[GRASSLIST:2636] Re: interpolation of area precipitation with s.surf.idw

[Paul Kelly]

Hello

On Mon, 16 Feb 2004, Alexander Fritz wrote:

> Dear GRASS users,
>
> I'm trying to interpolate area precipitation maps with s.surf.idw in
> 5-minute-steps. I have rain gage data in 5-minute steps for several
> stations. Unfortunately not every station has recorded the whole time I'm
> interested in.

> Is it possible to fill these gaps with something like "no
> data" and/or is there an option for s.surf.idw which excludes the stations
> with no recorded data at this time step. Any suggestions?

The whole purpose of interpolation is to fill regions with no recorded
data by interpolating from the surrounding data points. So the concept of
a 'no-data' area in the output doesn't really make sense, IMHO.

Maybe try to define more clearly what you need and think about how you
would define the area with no data. Perhaps s.voronoi and v.to.rast; use
that as a MASK? I presume you have a different sites file for each 5
minute interval and each has all the sites in it but some of them have
missing attributes? In that case, does s.surf.idw not simply ignore the
sites that have missing attributes and interpolate from the rest? (if it
doesn't, please report it as a bug and I might have time too look at it.) You
could run it like this and then mask your output using the results from
s.voronoi or something like that.

But I would think about how best to define your 'no-data' area for each
time interval and if it really makes sense to do that.