1D linear interpolation

hello,
i was using '1D linear interpolation' to generate constant-head boundary conditions along a river (FeFlow Classic). after river elevations at start point A and B were assigned, a*.shp (or *.lin) was used to join and the method of '1D linear interpolation' was used to generate the values for those nodes along the river between A and B. i checked the interpolated values after interpolation and found at some nodes the interpolated values were actually greater than the values at either A or B. in another word, the interpolated river elevations were 'mounded' somewhere between A and B. i was wondering what may cause this problem. were those node elevations (i.e. river bed topography) considered during the interpolation? your help is greatly appreciated!

thanks in advance

b 

i think i figured it out. the value of 'snap distance' under the '1D linear interpolation' caused the problem. reduced 'snap distance' value to make sure only two river elevation values at point A and point B would be used for interpolation (i.e. excluding any existing constant-head values that were previously assigned along the river but outside the river segment A-B).

b