[GRASS-user] [GRASSLIST:1074] Re: interpolation from vector points

[David Finlayson]

I understand the problem you are having. What we need is a v.hull with
a tension parameter so that it sticks closer to the data points.

Anyway, try this:

1. Buffer your data points with a sufficiently wide buffer that the
internal data area merges together into a single polygon. Note the
buffer distance.

2. Convert the polygon to a raster with a value = 1

3. Select all cells that are null (i.e., outside the data area)

4. buffer the "no data" area to trim off the excess data region
created in step 1. The idea is to cut back into the data region about
the same distance you buffered in the first place

5. Select all the null cells in step 4 (the data region) and make this your MASK

6. Trim the interpolated map by the MASK created in step 5.


On 5/19/06, Luca Penasa <luca.penasa at email.it> wrote:
> Thanks for your help...
> I tried using v.hull but the result wasn't what i hope... v.hull create
> a vector containing a polygon, a centroid and an area object... the
> command works fine but i need to obtain a more precise job... what i
> need is a polygon that connect all the external points without to
> calculate (like v.hull do) the minor length for the polygon.
> Is there a way to obtain this?
>
> besides, i had some problems buffering the vector polygon that iv
> created with v.hull... using the v.buffer on the vector with a
> buffer-value more than 70 meters i obtain a strange figure with a hole
> and too much centroid... any idea?
>
>
>
> Michael Barton wrote:
> > If you work with v.surf.rst parameters, you should be able to get very good
> > results for your interpolation.
> >
> > As you imply, however, you can sometimes get better (or more accurate)
> > results if you limit the raster interpolation to the area of the points. You
> > can do this by calculating a convex hull around the points. This is the
> > minimum polygon that will contain all the points. You can create a convex
> > hull in GRASS with v.hull.
> >
> > Use v.to.rast to convert the convex hull to a raster map. You can use
> > v.buffer to expand the convex hull a bit before you convert or use r.grow
> > to expand it after you convert to raster. Then specify this raster as a mask
> > in v.surf.rst. The resulting interpolated map will only exist in the area of
> > the points.
> >
> > Michael
> > __________________________________________
> > Michael Barton, Professor of Anthropology
> > School of Human Evolution & Social Change
> > Center for Social Dynamics & Complexity
> > Arizona State University
> >
> > phone: 480-965-6213
> > fax: 480-965-7671
> > www: http://www.public.asu.edu/~cmbarton
> >
> >
> >
> >
> >> From: Luca Penasa <luca.penasa at email.it>
> >> Date: Mon, 08 May 2006 11:22:32 +0200
> >> To: <GRASSLIST at inet.baylor.edu>
> >> Subject: [GRASSLIST:1031] interpolation from vector points
> >>
> >> Iv a mapset with a lot (200) of point. Every point is a pollution
> >> indicator (a tree, a bush...) with many informations, one of these is a
> >> lichens biodiversity index.
> >> What i want to do is to interpolate this index value to obtain a map of
> >> biodiversity of the city covered by the survey.
> >> Iv done the interpolation (with no so good results) but now i need to
> >> cut the raster obtained around the external points... maybe i have to do
> >> something like a buffer around the external points of survey and cut the
> >> map obtained from interpolation. How can i do that without have 'holes'
> >> in the internal part of the map? is there a way to do that?
> >>
> >>
> >> Sorry for my English
> >>
> >> Luca
> >>
> >>
> >>
> >
> >
>
>
>
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-- 
David Finlayson