1. A stereoscopic sensor comprising: a first camera pair for capturing a first and second image, wherein the stereoscopic sensor is adapted to monitor and define a main surveillance zone in a surveillance plane at a predetermined distance from the stereoscopic sensor, said main surveillance zone comprising a first and a second surveillance zone, and the first camera pair defines the first surveillance zone with a primary coverage in a first direction and a secondary coverage in a second direction, wherein the stereoscopic sensor further comprises a second camera pair for capturing a first and a second image, said images being processable into a height image, wherein the coverage of the second camera pair defines the second surveillance zone with a primary coverage in the surveillance plane in the second direction and a secondary coverage in the first direction, wherein said secondary coverage of the first and the second camera pairs overlap the primary coverage of the other camera pair respectively, wherein said stereoscopic sensor is arranged above said overlap.