modify the the bilinear UDF for extrapolation

there is an excellent UDF for bilinear interpolation in this link

http://www.excelforum.com/excel-prog...rpolation.html

but if if the x value for example is beyond or less than the max or min values in the table the function gives
error value and in other version of the same function it gives ( x outside table extents)

is there any possibility to overcome this limit, i mean to modify the macro to deal with these values and calculate also extrapolation.
this off course with shg permission
or any other solution to do so

thanks

where are you shg