Combination of a stage and a level sensor configured to sense a height level at a target location on an object is described, the stage comprising an object table configured to hold the object and a positioning device for displacing the object table relative to the level sensor in a first direction, the level sensor comprising a projection system configured to project a measurement beam onto a measurement area of the object, the measurement area having a measurement area length in the first direction, a detector system configured to receive different portions of the measurement beam after being reflected off different sub-areas within the measurement area, the different sub-areas being arranged in the first direction, and to supply output signals representative of the different portions received, a signal processing system configured to process the output signals from the detector system.