test1<-function(x,z,sx=0.3,sz=0.4){ x<-x*20
  (pi**sx*sz)*(1.2*exp(-(x-0.2)^2/sx^2-(z-0.3)^2/sz^2)+
             0.8*exp(-(x-0.7)^2/sx^2-(z-0.8)^2/sz^2))
}
n<-500

x<-runif(n)/20;z<-runif(n);
xs<-seq(0,1,length=30)/20;zs<-seq(0,1,length=30)
pr<-data.frame(x=rep(xs,30),z=rep(zs,rep(30,30)))
truth<-matrix(test1(pr$x,pr$z),30,30)
f <- test1(x,z)
y <- f + rnorm(n)*0.2

b1<-gam(y~t2(x,z,bs=c("ps","ps")),drop.intercept=T)

b1$sp

 # t2(x,z)rr       t2(x,z)nr        t2(x,z)rn

# 4.280221e-04     3.884871e+06     1.386065e+06