[R] Contour Plot on a non Rectangular Grid

[Uwe Ligges]

On 24.10.2010 14:14, Lorenzo Isella wrote:
> 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


What about the facilities in package rgl then?

Uwe Ligges


> Lorenzo
>
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