The rolling straight-edge (also rolling straightedge or planograph) is an instrument used to measure the surface regularity of roads and similar structures such as airport runways. It consists of a straightedge of a fixed distance mounted on wheels with a sensor at the centrepoint measuring deviation in height. It is rolled along the road surface and set to specific trigger levels which can be logged automatically or by means of an audible alarm. The rolling straight-edge was developed by the British Road Research Laboratory to replace earlier manual methods of measurement using rulers. It has been used by several countries and remains in use in the United Kingdom, Germany and Taiwan. Description The rolling straight-edge is a piece of equipment used to measure the surface regularity of roads and similar structures, such as airport runways. The equipment consists of a long beam (the straight edge) mounted on wheels with a measuring wheel at the midpoint. The measuring wheel moves up and down according to depressions in the road surface, a sensor measures the vertical movement, which is recorded on a graduated scale. The equipment is rolled longitudinally down the surface being measured. The equipment can be fitted with a bell or buzzer that alerts the user when a set limit of vertical deviation is breached. The recording of data, typically chainage and magnitude or number of deviations, can be done by hand, though some units contain automated dataloggers. A smoothness index for the road can be derived from a standard deviation analysis of the results. As an example of a unit the Road Research Laboratory rolling straight-edge measures in length with the sensor mounted at the midpoint. The unit was mounted on forty diameter road wheels mounted in pairs apart. The whole unit is dismantlable into three parts for transport and the frame was made of glass fibre. it was designed to be pulled along the road by hand at a slow walking pace, approximately . History The rolling straight-edge is one of the earliest methods of measuring surface regularity. It was a development of an earlier technique of measuring road depressions manually with a ruler under a straight-edge. This method had later been developed by the British Department of Transport with the use of graduated wedges which were pushed under the ruler to measure the height, but remained a slow and cumbersome technique. The government's Road Research Laboratory developed