strcmp(border,'top')
                ref = max(this.NODE(:,2));
            elseif strcmp(border,'bottom')
                ref = min(this.NODE(:,2));
            else
                disp('Warning: non-supported border.');
                disp('Available borders tag: ''left'',''right'', ''top'',''bottom''');
                return;
            end
            
            for el = 1:this.nelem 

                % Get number of linear interpolation points
                nLinNodes = length(this.ELEM{el});
  