A virtual gauging method for use in a lithographic process includes gauging a region at a surface of a wafer when the region is located away from an axis of illumination producing wafer surface data, while other portions of the wafer are being illuminated. The method also includes acquiring time-domain measurements representing the wafer surface data and converting the time-domain measurements into space-domain measurements. This conversion can be done using a finite-impulse-response (FIR) filter. The FIR filter can be triggered with a spatial interrupt, and a width of the FIR filter is modified in response to a velocity of travel of the wafer. The method further includes converting space-domain measurements into wafer correction data.