[GRASS-user] Re: r.surf.nnbathy interpolation to whole region

[Maciej Sieczka]

Luigi Ponti wrote:
> Maciej Sieczka wrote:
>> Luigi Ponti wrote:

>>> What I meant is that the outer Voronoi cells may extend up to
>>> the region's bounding box, with consequent new Voronoi cells for
>>>  interpolation points outside the convex hull, please see 
>>> http://quartese.googlepages.com/voronoi

>> Anyway - what could be done to overcome the trimming to convex
>> hull effect: you need to make sure that the convex hull for your
>> input cells covers your whole area of interest - ie. enlarge
>> region enough for important input cells not to be omitted. If
>> cells in the input raster are distributed so that a convex hull
>> covering the whole area of interest is not possible, you need to
>> enhance your input raster.

> Please, have also a look at 
> http://www.ems-i.com/gmshelp/Interpolation/Interpolation_Schemes/Natural_Neighbor_Interpolation.htm
> 
> This web page suggests an approach for extrapolation that would get
> a non-null value to all the points in the region: four additional
> input points are included in the NN interpolation that correspond to
> the corners of a user-defined region (a bounding box that includes
> all input points), by assigning them a value based on a different
> interpolation method (IDW). This way, the convex hull of input
> points has the same extent of the bounding box of the region. Also,
> I think that there is no need of giving the -W nnbathy flag a
> negative value and the resulting interpolated raster would still be
> inside the range of the input data because of the way IDW works.
> 
> It sounds feasible to add this functionality to r.surf.nnbathy, but
> I don't know if the approach is correct from the GIS point of view
> (i.e., mixing two interpolation methods) or even more useful/better
> than just using IDW for the whole region.

It is possible to add such a functionality to r.surf.nnbathy, but I'd
rather leave it to the user to enhance his input raster the way he
prefers to, if necessary. Different interpolation methods or additional
data sources can be used to add the 4 corner points in question.

Cheers
Maciek