs(LST_Day_2016_lwr1 { -, contains }
> > LST_Day_2016_orig)"  ??
> >
> > Shouldn't a warning be raised when granularities are different and no
> > temporal topology operator is used? Dunno if that's possible at all.
> >
> > Thanks for any tips
> >
> > Vero
> >
> > _______________________________________________
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> > grass-user at lists.osgeo.org
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>
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