We propose here algorithms to perform two new operations on an arrangement of line segments in the plane, represented by a trapezoidal map: the split of the map along a given vertical line $D$, and the union of two trapezoidal maps computed in two vertical slabs of the plane that are adjacent through a vertical line $D$. The data structure is a modified Influence graph, still allowing dynamic insertions and deletions of line segments in themap. The algorithms for both operations run in $O(s_D \log n +\log^2 n)$ time, where $n$ is the number of line segments in the map, and $s_D$ is the number of line segments intersected by $D$.