[R] Problem with contour plot

[Berthold Kramm]

Using the code below (based on 'Modern Applied Statistics with S (4th 
Edition)from Venables,Ripley) for a 2*2 contour (plus color) plot, the 
top left contour plot is distorted in the sense that the contour plot is 
compressed in x-direction, while the axis and the contour plot in 
y-direction display correctly.
Any idea how to fix this anomaly ?

Best wishes,
Berthold

...
par(mfrow = c(2,2), pty ="s")

topo.ls <- surf.ls(3,RALV,RARV,PP)
trsurf <- trmat(topo.ls,80,160,80,160,100)
eqscplot(trsurf,xlab="RALV",ylab="RARV",type="n")
image(trsurf,col=terrain.colors(100),add=T)
contour(trsurf, lty="solid", add=TRUE)
points(RALV,RARV)
title("PP")

topo.ls <- (replication of the same block as above 3 times with 
different variables) ...