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Length measurement, distance measurement, or range measurement (ranging) refers to the many ways in which
length Length is a measure of distance. In the International System of Quantities, length is a quantity with dimension distance. In most systems of measurement a base unit for length is chosen, from which all other units are derived. In the Inte ...
,
distance Distance is a numerical or occasionally qualitative measurement of how far apart objects or points are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two counties over"). ...
, or range can be measured. The most commonly used approaches are the rulers, followed by transit-time methods and the interferometer methods based upon the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit fo ...
. For objects such as crystals and
diffraction grating In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structur ...
s,
diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
is used with
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
s and
electron beam Cathode rays or electron beam (e-beam) are streams of electrons observed in discharge tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, glass behind the positive electrode is observed to glow, due to el ...
s. Measurement techniques for three-dimensional structures very small in every dimension use specialized instruments such as ion microscopy coupled with intensive computer modeling.


Standard rulers

The
ruler A ruler, sometimes called a rule, line gauge, or scale, is a device used in geometry and technical drawing, as well as the engineering and construction industries, to measure distances or draw straight lines. Variants Rulers have long ...
the simplest kind of length measurement tool: lengths are defined by printed marks or engravings on a stick. The
metre The metre ( British spelling) or meter ( American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its pre ...
was initially defined using a ruler before more accurate methods became available.
Gauge block Gauge blocks (also known as gage blocks, Johansson gauges, slip gauges, or Jo blocks) are a system for producing precision lengths. The individual gauge block is a metal or ceramic block that has been precision ground and lapped to a specific ...
s are a common method for precise measurement or calibration of measurement tools. For small or microscopic objects, microphotography where the length is calibrated using a graticule can be used. A graticule is a piece that has lines for precise lengths etched into it. Graticules may be fitted into the eyepiece or they may be used on the measurement plane.


Transit-time measurement

The basic idea behind a transit-time measurement of length is to send a signal from one end of the length to be measured to the other, and back again. The time for the round trip is the transit time Δt, and the length ℓ is then 2ℓ = Δt*"v",with ''v'' the speed of propagation of the signal, assuming that is the same in both directions. If light is used for the signal, its
speed In everyday use and in kinematics, the speed (commonly referred to as ''v'') of an object is the magnitude of the change of its position over time or the magnitude of the change of its position per unit of time; it is thus a scalar quant ...
depends upon the medium in which it propagates; in
SI units The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. ...
the speed is a defined value ''c''0 in the reference medium of classical vacuum. Thus, when light is used in a transit-time approach, length measurements are not subject to knowledge of the source frequency (apart from possible frequency dependence of the correction to relate the medium to classical vacuum), but are subject to the error in measuring transit times, in particular, errors introduced by the response times of the pulse emission and detection instrumentation. An additional uncertainty is the ''refractive index correction'' relating the medium used to the reference vacuum, taken in SI units to be the classical vacuum. A
refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, ...
of the medium larger than one slows the light. Transit-time measurement underlies most
radio navigation Radio navigation or radionavigation is the application of radio frequencies to determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of radiodetermination. The basic principles a ...
systems for boats and aircraft, for example,
radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor v ...
and the nearly obsolete Long Range Aid to Navigation
LORAN-C Loran-C is a hyperbolic radio navigation system that allows a receiver to determine its position by listening to low frequency radio signals that are transmitted by fixed land-based radio beacons. Loran-C combined two different techniques to ...
. For example, in one radar system, pulses of electromagnetic radiation are sent out by the vehicle (interrogating pulses) and trigger a response from a ''responder beacon''. The time interval between the sending and the receiving of a pulse is monitored and used to determine a distance. In the
global positioning system The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite ...
a code of ones and zeros is emitted at a known time from multiple satellites, and their times of arrival are noted at a receiver along with the time they were sent (encoded in the messages). Assuming the receiver clock can be related to the synchronized clocks on the satellites, the ''transit time'' can be found and used to provide the distance to each satellite. Receiver clock error is corrected by combining the data from four satellites. Such techniques vary in accuracy according to the distances over which they are intended for use. For example, LORAN-C is accurate to about GPS about enhanced GPS, in which a correction signal is transmitted from terrestrial stations (that is,
differential GPS Differential Global Positioning Systems (DGPSs) supplement and enhance the positional data available from global navigation satellite systems (GNSSs). A DGPS for GPS can increase accuracy by about a thousandfold, from approximately to . DGPSs ...
(DGPS)) or via satellites (that is,
Wide Area Augmentation System The Wide Area Augmentation System (WAAS) is an air navigation aid developed by the Federal Aviation Administration to augment the Global Positioning System (GPS), with the goal of improving its accuracy, integrity, and availability. Essentia ...
(WAAS)) can bring accuracy to a few meters or or, in specific applications, tens of centimeters. Time-of-flight systems for robotics (for example, Laser Detection and Ranging
LADAR Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
and Light Detection and Ranging
LIDAR Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
) aim at lengths of and have an accuracy of about


Interferometer measurements

In many practical circumstances, and for precision work, measurement of dimension using transit-time measurements is used only as an initial indicator of length and is refined using an interferometer. Generally, transit time measurements are preferred for longer lengths, and interferometers for shorter lengths. The figure shows schematically how length is determined using a
Michelson interferometer The Michelson interferometer is a common configuration for optical interferometry and was invented by the 19/20th-century American physicist Albert Abraham Michelson. Using a beam splitter, a light source is split into two arms. Each of those ...
: the two panels show a laser source emitting a light beam split by a ''
beam splitter A beam splitter or ''beamsplitter'' is an optical device that splits a beam of light into a transmitted and a reflected beam. It is a crucial part of many optical experimental and measurement systems, such as interferometers, also finding wid ...
'' (BS) to travel two paths. The light is recombined by bouncing the two components off a pair of ''
corner cube A corner reflector is a retroreflector consisting of three mutually perpendicular, intersecting flat surfaces, which reflects waves directly towards the source, but translated. The three intersecting surfaces often have square shapes. Radar ...
s'' (CC) that return the two components to the beam splitter again to be reassembled. The corner cube serves to displace the incident from the reflected beam, which avoids some complications caused by superposing the two beams. The distance between the left-hand corner cube and the beam splitter is compared to that separation on the fixed leg as the left-hand spacing is adjusted to compare the length of the object to be measured. In the top panel the path is such that the two beams reinforce each other after reassembly, leading to a strong light pattern (sun). The bottom panel shows a path that is made a half wavelength longer by moving the left-hand mirror a quarter wavelength further away, increasing the path difference by a half wavelength. The result is the two beams are in opposition to each other at reassembly, and the recombined light intensity drops to zero (clouds). Thus, as the spacing between the mirrors is adjusted, the observed light intensity cycles between reinforcement and cancellation as the number of wavelengths of path difference changes, and the observed intensity alternately peaks (bright sun) and dims (dark clouds). This behavior is called interference and the machine is called an
interferometer Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber o ...
. By ''counting fringes'' it is found how many wavelengths long the measured path is compared to the fixed leg. In this way, measurements are made in units of wavelengths ''λ'' corresponding to a particular atomic transition. The length in wavelengths can be converted to a length in units of metres if the selected transition has a known frequency ''f''. The length as a certain number of wavelengths ''λ'' is related to the metre using ''λ'' = . With ''c0'' a defined value of 299,792,458 m/s, the error in a measured length in wavelengths is increased by this conversion to metres by the error in measuring the frequency of the light source. By using sources of several wavelengths to generate sum and difference beat frequencies, absolute distance measurements become possible. This methodology for length determination requires a careful specification of the wavelength of the light used, and is one reason for employing a
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The ...
source where the wavelength can be held stable. Regardless of stability, however, the precise frequency of any source has linewidth limitations. Other significant errors are introduced by the interferometer itself; in particular: errors in light beam alignment, collimation and fractional fringe determination. Corrections also are made to account for departures of the medium (for example, air) from the reference medium of classical vacuum. Resolution using wavelengths is in the range of ΔL/L ≈ depending upon the length measured, the wavelength and the type of interferometer used. The measurement also requires careful specification of the medium in which the light propagates. A ''refractive index correction'' is made to relate the medium used to the reference vacuum, taken in SI units to be the classical vacuum. These refractive index corrections can be found more accurately by adding frequencies, for example, frequencies at which propagation is sensitive to the presence of water vapor. This way non-ideal contributions to the refractive index can be measured and corrected for at another frequency using established theoretical models. It may be noted again, by way of contrast, that the transit-time measurement of length is independent of any knowledge of the source frequency, except for a possible dependence of the correction relating the measurement medium to the reference medium of classical vacuum, which may indeed depend on the frequency of the source. Where a pulse train or some other wave-shaping is used, a range of frequencies may be involved.


Diffraction measurements

For small objects, different methods are used that also depend upon determining size in units of wavelengths. For instance, in the case of a crystal, atomic spacings can be determined using
X-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
. The present best value for the lattice parameter of silicon, denoted ''a'', is: ::a = 543.102 0504(89) × 10−12 m, corresponding to a resolution of ΔL/L ≈ Similar techniques can provide the dimensions of small structures repeated in large periodic arrays like a
diffraction grating In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structur ...
. Such measurements allow the calibration of
electron microscope An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hi ...
s, extending measurement capabilities. For non-relativistic electrons in an electron microscope, the de Broglie wavelength is: :\lambda_e = \frac \ , with ''V'' the electrical voltage drop traversed by the electron, ''me'' the electron mass, ''e'' the
elementary charge The elementary charge, usually denoted by is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charge is a fundam ...
, and ''h'' the
Planck constant The Planck constant, or Planck's constant, is a fundamental physical constant of foundational importance in quantum mechanics. The constant gives the relationship between the energy of a photon and its frequency, and by the mass-energy equivale ...
. This wavelength can be measured in terms of inter-atomic spacing using a crystal diffraction pattern, and related to the metre through an optical measurement of the lattice spacing on the same crystal. This process of extending calibration is called '' metrological traceability''. See The use of metrological traceability to connect different regimes of measurement is similar to the idea behind the
cosmic distance ladder The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible o ...
for different ranges of astronomical length. Both calibrate different methods for length measurement using overlapping ranges of applicability.


Far and moving targets

Ranging is technique that measures distance or
slant range In radio electronics, especially radar terminology, slant range or slant distance is the distance along the relative direction between two points. If the two points are at the same level (relative to a specific datum), the slant distance equals t ...
from the observer to a target, especially a far and moving target. Active methods use unilateral transmission and passive reflection. Active rangefinding methods include
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The ...
(
lidar Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
),
radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor v ...
,
sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances (ranging), communicate with or detect objects on o ...
, and ultrasonic rangefinding. Other devices which measure distance using
trigonometry Trigonometry () is a branch of mathematics that studies relationships between side lengths and angles of triangles. The field emerged in the Hellenistic world during the 3rd century BC from applications of geometry to astronomical studies. ...
are
stadiametric Stadiametric rangefinding, or the stadia method, is a technique of measuring distances with a telescopic instrument. The term stadia comes from a Greek unit of length '' Stadion'' (equal to 600 Greek feet, '' pous'') which was the typical length ...
,
coincidence A coincidence is a remarkable concurrence of events or circumstances that have no apparent causal connection with one another. The perception of remarkable coincidences may lead to supernatural, occult, or paranormal claims, or it may lead t ...
and
stereoscopic rangefinder A stereoscopic rangefinder or stereoscopic telemeter is an optical device that measures distance from the observer to a target, using the observer's capability of binocular vision. It looks similar to a coincidence rangefinder, which uses differen ...
s. Older methodologies that use a set of known information (usually distance or target sizes) to make the measurement, have been in regular use since the 18th century. Special ranging makes use of actively synchronized transmission and
travel time ''Travel Time'' is a Philippine television travel documentary show broadcast by GMA Network GMA Network (Global Media Arts or simply GMA) is a Philippine free-to-air television and radio network. It is the flagship property of publicly t ...
measurements. The time difference between several received signals is used to determine exact distances (upon multiplication by the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit fo ...
). This principle is used in
satellite navigation A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location ( longitude, latitude, and altitude/ elevation) to hig ...
. In conjunction with a standardized model of the Earth's surface, a location on that surface may be determined with high accuracy. Ranging methods without accurate time synchronization of the receiver are called
pseudorange The pseudorange (from pseudo- and range) is the ''pseudo'' distance between a satellite and a navigation satellite receiver (see GNSS positioning calculation), for instance Global Positioning System (GPS) receivers. To determine its position ...
, used, for example, in GPS positioning. With other systems ranging is obtained from passive radiation measurements only: the
noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...
or
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
signature A signature (; from la, signare, "to sign") is a Handwriting, handwritten (and often Stylization, stylized) depiction of someone's name, nickname, or even a simple "X" or other mark that a person writes on documents as a proof of identity and ...
of the object generates the signal that is used to determine range. This
asynchronous Asynchrony is the state of not being in synchronization. Asynchrony or asynchronous may refer to: Electronics and computing * Asynchrony (computer programming), the occurrence of events independent of the main program flow, and ways to deal wit ...
method requires multiple measurements to obtain a range by taking multiple bearings instead of appropriate scaling of active pings, otherwise the system is just capable of providing a simple bearing from any single measurement. Combining several measurements in a time sequence leads to tracking and tracing. A commonly used term for residing terrestrial objects is
surveying Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. A land surveying professional is ...
.


Other techniques

Measuring dimensions of localized structures (as opposed to large arrays of atoms like a crystal), as in modern
integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
s, is done using the scanning electron microscope. This instrument bounces electrons off the object to be measured in a high vacuum enclosure, and the reflected electrons are collected as a photodetector image that is interpreted by a computer. These are not transit-time measurements, but are based upon comparison of
Fourier transform A Fourier transform (FT) is a mathematical transform that decomposes functions into frequency components, which are represented by the output of the transform as a function of frequency. Most commonly functions of time or space are transformed ...
s of images with theoretical results from computer modeling. Such elaborate methods are required because the image depends on the three-dimensional geometry of the measured feature, for example, the contour of an edge, and not just upon one- or two-dimensional properties. The underlying limitations are the beam width and the wavelength of the electron beam (determining
diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
), determined, as already discussed, by the electron beam energy. and The calibration of these scanning electron microscope measurements is tricky, as results depend upon the material measured and its geometry. A typical wavelength is and a typical resolution is about Other small dimension techniques are the
atomic force microscope Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very-high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the op ...
, the focused ion beam and the helium ion microscope. Calibration is attempted using standard samples measured by
transmission electron microscope Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a gr ...
(TEM).
Nuclear Overhauser effect The nuclear Overhauser effect (NOE) is the transfer of nuclear spin polarization from one population of spin-active nuclei (e.g. 1H, 13C, 15N etc.) to another via cross-relaxation. A phenomenological definition of the NOE in nuclear magnetic res ...
spectroscopy (NOESY) is a specialized type of
nuclear magnetic resonance spectroscopy Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fie ...
where distances between atoms can be measured. It is based on the effect where nuclear spin cross-relaxation after excitation by a radio pulse depends on the distance between the nuclei. Unlike spin-spin coupling, NOE propagates through space and does not require that the atoms are connected by bonds, so it is a true distance measurement instead of a chemical measurement. Unlike diffraction measurements, NOESY does not require a crystalline sample, but is done in solution state and can be applied to substances that are difficult to crystallize.


Astronomical distance measurement


Other systems of units

In some systems of units, unlike the current SI system, lengths are fundamental units (for example, ''wavelengths'' in the older SI units and ''bohrs'' in
atomic units The Hartree atomic units are a system of natural units of measurement which is especially convenient for atomic physics and computational chemistry calculations. They are named after the physicist Douglas Hartree. By definition, the following four ...
) and are not defined by times of transit. Even in such units, however, the ''comparison'' of two lengths can be made by comparing the two transit times of light along the lengths. Such time-of-flight methodology may or may not be more accurate than the determination of a length as a multiple of the fundamental length unit.


List of devices


Contact devices

* Architect's scale *
Caliper A caliper (British spelling also calliper, or in plurale tantum sense a pair of calipers) is a device used to measure the dimensions of an object. Many types of calipers permit reading out a measurement on a ruled scale, a dial, or a digital d ...
* Diagonal scale * Engineer's scale *
Feeler gauge A feeler gauge is a tool used to measure gap widths. Feeler gauges are mostly used in engineering to measure the clearance between two parts. Description They consist of a number of small lengths of steel of different thicknesses with measureme ...
,
used in metal working to measure size of gaps * Gauge blocks *
Gunter's chain Gunter's chain (also known as Gunter’s measurement) is a distance measuring device used for surveying. It was designed and introduced in 1620 by English clergyman and mathematician Edmund Gunter (1581–1626). It enabled plots of land to be a ...
*
Measuring rod A measuring rod is a tool used to physically measure lengths and survey areas of various sizes. Most measuring rods are round or square sectioned; however, they can also be flat boards. Some have markings at regular intervals. It is likely th ...
* Meter stick * Metric scale *
Micrometer Micrometer can mean: * Micrometer (device), used for accurate measurements by means of a calibrated screw * American spelling of micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...
*
Opisometer An opisometer, also called a curvimeter, meilograph, or map measurer, is an instrument for measuring the lengths of arbitrary curved lines. Description A simple opisometer consists of a toothed wheel of known circumference on a handle. The whee ...
or
curvimeter An opisometer, also called a curvimeter, meilograph, or map measurer, is an instrument for measuring the lengths of arbitrary curved lines. Description A simple opisometer consists of a toothed wheel of known circumference on a handle. The whee ...
*
Pacing (surveying) Pacing is a reasonably easy and quick method of measuring distance in the field. It is used to measure a distance and is often used with a sighting or a hand compass. Most commonly, pacing is split up into segments, such as chains, which are set me ...
*
Ruler A ruler, sometimes called a rule, line gauge, or scale, is a device used in geometry and technical drawing, as well as the engineering and construction industries, to measure distances or draw straight lines. Variants Rulers have long ...
*
Stadimeter A stadimeter is an optical device for estimating the range to an object of known height by measuring the angle between the top and bottom of the object as observed at the device. It is similar to a sextant, in that the device is using mirrors to ...
*
Surveyor's wheel A surveyor's wheel, also called a clickwheel, hodometer, waywiser, trundle wheel, measuring wheel or perambulator is a device for measuring distance. Origin The origins of the surveyor's wheel are connected to the origins of the odometer. W ...
*
Tape measure A tape measure or measuring tape is a flexible ruler used to measure length or distance. It consists of a ribbon of cloth, plastic, fibre glass, or metal strip with linear measurement markings. It is a common measuring tool. Its design all ...
*
Thread pitch gauge A thread gauge, also known as a screw gauge or pitch gauge, is used to measure the pitch or lead of a screw thread. Thread pitch gauges are used as a reference tool in determining the pitch of a thread that is on a screw or in a tapped hole. T ...
* Ultrasonic thickness gauge * Yard stick


Non-contact devices

*
Ranging Length measurement, distance measurement, or range measurement (ranging) refers to the many ways in which length, distance, or range can be measured. The most commonly used approaches are the rulers, followed by transit-time methods and the in ...


Based on time-of-flight

* Electronic distance meter * Ultrasonic ranging module (
sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances (ranging), communicate with or detect objects on o ...
, echo sounding) * Radar distance measurement *
Laser rangefinder A laser rangefinder, also known as a laser telemeter, is a rangefinder that uses a laser beam to determine the distance to an object. The most common form of laser rangefinder operates on the time of flight principle by sending a laser pulse in ...
,
lidar Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...


See also

* Distance-based road exit numbers *
Linear referencing Linear referencing, also called linear reference system or linear referencing system (LRS), is a method of spatial referencing in engineering and construction, in which the locations of physical features along a linear element are described in te ...
*
Meridian arc In geodesy and navigation, a meridian arc is the curve between two points on the Earth's surface having the same longitude. The term may refer either to a segment of the meridian, or to its length. The purpose of measuring meridian arcs is to ...
*
Milestone A milestone is a numbered marker placed on a route such as a road, railway line, canal or boundary. They can indicate the distance to towns, cities, and other places or landmarks; or they can give their position on the route relative to so ...
*
Rangefinder A rangefinder (also rangefinding telemeter, depending on the context) is a device used to measure distances to remote objects. Originally optical devices used in surveying, they soon found applications in other fields, such as photography an ...
* GPS,
indirect by runtime measurement of electromagnetic waves in the GHz-range *
Hypsometer A hypsometer is an instrument for measuring height or elevation. Two different principles may be used: trigonometry and atmospheric pressure. Etymology The English word ''hypsometer'' originates from the Ancient Greek words ὕψος (húpsos, ...
*
Interferometer Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber o ...
* Macrometer *
Odometer An odometer or odograph is an instrument used for measuring the distance traveled by a vehicle, such as a bicycle or car. The device may be electronic, mechanical, or a combination of the two ( electromechanical). The noun derives from ancient G ...
*
Position sensor A position sensor is a sensor that detects an object's position. A position sensor may indicate the absolute position of the object (its location) or its relative position (displacement) in terms of linear travel, rotational angle or three-dimension ...
*
Positioning system A positioning system is a system for determining the position of an object in space. One of the most well-known and commonly used positioning systems is the Global Positioning System (GPS). Positioning system technologies exist ranging from worl ...
*
Standard ruler A standard ruler is an astronomical object for which the actual physical size is known. By measuring its angular size in the sky, one can use simple trigonometry to determine its distance from Earth. In simple terms, this is because objects of a f ...
, in astronomy * Tachymeter *
Taximeter A taximeter or fare meter is a mechanical or electronic device installed in taxicabs and auto rickshaws that calculates passenger fares based on a combination of distance travelled and waiting time. Its shortened form, "taxi", is also a metony ...
, measure usually includes a time component as well *
Tellurometer The tellurometer was the first successful microwave electronic distance measurement equipment. The name derives from the Latin ''tellus'', meaning Earth. History The original tellurometer, known as the Micro-Distancer MRA 1, was introduced in 1 ...
*
Travelling microscope A travelling microscope is an instrument for measuring length with a resolution typically in the order of 0.01mm. The precision is such that better-quality instruments have measuring scales made from Invar to avoid misreadings due to thermal effe ...
* Angular measuring instrument *
Altimeter An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water. The m ...
, height *
Distance measuring equipment (aviation) In aviation, distance measuring equipment (DME) is a radio navigation technology that measures the slant range (distance) between an aircraft and a ground station by timing the propagation delay of radio signals in the frequency band between 9 ...
* Ellipsometry#Imaging ellipsometry * Frequency-modulated continuous-wave radar (FMCW) *
Length scale In physics, length scale is a particular length or distance determined with the precision of at most a few orders of magnitude. The concept of length scale is particularly important because physical phenomena of different length scales cannot ...
*
Low-energy electron microscopy Low-energy electron microscopy, or LEEM, is an analytical surface science technique used to image atomically clean surfaces, atom-surface interactions, and thin (crystalline) films. In LEEM, high-energy electrons (15-20 keV) are emitted from an e ...
*
Orders of magnitude (length) The following are examples of orders of magnitude for different lengths. __TOC__ Overview Detailed list To help compare different orders of magnitude, the following list describes various lengths between 1.6 \times 10^ metres and 10 ...
*
Pulse-Doppler radar A pulse-Doppler radar is a radar system that determines the range to a target using pulse-timing techniques, and uses the Doppler effect of the returned signal to determine the target object's velocity. It combines the features of pulse radars and ...
* Range ambiguity resolution *
Cosmic distance ladder The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible o ...
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Bradley A. Fiske Rear Admiral Bradley Allen Fiske (June 13, 1854 – April 6, 1942) was an officer in the United States Navy who was noted as a technical innovator. During his long career, Fiske invented more than a hundred and thirty electrical and mechanica ...
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Dazzle camouflage Dazzle camouflage, also known as razzle dazzle (in the U.S.) or dazzle painting, is a family of ship camouflage that was used extensively in World War I, and to a lesser extent in World War II and afterwards. Credited to the British marine ...
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Depression range finder The depression range finder (DRF) was a fire control device used to observe the target's range and bearing to calculate firing solutions when gun laying in coastal artillery. It was the main component of a vertical base rangefinding system. ...
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Fire-control system A fire-control system (FCS) is a number of components working together, usually a gun data computer, a director, and radar, which is designed to assist a ranged weapon system to target, track, and hit a target. It performs the same task as a ...
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Range-finder painting A range-finder painting, sometimes called range-finding painting, is a large landscape painting produced as a training device to help gunners improve their accuracy. Historically, the best-documented use of such paintings was in the United States d ...
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Rangefinding telemeter A rangefinder (also rangefinding telemeter, depending on the context) is a device used to measure distances to remote objects. Originally optical devices used in surveying, they soon found applications in other fields, such as photography an ...
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Slant range In radio electronics, especially radar terminology, slant range or slant distance is the distance along the relative direction between two points. If the two points are at the same level (relative to a specific datum), the slant distance equals t ...
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Tacheometry Tacheometry (; from Greek for "quick measure") is a system of rapid surveying, by which the horizontal and vertical positions of points on the earth's surface relative to one another are determined without using a chain or tape, or a separate l ...
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Telemeter chronograph A chronograph is a specific type of watch that is used as a stopwatch combined with a display watch. A basic chronograph has an independent sweep second hand and a minute sub-dial; it can be started, stopped, and returned to zero by successive ...
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Tellurometer The tellurometer was the first successful microwave electronic distance measurement equipment. The name derives from the Latin ''tellus'', meaning Earth. History The original tellurometer, known as the Micro-Distancer MRA 1, was introduced in 1 ...


References


Further reading

* {{citizendium, title=Metre (unit) SI units Metrology Scientific techniques Length, distance, or range measuring devices