The interpolation will take some time, depending on the speed of your computer. Contour interpolation first rasterizes contour lines in the segment map. This results in values for all pixels that are located on the segments. Then values have to be calculated for pixels that fall in between the segments. For each undefined pixel, the distance is calculated towards the two nearest contour lines. The distances are calculated forwards and backwards, until no more changes occur. Then a linear interpolation is performed using the two distance values. This returns the value for the undefined pixel. For more information see the ILWIS help (search for contour interpolation)