[R] dose-response on a grid

[William Simpson]

I have the following problem. I have measured a dose response curve
(binary response, continuous dose) on a grid of x,y positions. I would
like to produce a grey-level plot that shows the LD50 at each (x,y)
position.

I am thinking that I have to do something like
fit<-glm(resp ~ x*y + dose, family = binomial)
Corrections welcome.

But from here I don't know how to get LD50, and certainly not at each
x,y, position.

Thanks very much for any help.

Bill