[GRASS-user] Raster surface from regularly spaced points

[Luigi Ponti]

Thanks Markus!

On 19/02/2010 11:52, Markus Metz wrote:
> You can try the overhauled v.surf.bspline module. You would need to 
> svn up to get the new version, available in grass64 and higher.
I was browsing the source code but did not see it
http://trac.osgeo.org/grass/browser/grass/branches/releasebranch_6_4/vector
http://trac.osgeo.org/grass/browser/grass/trunk/vector

Am I looking at the wrong spot?

I will probably get it in the next grasswin standalone installer, but I 
also need source code to update an heritage cygwin/grass version.

>
> run v.surf.bspline with the -e flag first to get estimated mean 
> distance between points. If your points are not completely but almost 
> regularly spaced, multiply the estimated distance by 1.5 and use that 
> value for sin and sie.

This is great fine-tuning get me up and running -- thanks.

>
> BTW, method=bicubic gives much nicer results than method=bilinear.

I think it is slower, isn't it? Not sure how much slower, though.

>
> To conserve the range of the original data, use a low lambda_i value 
> somewhere between 0.01 and 0.1.
>
> I would be really interested if it works for you!

I will let you know. Thanks again,

Luigi

>
> Markus M
>
>
>
> Luigi Ponti wrote:
>> Hello,
>>
>> I have a grid of (almost) regularly spaced vector points from which I 
>> would like to get a continuous raster surface. I say "almost" 
>> regularly spaced, because the grid is almost square in that the 
>> position of points derives from a netCDF grid with a different 
>> projection than the current GRASS location. One cell of this 
>> non-aligned grid covers several raster cells at the current region 
>> resolution.
>>
>> So far I have been using a v.surf.idw approach but I am not sure 
>> whether that is appropriate. An exact interpolator would be best, but 
>> at least I want conserve the range of the original data.
>>
>> According the the GRASS book (p. 228), my options would be v.surf.idw 
>> and v.surf.rst but the latter does not conserve data range, I think. 
>> Another idea would be to use bilinear interpolation.
>>
>> Any hints very welcome and thanks for your attention if you got so far,
>>
>> Luigi
>>
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>