Nuclear-based basis weight sensors are passline-sensitive. Error in measurement is induced when the sheet moves up and down in the gap between the radiation source and detector. A passline-insensitive basis weight sensor includes a triangulation sensor to measure the position of the sheet within the gap. The sensor and gap is characterized in the laboratory for its passline behavior over a range of basis weights. The curves are either parameterized or a lookup table is created for each weight and passline position and the data added to the sensor's processor. The basis weight measured can be automatically corrected to account for deviations from the passline or nominal path through the sensor.