Measurement of liquid flow in open channels - Velocity-area methods (ISO 748:1997)

Durchflußmessung in offenen Gerinnen - Geschwindigkeitsflächen-Verfahren (ISO 748:1997)

Mesure de débit des liquides dans les canaux découverts - Méthodes d'exploration du champ des vitesses (ISO 748:1997)

Merjenje pretoka tekočin v odprtih kanalih - Metode hitrost-površina (ISO 748:1997)

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Publication Date
Withdrawal Date
Technical Committee
Drafting Committee
Current Stage
9960 - Withdrawal effective - Withdrawal
Due Date
Completion Date


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SIST EN ISO 748:2001

Measurement of liquid flow in open channels - Velocity-area methods (ISO 748:1997)

Durchflußmessung in offenen Gerinnen - Geschwindigkeitsflächen-Verfahren (ISO

Mesure de débit des liquides dans les canaux découverts - Méthodes d'exploration du

champ des vitesses (ISO 748:1997)
Ta slovenski standard je istoveten z: EN ISO 748:2000
17.120.20 Pretok v odprtih kanalih Flow in open channels
SIST EN ISO 748:2001 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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Third edition
Measurement of liquid flow in open
channels - Velocity-area methods
Mesure de dkbit des liquides dans /es canaux d&ouverts - Mkthodes
d ’exploration du champ des vitesses
Reference number
IS0 748: 1997(E)
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IS0 748: 1997(E)
Contents Page

1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Normative reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... ‘.

3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

4 Principle of the methods of measurements . . . . . . . . . . . . . . . . . . . . . . . . . . ...*. 1

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

5 Selection and demarcation of site

6 Measurement of cross-sectional area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

7 Measurement of velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

a Computation of discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9 Uncertainties in flow measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

A Correction for sag, pull, slope and temperature in measurement

of cross-section width by tape or wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
B Measurement across the cross-section
C Corrections for wetted length of wire when measuring depths

with wire not normal to surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

D Correction for drift

E Uncertainty of a velocity-area measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

F Determination of mean velocity from float measurements . . . . . . . . 39

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

G Bibliography
0 IS0 1997

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced

or utilized in any form or by any means, electronic or mechanical, including photocopying and

microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 l CH-1211 Geneve 20 l Switzerland
Internet central @
c=ch; a=400net; p=iso; o=isocs; s=central
Printed in Switzerland
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0 IS0
IS0 7483 997(E)

IS0 (the International Organization for Standardization) is a worldwide federation of national standards bodies (IS0

member bodies). The work of preparing International Standards is normally carried out through IS0 technical

committees. Each member body interested in a subject for which a technical committee has been established has

the right to be represented on that committee. International organizations, governmental and non-governmental, in

liaison with ISO, also take part in the work. IS0 collaborates closely with the International Electrotechnical

Commission (IEC) on all matters of electrotechnical standardization.

Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.

Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.

International Standard IS0 748 was prepared by Technical Committee lSO/TC 113, Hydrometric determinations,

Subcommittee SC 1, Velocity area methods.

This third edition cancels and replaces the second edition (IS0 748:1979), which has been technically revised.

Annexes A to G of this International Standard are for information only.
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IS0 748: 1997(E)
Measurement of liquid flow in open channels - Velocity-area
1 Scope

This International Standard specifies methods for determining the velocity and cross-sectional area of water flowing

in open channels without ice cover, and for computing the discharge therefrom.

It covers methods of employing current-meters and floats to measure the velocities. Although, in most cases, these

measurements are intended to determine the stage-discharge relation of a gauging station, this International

Standard deals only with single measurements of the discharge; the continuous recording of discharges over a

period of time is covered in IS0 1100-I and IS0 1100-2.

NOTE The methods for determining the velocity and cross-sectional area of water flowing in open channels with ice cover are

specified in IS0 9196.
2 Normative reference
constitute provisions of this

The following standard contains provisions which, through reference in this text,

International Standard. At the time of publication, the edition indicated was valid. All standards are subject to

revision, and parties to agreements based on this International Standard are encouraged to investigate the

possibility of applying the most recent edition of the standard indicated below. Members of IEC and IS0 maintain

registers of currently valid International Standards.
IS0 7723 996, Hydrometric determinations - Vocabulary and symbols.
3 Definitions

For the purposes of this International Standard, the definitions given in IS0 772 and the following definition apply.

3.1 unit-width discharge
discharge through a unit width of a section at a given vertical
4 Principle of the methods of measurements

4.1 The principle of these methods consists of measuring velocity and cross-sectional area. A measuring site is

chosen conforming to the specified requirements; the width, depending on its magnitude, is measured either by

means of steel tape or by some other surveying method, and the depth is measured at a number of points across

the width, sufficient to determine the shape and area of the cross-section.

Velocity observations are made at each vertical preferably at the same time as measurement of depth, especially in

the case of unstable beds. They are made by any one of the standard methods using current-meters. If unit width

discharge is required, it is generally computed from the individual observations.

In the integration method, the mean velocity is obtained directly.
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0 IS0
IS0 748:1997(E)

Under certain circumstances, velocity observations can also be made using surface floats or velocity-rods. Other

methods consist of measuring the velocity along one or several horizontal lines of the section (e.g. moving-boat and

ultrasonic methods.)

4.2 The discharge is computed either arithmetically or graphically by summing the products of the velocity and

corresponding area for a series of observations in a cross-section.
5 Selection and demarcation of site
5.1 Selection of site

The site selected should comply as far as possible with the following requirements:

a) The channel at the measuring site should be straight and of uniform cross-section and slope in order to

minimize abnormal velocity distribution.

NOTE When the length of the channel is restricted, it is recommended for current-meter measurements, or other velocity-

meter measurements, that the straight length upstream should be at least twice that downstream.

Flow directions for all points on any vertical across the width should be parallel to one another and at right

angles to the measurement section.

The bed and margins of the channels should be stable and well defined at all stages of flow in order to facilitate

accurate measurement of the cross section and ensure uniformity of conditions during and between discharge


The curves of the distribution of velocities should be regular in the vertical and horizontal planes of


Conditions at the section and in its vicinity should also be such as to preclude changes taking place in the

velocity distribution during the period of measurement.
Sites displaying vortices, reverse flow or dead water should be avoided.

The measurement section should be clearly visible across its width and unobstructed by trees, aquatic growth

or other obstacles. When gauging from a bridge with divide piers, each section of the channel should be treated


The depth of water at the section should be sufficient at all stages to provide for the effective immersion of the

current-meter or float, whichever is to be used.

The site should be easily accessible at all times with all necessary measurement equipment.

The section should be sited away from pumps, sluices and outfalls, if their operation during a measurement is

likely to create flow conditions inconsistent with the natural stage-discharge relationship for the station.

Sites where there is converging or diverging flow should be avoided.

In those instances where it is necessary to make measurements in the vicinity of a bridge, it is preferable that

the measuring site be upstream of the bridge. However in special cases and where accumulation of ice, logs

or debris is liable to occur, it is acceptable that the measuring site be downstream of the bridge. Particular care

should be taken in determining the velocity distribution when bridge apertures are surcharged.

The measurement of flow under ice cover is dealt with in IS0 9196 but for streams subject to formation of ice

cover, requirements of measurement specified in this International Standard can be used during the free water

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0 IS0
IS0 748: 1997(E)

n) It may, at certain states of river flow or level, prove necessary to carry out current-meter measurements on

sections other than that selected for the station. This is quite acceptable if there are no substantial ungauged

losses or gains to the river in the intervening reach and so long as all flow measurements are related to levels

recorded at the principal reference section.
5.2 Demarcation of site

NOTE If the site is to be established as a permanent station or likely to be used for future measurement, it should be provided

with means for demarcation of the cross-section and for determination of stage.

52.1 The position of each cross-section, normal to the mean direction of flow, shall be defined on the two banks

by clearly visible and readily identifiable markers. Where a site is subject to considerable snow cover, the section

line-markers may be referenced to other objects such as rock cairns.

5.2.2 The stage shall be read from a gauge at intervals throughout the period of measurement and the gauge

datum shall be related by precise levelling to a standard datum.

52.3 An auxiliary gauge on the opposite bank shall be installed where there is likelihood of a difference in the

level of water surface between the two banks. This is particularly important in the case of very wide rivers. The

mean of the measurements taken from the two gauges shall be used as the mean level of the water surface and as

a base for the cross-sectional profile of the stream.
6 Measurement of cross-sectional area
6.1 General

The cross-sectional profile of the open channel at the gauging-site shall be determined at a sufficient number of

points to establish the shape of the bed.

The location of each point is determined by measuring its horizontal distance to a fixed reference point on one bank

of the channel, in line with the cross-section. This in turn allows calculation of the area of individual segments

separating successive verticals where velocities are measured.
6.2 Measurement of width

Measurement of the width of the channel and the width of the individual segments may be obtained by measuring

the horizontal distance from or to a fixed reference point which shall be in the same plane as the cross-section at

the measuring site.

6.2.1 Where the width of the channel permits, these horizontal distances shall be measured by direct means, for

example a graduated tape or suitable marked wire, care being taken to apply the necessary corrections given in

annex A. The intervals between the verticals, i.e. the widths of the segments, shall be similarly measured.

6.2.2 Where the channel is too wide for the above methods of measurement, the horizontal distance shall be

determined by optical or electronic distance-meters, or by one of the surveying methods given in annex B.

6.3 Measurement of depth

6.3.1 Measurement of depth shall be made at intervals close enough to define the cross-sectional profile

accurately. In general, the intervals shall not be greater than l/20 of the width.

NOTE 1 For small channels with a regula r bed profile, the num ber of i ntervals may be reduced. This may, h

the accu racy of the determinati on of the bed profile (see 7.1.3 and clause

NOTE 2 Accuracy of measurement of discharge is increased by decreasing the spacing between verticals.

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0 IS0
IS0 748:1997(E)

6.3.2 The depth shall be measured by employing either sounding-rods or sounding-lines or other suitable devices.

Where the channel is of sufficient depth, an echo-sounder may be used. If the velocity is high and the channel is

sufficiently deep, it is preferable to use an echo-sounder or other device which will not require large corrections.

6.3.3 When a sounding-rod or sounding-line is used, it is desirable that at least two readings be taken at each

point and the mean value adopted for calculations, unless the difference between the two values is more than 5 %,

in which case two further readings shall be taken. If these are within 5 %, they shall be accepted for the

measurement and the two earlier readings discarded. If they are again different by more than 5%, no further

readings shall be taken but the average of all four readings shall be adopted for the measurement, noting that the

accuracy of this measurement is reduced.

When an echo-sounder is used, the average of several readings shall always be taken at each point. Regular

calibrations of the instrument shall be carried out under the same conditions of salinity and temperature of the water

to be measured.

NOTE Where it is imp #racticable to take more than one reading of the depth, the uncertainty in measurement may be

increased (see clause 9).

6.3.4 Where measurements of the depths are made separately from the velocity measurements and the water

level is not steady, the water level shall be observed at the time of each measurement of the depth. When this is

not possible, the water level shall be observed at intervals of 15 min and the value of the level at the time of each

determination of depth shall be obtained by interpolation.

NOTE 1 When, during the measurement of discharge, the bed profile changes appreciably, depth measurements should be

carried out by taking one depth reading at each point at the beginning and one at the end of the velocity measurement at each

vertical, and the mean value of these two measurements shall be taken as the effective depth. Care should be exercised when

taking repeated soundings to avoid disturbance of the bed.
NOTE 2 Inaccuracies in soundings are most likely to occur owing to:

the departure from the vertical of the sounding-rod or line, particularly in deep water, when the velocity is high;

the penetration of the bed by the sounding-weight or -rod;
the nature of the bed when an echo-sounder is used.

Errors due to a) may be minimized by the use, where practicable, of an echo-sounder, or pressure-measuring device. The

effect of drag on a sounding-line may be reduced by using a streamlined lead weight at the end of a fine wire. A correction

shall be applied to the wetted length of wire if the wire is not normal to the water-surface. It is recommended that the angle of

departure from the vertical of the sounding line should not be greater than 300 in view of the inaccuracies involved. Methods of

applying the correction are given in annex C.

Errors due to b) may be reduced by fitting a baseplate to the lower end of the sounding-rod, or by fastening a disk to the end of

the sounding-line, provided they will not cause additional scour of fine bed material due to high velocities.

Errors due to c) may be reduced by selecting an echo-sounder frequency that most adequately depicts the bed-water interface.

NOTE 3 In certain cases, for example floods, it may be impossible to determine an adequate profile of cross-section during

For those cases, the full profile shall be determined by surveying methods, either before or after the

the measurement.

measurement. However, it should be recognized that this method is subject to errors due to possible erosion or deposition in

the cross-section between the time the profile is determined and the time of discharge measurement.

7 Measurement of velocity
7.1 Measurement of velocity using current-meters
7.1 .I Rotating-element current-meters

Rotating-element current-meters should be constructed, calibrated and maintained according to IS0 2537 and

IS0 3455.They should be used only within their calibrated range and fitted on suspension equipment similar to that

used during calibration.
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63 IS0 IS0 748: 1997(E)

In the vicinity of the mini mum speed of response , the uncertainty in determin ing the veloc ity is high. Care should be

exerci sed when measuri ng velocities near the mi nimu m speed of response.

For high velocities, the propeller, in the case of propeller-type current-meters, or the reduction ratio where available,

shall be chosen in order that the maximum speed of rotation can be correctly measured by the revolution counter.

No rotating-element current-meter shall be selected for use in water where the mean depth is less than 4 times the

diameter of the impeller that is to be used, or of the body of the meter itself, whichever is the greater. No part of the

meter shall break the surface of the water.
7.1.2 Electromagnetic current-meters

Electromagnetic current-meters are acceptable for making measurements of point velocity. These current-meters

have the advantage that they have no moving parts and thereby eliminate all friction and resistance. They should

be calibrated throughout the range of velocity for which they are to be used, and should meet accuracy

requirements similar to rotating-element current-meters. They should not be used outside the range of calibration.

Electromagnetic current-meters are capable of operation in shallow depths and of detecting and measuring flow

reversal. No electromagnetic current-meter shall be selected for use in water whose mean depth is less than 3

times the vertical dimension of the probe.
and provide a digital readout of velocity
The control box of the electromagnetic meter should be splashproof
instantaneously or averaged over preset time periods.

The sensor of the electromagnetic meter should have a moulded epoxy resin pod with no protrusions, containing an

It shall be relatively immune to fouling or damage,
electromagnetic sensor and solid-state encapsulated circuitry.
simple to clean and maintain and be readily interchangeable.
7.1.3 Measurement procedure

Velocity observations are normally made at the same time as measurements of the depth. This method shall be

used in the case of unstable beds. Where, however, the two measurements are made at different times, the

velocity observations shall be taken at a sufficient number of places, and the horizontal distance between

observations shall be measured as described in 6.2.1 and 6.2.2.

In judging the specific number n of verticals that are to be defined for the purpose of gauging flow at a particular

location, the following criteria shall be applied.
Channel width > 0 and < 0,5 m n=3to4
n = 4 to 5
Channel width > 0,5 m and < 1 m
Channel width > 1 m and < 3 m n=5to8
Channel width > 3 m and < 5 m
Channelwidth>5mand Channel width >I0 m n 2 20

In all instances, measurements of depth or velocity made at the water ’s edge are additional to the above.

It is further recommended that the location of the verticals be selected after a previous cross-section survey. When

the channel is sufficiently uniform it may be possible to reduce the number of verticals and to allocate equal

distance spacing between the verticals without conflicting with the above requirement.

The verticals uld be chosen so that the discharge in each segment
sho is less than 5 % of the total, insofar as
possible, and in no case sho luld it exceed 10 %.
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IS0 748: 1997(E)

The current-meter shall be held in the desired position in each vertical by means of a wading-rod in the case of

shallow channels, or by suspending it from a cable or rod in the case of deeper channels. When a boat is used, the

current-meter shall be held so that it is not affected by disturbances of flow caused by the boat.

The current-meter shall be placed at the selected point in the vertical so that the horizontal axis of the meter is

parallel to the direction of flow at that point. The meter shall be allowed to adjust to the flow before the readings are


NOTE Care should be taken to ensure that the current-meter observations are not affected by random surface-waves and


NOTE 2 When a number of points in a vertical are to be measured, a battery of current-meters fixed to the same rod can be

used to measure corresponding velocities simultaneously whilst ensuring that there is no mutual interference.

If there is any appreciable deflection of the cable on which the meter is suspended, a correction shall be applied for

the depth of the measuring-point. No generally applicable correction factor can be given, but it shall be determined

by the user for the particular instrument and conditions of measurement. However, the values given in annex C

may serve as a guide.

NOTE 3 The selection and use of appropriate suspension equipment is described in IS0 3454 and IS0 4375.

The velocity at each selected point shall be observed by exposing
a) a rotating-element current-meter for a minimum of 30 s, or
b) an electromagnetic current-meter for a minimum of 10 s.

Where the velocity is subject to periodic pulsations, the exposure time should be increased accordingly. (See

ISO/TR 7178.)

current-meter shall be removed from the water or brought to the surface at intervals for exam ination, usually

whe n passing from vertical to another.

A spin test, where appropriate, should be performed after each discharge measurement to ensure that the

mechanism operates freely (see IS0 2537).

More than one current-meter may be used in determining velocities in the individual verticals, different current-

meters being used for consecutive verticals.

In channels where the flow is unsteady, it is possible to correct for the variations in the total discharge during the

period of the measurement not only by observing the change in stage, but also by continuously measuring the

velocity at some conveniently chosen point in the main current.
7.1.4 Oblique flow

If oblique flow is unavoidable, the angle of the direction of the flow to the perpendicular to the cross-section shall be

measured and the measured velocity adjusted. Special instruments have been developed for measuring the angle

and velocity at a point simultaneously. Where, however, these are not available and there is insignificant wind, the

angle of flow throughout the vertical can be taken to be the same as that observed on the surface. This angle can

be measured with appropriate equipment provided that the operator is located above the measurement vertical. If

the channel is very deep or if the local bed profile is changing rapidly, this assumption shall not be accepted without


If the measured angle to the perpendicular to the cross section is g ,the velocity used for computation of flow

discharge shall be:
corrected = ‘measured ‘OS r

NOTE Some current-meters are equipped to measure the normal component of ve ‘locity directly when held perpendicular

to the measurement cross-section. This correction should not be applied in such cases.

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0 IS0 IS0 748:1997(E)
7.1.5 Method for mean velocity measurement in a vertical Choice and classification
These are: time available, width
The choice of the method for velocity measurement depends on certain factors.

and depth of the channel, bed conditions in the measuring section and the upstream reach, rate of variation of level,

degree of accuracy wanted and equipment used.
These methods are classified as follows:
a) Velocity distribution method (see
b) Reduced point methods (see
c) Integration method (see
d) Other methods (see Velocity distribution method

Using this method, the values of the velocity are obtained from observations at a number of points on each vertical

between the surface of the water and the bed of the channel. The number and spacing of the points should be so

chosen as to define accurately the velocity distribution on each vertical with a difference in readings between two

The location of the top and the bottom
adjacent points of not more than 20 % with respect to the higher value.

readings should be chosen, taking into account the specification under 7.1 .I (see IS0 1088).

The velocity observations at each position are then plotted and the unit width discharge or mean velocity

determined by planimeter, digitizer or equivalent method.

NOTE 1 This method may not be suitable for routine discharge measurements beta use the apparent gain in precision


be offset by errors resulting from change of stage during the long period of time ne eded for making the measurement.

NOTE 2 The velocity curve can be extrapolated from the last measuring point to the bed or wall by calculating vx from the

= vu -

vx is the open point velocity in the extrapolated zone at a distance Xfrom the bed or wall.

va is the velocity at the last measuring point at a distance a from the bed or wall.

The mean velocity V between the bottom r a ve rtical side) of the cha nnel and the nearest point of measurement (where the

measured velocity is v,) can be ca lculated di rectly from the equation
m is an exponent
d is the total depth of flow

Generally m lies between 5 and 7 but it may vary over a wider range depending on the

hydraulic resistance. The value m = 2
applies to coarse beds or walls while m = 10 is characteristic of smoot
h beds or walls.
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0 IS0
IS0 748: 1997(E)
m is obtained as follows:
m = 2 h + Cv,r +Q3
L 1
g = acceleration due to gravity (m/s ’);
C vert = Chezy ’s coefficient on a vertical (m ”*“/s).

NOTE 3 An alternative method of obtaining the velocity in the region beyond the last measuring-point is based on the

assumption that the velocity for some distance up from the bed of the channel is proportional to the logarithm of the distance X

from that boundary. If the observed velocities at points approaching the bed are plotted against log X, then the best-fitting

straight line through these points can be extended to the boundary. The velocities close to the boundary can then be read from

the graph. Reduced point methods

These methods, less strict than methods exploring the entire field of velocity, are used frequently because they

require less time than the velocity-distribution method ( They are based, however, on assumed velocity


:hat for a new gaug ing section the accuracy of the selected method be assessed comparing

It IS recommended t
those obtai ned from the velocity distribution method.
the results of prelimi nary gaugings with
a) Two-point method
each vertical by exposing the current-meter at 0,2 and 0,8 of the depth
Velocity observations shall be made at
two values shall be taken as the mean velocity in the ve rtical.
below the surface. The average of the
One-point method

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