Refer to figure.

ICAO Doc 9426
3.3 Minimizing the Effect of Wake Turbulence

3.3.3.6 Ground effect becomes an important factor when considering the movement and decay of the vortices. The ground acts as a plane of reflection; as the trailing vortices descend toward the ground their vertical velocity decreases and, with no or little wind, they begin to travel horizontally over the ground away from each other at a height approximately half the wing span of the generating aircraft.

Area affected by wake turbulence behind an aircraft:

• Laterally 2 x wingspan (so that means another ½ wingspan from the wing tip)
• Vertically 1 x wingspan (so we will assume ½ wingspan above and below the aircraft)