Glossary

Some useful terminology that commonly appears in smoothness specifications:

Accelerometer: A sensor that measures acceleration. An inertial profiler uses the signal from the on-board accelerometer(s) to compute the inertial reference value (i.e., relative height) that can be combined with the height sensor data to produce a complete profile. See also Height Sensor.
Accuracy: Lack of error. The root-mean-square value of the error when comparing measured values with “deemed correct” or reference values. See also Repeatability.
Amplitude: The maximum value of a periodic curve measured along its vertical axis with reference to the horizontal axis.
Bias Error: Error stemming from systematic problems, including inaccurate calibration, physical damage, or a defect in the profiler’s design.
Blanking Band: A band that is placed over a profilograph trace to “mask” the portion of the trace that is not counted as roughness.
Bode Plot: A frequency response plot made on log-log axes.
Bounce Test: A test performed on an inertial profiler when stationary in order to check the measurement system.
Calibration: The process of correcting the scale of a transducer.
Correlation Analysis: Analysis that describes how one variable relates to another.
Digital Filter: A calculation procedure that transforms one signal into another to eliminate irrelevant data.
Dipstick: A static, inclinometer-based proprietary device used to measure elevations at individual points (originally used for measuring floor flatness).
Distance Measurement Instrument (DMI): A transducer used to determine the longitudinal distance that the measurement vehicle has traveled.
Frequency Response: The ratio of output amplitude to input amplitude for a sinusoid.
Gain: The ratio that a filter amplifies or attenuates an input signal component at a given wavelength or frequency.
Golden Car: The quarter-car model used in IRI computation. See also IRI.
Half-car Roughness Index (HRI): A number calculated by applying the IRI algorithm to the average of two profiles.
Height Sensor: A sensor used in an inertial profiler to measure the vertical distance between the sensor and the pavement surface.
High-Pass Filter: A digital filter that produces an anti-smoothing effect that reduces the effect of long wavelengths. See also Low-Pass Filter.
High-Speed Profiler: An inertial profiler (vans, trucks, cars, etc.) that is operated at prevailing highway speed. See also Inertial Profiler and Light-Weight Profiler.
Inertial Profiler: A profile measurement device that measures the pavement profile using an accelerometer to form an inertial reference and a height sensor to measure the pavement surface height relative to that reference.
International Roughness Index (IRI): A statistic that summarizes the roughness qualities impacting vehicle response based on the Golden-Car vehicle model at a standard simulation speed of 49.7 mph (80 km/h).
Lead-In: The distance required for an inertial profiler to reach an acceptable speed and for the data collection filters used in the profile computation to stabilize. See also Lead-out.
Lead-Out: A safe distance for an inertial profiler to operate to a stop position or until the data collection system is turned off. See also Lead-In.
Light-Weight Profiler: An inertial profiler that is relatively light-weight (golf cart, ATV, etc.) compared with high-speed profilers. It is often operated much slower than prevailing traffic speed. See also Inertial Profiler and High-Speed Profiler.
Linear System: A system in which the output is directly proportional to the input.
Localized Roughness: Short sections of roadway that contribute disproportionately to the overall roughness index value. Also referenced as “hot spots”
Longitudinal Profile: A longitudinal two-dimensional slice of a road surface taken along an imaginary line that consists of elevation values and a distance reference for each elevation.
Low-Pass Filter: A smoothing filter, such as moving-average, that reduces the effect of short wavelengths. See also High-pass Filter.
Mean Panel Ratings: A subjective rating system whereby automobile passengers rate the smoothness of a given road after driving on it.
Mean Roughness Index (MRI): A number calculated by averaging the IRI values from the two wheelpath profiles.
Moving Average Filter: A digital filter that replaces each profile elevation point with the average of several adjacent points.
Portability: The ability to obtain consistent results when using different types of measurement devices.
Power Spectral Density (PSD): A method that describes how the power of a signal (i.e., a time average of signal energy) or time series is distributed with frequency.
Precision: The measure of variation between multiple measurements, expressed in standard deviation.
Profile Index: A generic summary number calculated from a profile. This term should be avoided since it may be confused with PrI or RN. See also PrI and RN.
Profiler: An instrument used to measure road profiles.
Profiler Certification: A procedure used to determine whether a profiler satisfies a specific set of performance criteria.
Profiler Operator Certification: A procedure used to determine whether a profiler operator passes a specific set of written and field operational tests in order to be certified for operating profilers.
Profilograph: A device used to measure smoothness by measuring the deviations of a pavement surface using a mid-point measuring wheel from the reference established by a set of wheels (6 for a California-type) at either end of the device.
Profilograph Index (PrI): A smoothness index that is computed from a profilograph trace. This is sometimes called Profile Index (PI), but is more specifically called PrI.
Reference Device: A device used to obtain the true profile of a pavement. Devices such as rod and level, Dipstick(TM) and walking profiler are considered reference devices.
Repeatability: The expected standard deviation of measurements obtained in repeated tests using the same device on a single, randomly-selected pavement surface.
Reproducibility: The standard deviation of the error included in a single measurement, relative to a reference measurement. The reproducibility of a device includes errors that are systematic with respect to that device, but random with respect to a particular test.
Ride Number (RN): A calculated roughness index, between 0 and 5, that approximates the Mean Panel Rating for a pavement surface. See also Mean Panel Ratings, IRI, and PrI.
Ride: Also spelled out as “Ride Quality”. Measured as accelerations in the vehicle body.
Road Roughness: The deviations of a pavement surface from a true plane surface with characteristic dimensions that affect vehicle dynamics and ride quality.
Rod and Level: Static equipment used to measure elevations at individual points, commonly used for land surveying.
Rolling Straightedge: Profiling equipment generally consisting of a rigid beam (or frame) with support wheels on either end and a measuring wheel at the middle, which is rolled over the surface to be profiled.
Roughness Profile: A plot that shows the variation of roughness over a section of pavement. This is also referred to as a “continuous roughness report”.
Response-type Road Roughness Measuring System (RSRRMS): A system that measures suspension deflection of either one or two wheels of either a passenger car or a towed trailer and records these deflections as "counts" or as actual measured deflection.
Sample Interval: The longitudinal distance between captured data points.
Segment Length: The length of section of pavement where a smoothness index is recorded.
Signal Processing: The mathematical transformation and analysis of signals.
Signal: A series of numbers.
Sinusoid: A periodic curve defined by wavelength, amplitude and phase.
Smoothness: Lack of roughness or lack of significant bumps and dips from the pavement surface that cause discomfort to motorists.
Standard Error: The portion of the total error due to random effects.
True Profile: The undistorted shape of a pavement surface.
Valid Profiler: A profiler that provides the same statistical values that would be obtained from the true profile.
Verification Site: A pavement section used to periodically check if an inertial profiler is functioning properly.
Wave Band: A range of frequencies. In profile analysis, wave band often refers to spatial frequencies. See also Wave Number.
Wave Number: The number of wave cycles per unit length.
Wavelength: The distance between peaks or crests of a wave or sinusoid.