[R] Contour Plot on a non Rectangular Grid

Lorenzo Isella lorenzo.isella at gmail.com
Sun Oct 24 15:12:24 CEST 2010

On 10/24/2010 02:55 PM, David Winsemius wrote:
> On Oct 24, 2010, at 4:30 AM, Lorenzo Isella wrote:
>> Dear All,
>> I would like to plot a scalar (e.g. a temperature) on a
>> non-rectangular domain (or even better: I would simply like to be able
>> to draw a contour plot on an arbitrary 2D domain). I wonder if there
>> is any tool to achieve that with R. I did some online search in
>> particular on the list archives, found several queries similar to this
>> one but was not able to find any conclusive answer.
> One implemented approach to this exists with the rms/Hmisc package
> combination. The perimeter function is used to define a region within
> which the are a sufficient number of cases and the perimeter object is
> passed to the bplot function, which is a wrapper for a lattice
> contourplot call. There is no reason you couldn't emulate
>> I am interested in the following 2 options
>> (1) just read a file of the form
>> x1 y1 z1
>> x2 y2 x2
>> ... ... ...
>> xn yn zn
>> where the set of {xi} and {yi} are coordinates on an arbitrary domain
>> and {zi} are the values of the scalar for the corresponding {x,y}
>> coordinates.
>> (2) Sometimes the domain where I want to draw a contour plot is
>> nothing too fancy and the scalar itself is given by an analytical
>> function. Consider e.g. the case of a circle of radius R=pi/2 centered
>> about the origin and a function like
>> z=f(x,y)=abs(cos(y))
> That defines the contours but does not restrict the domain.
>> NB: in this case a satisfactory solution could be to plot z on a
>> rectangular grid and then clip a circular region
>> To fix the ideas, the final result in this case (with a colorjet map)
>> should look like this
>> http://dl.dropbox.com/u/5685598/scalar_plot.pdf
> And that color encoded output would not be the output of a contourplot
> but is more like a levelplot or an image plot. Nonetheless, the
> perimeter and bplot combination can deliver a similar result if you
> supply either code or data as a suitable test case for analysis and
> display.

I agree that contour plot was a misleading name for what I had in mind. 
I'll try your suggestion and the one by Uwe about rgl and post again if 
I had troubles. As to the domain of the function, at least in case (1), 
that should arise from the collected data points in (x,y) if the 
sampling is dense enough.


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