[R] How can I get the interpolated data?

[pinwheels]

Hello,everybody!

I am a beginner of R.

And I want to ask a question. If anybody would help me, thank you very much
ahead.
I want to plot something like a response surface, and I find the "rsm"
package.

Some commands are like this:

#code head
library(rsm)
CR = coded.data(ChemReact, x1 ~ Time, x2 ~ Temp)
CR.rsm = rsm(Yield ~ SO(x1,x2), data = CR)
summary(CR.rsm)
contour(CR.rsm,x1~x2)
#code end

What if I want the data interpolated, what should I do?
For example:
There is a data frame like:

xa1<-seq(86,88,len=21)
xa2<-seq(175,179,len=41)
z<-  ... # referring site(xa1,xa2) from the contour plotted above

or 

xa1      xa2        z
86        175      ???
86.1      175      ???
...         ...        ...
86.7       177.3   ???
...        ....         ...
88        179       ???

or  something alike.

How could I get the z value(???) from the CR.rsm or the plotted contour? 
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