An apparatus, such as a lithographic apparatus, has a metrology frame that has a reference frame mounted thereon that includes a reference surface. A gas gauge is movable relative to the reference frame, metrology frame, and a measured surface. A reference nozzle in the gas gauge provides gas to the reference surface and a measurement nozzle provides gas to the measured surface. A microelectromechanical (MEM) sensor may be used with the gas gauge to sense a difference in backpressure from each of the reference nozzle and the measurement nozzle. Optionally, multiple gas gauges are positioned in an array, which may extend in a direction that is substantially parallel to a plane of the measured surface. The gauges may be fluidly connected to a reference nozzle of the gas gauge. A channel may distribute gas across the array.