The pressure corresponds to the force, which is applied vertically to the surface area. In this case, the force is relative to the pressed surface, thus, is divided by the area of the surface. Absolute pressure is assumed to be 0 bar. This, at the same time, is also the maximum (theoretically) achievable negative pressure. The atmospheric pressure at sea-level is exactly 1013 hPa. All technical measuring instruments- and values show the relative pressure. The atmospheric pressure is used as the basis.