extrapolation of surface

[Alexander Rauscher <e9121161(at)stud4.tuwien.ac.at>]

hi everybody, 

i have a function f(x,y) which is f(x,y) != 0 on a more or less circular
region which lies within an e.g. 128x128 array and 0 outside this region.
now i want to extrapolate this function (over a range of e.g 10 pixles)
into the region outside this circular region without the result being
influenced by all the f(x,y)=0. 
sfit also takes into account where f(x,y)=0, which makes the fitted
surface fit(x,y) decrease towards the edge of my array which is not what i
want. (knowing from the shape of the original f(x,y) that it should
increase...) 
anyone an idea what to do? 

thanks a lot in advance