diff --git a/VolZmapCalculus.cpp b/VolZmapCalculus.cpp
index 24762d9..fe7f2ca 100644
--- a/VolZmapCalculus.cpp
+++ b/VolZmapCalculus.cpp
@@ -146,7 +146,11 @@ VolZmap::IntersLineZMapBBox( const Point3d& ptP, const Vector3d& vtV, double& dU
 {
   // Punti estremi del box dello Zmap
    Point3d ptMin = ORIG ;
-   Point3d ptMax = ptMin + Vector3d( m_nNx[0] * m_dStep, m_nNy[0] * m_dStep, m_dMaxZ[0]) ;
+   Point3d ptMax ;
+   if ( m_nMapNum == 3)
+      ptMax = ptMin + Vector3d( m_dMaxZ[1], m_dMaxZ[2], m_dMaxZ[0]) ;
+   else
+      ptMax = ptMin + Vector3d( ( m_nNx[0] + 0.5) * m_dStep, ( m_nNy[0] + 0.5) * m_dStep, m_dMaxZ[0]) ;
 
    return IntersLineBox( ptP, vtV, ptMin, ptMax, dU1, dU2) ;
 }