IDW was indeed a poor choice, but I'd say that splines should generally work
OK. Robert

On Fri, Jul 8, 2011 at 8:09 AM, Roger Bivand <Roger.Bivand@nhh.no> wrote:

> On Fri, 8 Jul 2011, Barry Rowlingson wrote:
>
>  On Fri, Jul 8, 2011 at 2:26 PM, Robert Hijmans <r.hijmans@gmail.com>
>> wrote:
>>
>>
>>
>>  # now there are many ways to interpolate. See, e.g., the 'gstat' and
>>> 'automap' packages. See raster::interpolate for an example with splines.
>>> # using gstat and inverse distrance weighted interpolation:
>>>
>>> library(gstat)
>>> g <- gstat(id="level", formula = level~1, data=p, nmax=7, set=list(idp =
>>> .5))
>>> x1 <- interpolate(r, g)
>>>
>>
>> Statistical interpolation techniques may be right for this, but
>> thinking about it last night made me realise that contours are more
>> than just linear estimates of height at location. There's the
>> implication that between any two contour lines of height H1 and H2
>> there are no locations with height outside the bounds of (H1,H2).
>> Otherwise there would be a contour line there.
>>
>> And this may not be so uncommon in elevation models. Consider a steep
>> sided valley with a wide flood plain. You have close contours on
>> either side with a big gap between. Would a "statistical"
>> interpolation run down the valley side and plummet on down, then back
>> up the other side, turning the flood plain into a deep rounded valley
>> bottom? Sure it all depends on the parameters of the smoothing, but a
>> method that knew it was dealing with contours would constrain the
>> surface such that points between contour lines were always between the
>> contour line values.
>>
>> I think at least one of the algorithms in GRASS-GIS does this by
>> "drawing" the contour lines on the raster and then doing a 'flood
>> fill' operation between them.
>>
>> I'll have to dig out some GIS books...
>>
>
> r.surf.contour in GRASS does what is required:
>
> http://grass.osgeo.org/**grass64/manuals/html64_user/r.**surf.contour.html<http://grass.osgeo.org/grass64/manuals/html64_user/r.surf.contour.html>
>
> gives its description; the input is a raster of the contours where the
> non-missing cells take the contour elevation values. I have found that
> spline interpolation can create artefacts, and geostatistical interpolation
> (or IDW for obvious reasons) does not like many neighbouring data points
> along a contour line.
>
> Roger
>
>
>> Barry
>>
>> ______________________________**_________________
>> R-sig-Geo mailing list
>> R-sig-Geo@r-project.org
>> https://stat.ethz.ch/mailman/**listinfo/r-sig-geo<https://stat.ethz.ch/mailman/listinfo/r-sig-geo>
>>
>>
> --
> Roger Bivand
> Department of Economics, NHH Norwegian School of Economics,
> Helleveien 30, N-5045 Bergen, Norway.
> voice: +47 55 95 93 55; fax +47 55 95 95 43
> e-mail: Roger.Bivand@nhh.no
>
>

	[[alternative HTML version deleted]]

