[R] Contour Plot on a non Rectangular Grid

[Lorenzo Isella]

On 10/24/2010 01:51 PM, (Ted Harding) wrote:
> On 24-Oct-10 11:30:57, 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.
>> 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))
>>
>> 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
>>
>> Any suggestion is appreciated.
>> Many thanks
>>
>> Lorenzo
>
> For your option (1), the fundamental issue is interpolation.
> There are many methods for this, with different proprties!
> An R Site Search on "interpolation" yields a lot of hits.
> One (which is fairly basic, but may suit your purposes) is
> the interpp() function in package akima:
>
> http://finzi.psych.upenn.edu/R/library/akima/html/interpp.html
>
> Hoping this helps,
> Ted.
>
> --------------------------------------------------------------------
> E-Mail: (Ted Harding)<ted.harding at wlandres.net>
> Fax-to-email: +44 (0)870 094 0861
> Date: 24-Oct-10                                       Time: 12:51:03
> ------------------------------ XFMail ------------------------------

Hi,
And thanks for helping. I am anyway a bit puzzled, since case (1) is not 
only a matter of interpolation. Probably the point I did not make clear 
(my fault) is that case (1) in my original email does not refer to an 
irregular grid on a rectangular domain; the set of (x,y) coordinate 
could stand e.g. a flat metal slab along which I have temperature 
measurements. The slab could be e.g. elliptical or any other funny 
shape. What also matters is that the final outcome should not look 
rectangular, but by eye one should be able to tell the shape of the slab.
Case (1) is a generalization of case (2) where I do not have either an 
analytical expression for the surface not for the scalar.
Cheers

Lorenzo