R help - Oct 2011 - A question about R image function

Dear folks,

I have a question about the image() function in R. I found the following
link talking about this
but the replies didn't help with my situations.

http://r.789695.n4.nabble.com/question-on-image-function-td839275.html#a839276

To be simple, I will keep using the example in the above link.

Suppose the data are like 

         x                y              mcpvalue
0.4603578     0.6247629       1.001
0.4603715     0.6247788        1.001
0.4603852     0.6247948       1.001
0.4110561     0.5664841       0.995 

So we have four points with coordinates given by the four pairs of (x,y)
values.
Each point is associated with a mcpvalue.

How do we use image() to plot "mcpvalue" as a two-dimensional plot?
I hope that the points are positioned by the coordinates and the color of
each point 
in that plot is changing with the value of mcpvalue.

Using image() does not work as the coordinates of the points
are not ascending.

Thanks a lot!
Jeff


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Hi,

On Wed, Oct 5, 2011 at 1:44 PM, JeffND <Zuofeng.Shang.5 at nd.edu>
wrote:
> Dear folks,
>
> I have a question about the image() function in R. I found the following
> link talking about this
> but the replies didn't help with my situations.
>
>
http://r.789695.n4.nabble.com/question-on-image-function-td839275.html#a839276
In fact, that link states exactly what needs to be done, and why.
> To be simple, I will keep using the example in the above link.
>
> Suppose the data are like
>
> ? ? ? ? x ? ? ? ? ? ? ? ?y ? ? ? ? ? ? ?mcpvalue
> 0.4603578 ? ? 0.6247629 ? ? ? 1.001
> 0.4603715 ? ? 0.6247788 ? ? ? ?1.001
> 0.4603852 ? ? 0.6247948 ? ? ? 1.001
> 0.4110561 ? ? 0.5664841 ? ? ? 0.995
>
> So we have four points with coordinates given by the four pairs of (x,y)
> values.
> Each point is associated with a mcpvalue.
> How do we use image() to plot "mcpvalue" as a two-dimensional
plot?
> I hope that the points are positioned by the coordinates and the color of
> each point
> in that plot is changing with the value of mcpvalue.
We don't. image() is intended to plot data on a regular grid, and that's
not a
regular grid. You need to use kriging or some other form of spatial
interpolation
to fit it to a regular grid before you can use image().

You can however use plot() with the col= argument to plot points at the
actual values of the coordinates, and to color them as desired.


testdata <- structure(list(x = c(0.4603578, 0.4603715, 0.4603852, 0.4110561
), y = c(0.6247629, 0.6247788, 0.6247948, 0.5664841), mcpvalue = c(1.001,
1.001, 1.001, 0.995)), .Names = c("x", "y",
"mcpvalue"), class "data.frame", row.names = c(NA, -4L))

testdata.colors <- cut(testdata$mcpvalue, c(0, 1, 2))
testdata.colors <- rainbow(length(testdata.colors))[testdata.colors]

with(testdata, plot(x, y, col=testdata.colors))


Which is kind of useless in this case, but may work with your actual
data, which is why I
wrote it out so elaborately.
> Using image() does not work as the coordinates of the points
> are not ascending.
Exactly. So you can't use image().



-- 
Sarah Goslee
http://www.functionaldiversity.org

Hi Sarah,

Thanks a lot! You are right, my data is not over a regular grid cell
locations.
One advantage of image() is that it can produce continuous color change for
the data value.
When the data value is over a large range, this will make it more
convenient.

Thanks!
Jeff

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