A key aspect to any quality concrete pavement is smoothness. To achieve the best possible quality, it is important to monitor pavement smoothness daily. Knowing the profile of the previous day's paving allows contractors to know immediately if a problem exists and, in most cases, allows it to be corrected before a new day of paving begins. The most common device used for measuring pavement smoothness is the profilograph. Although several other devices are available for measuring pavement profiles at high speed, most of them must be attached to vehicles, such as a pick up truck, or are self-contained in a van. This prevents them from being used until pavements have been in place for several days. An advantage of the profilograph, which generally weighs less than 500 pounds and is operated by one person walking alongside it, is that it can be used 4 to 6 hours after placement of conventional concrete pavements. An important aspect of the profilograph is that results are repeatable, even if different operators are used or the direction of travel is changed. The two main types or profilographs are the California and Rainhart. The major difference between a Rainhart- and California-type profilometer is how the reference plane is established.