[R-sig-Geo] Mapping contours from jpg map

[Dylan Beaudette]

I would second Roger's comments. I have found r.surf.contour does well
in complex terrain, especially at interfaces with water (i.e. alpine
lakes). There are several related threads on the GRASS mailing list.

Dylan

On Fri, Jul 8, 2011 at 8:09 AM, Roger Bivand <Roger.Bivand at nhh.no> wrote:
> On Fri, 8 Jul 2011, Barry Rowlingson wrote:
>
>> On Fri, Jul 8, 2011 at 2:26 PM, Robert Hijmans <r.hijmans at 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
>
> 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 at r-project.org
>> 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 at nhh.no
>
> _______________________________________________
> R-sig-Geo mailing list
> R-sig-Geo at r-project.org
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>