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1、International Standard If 6420 0 a 4 *lb INTERNATLONAL ORGANIZATION FOR STANOARDIZATION*MEYHAPOHAR OPAHbl3AUklR fl0 CTAHAAPTH3AUblWORGANlSATlON INTERNATIONALE DE NORMALISATION Liquid flow measurement in open channels - Position fixing equipment for hydrometric boats Mesure de dkbit des liquides dans
2、 les canaux d protective eyeglasses should be worn. Since the path of a laser beam in clear air is difficult to see, it is often necessary to use some other means, such as shore targets, to assist in manoeuvring the boat into the path of the laser. Another optical device that can be used to give lin
3、e depends on sets of flashing lights configured so that an observer on a boat will see different sequences of flashes if the boat is on either side of the line, and a third sequence of flashes if the boat is exactly on the line. Copyright International Organization for Standardization Provided by IH
4、S under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/22/2007 22:48:22 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 6420-1984 (E) It m V Initial point, IP , Figure 1 - Position fixing for stream gauging n n / / / Figure 2 - Positi
5、on fixing for morphological surveys Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/22/2007 22:48:22 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 6420-19
6、84 (El 6.3 Radio frequency devices Radio frequency devices that operate at microwave frequencies can also be used to give line. A transmitter produces trains of pulses having two slightly different frequencies (for example a difference of 300 Hz) and two different repetition rates, the energy being
7、fed alternately to horn antennae about 6O on either side of the centreline of the course. The signals can be received on the boat and processed to detect course deviations that are of the order of f O,l”; positioning accuracies of + 0,2O are feasible. As the use of the electromagnetic spectrum is go
8、verned by the International Telecommunication Union (ITU), any use of radio frequency devices shall be licensed by the appropriate national agency. 7 Equipment that measures angles 7.1 Theodolites Conventional theodolites placed on the river bank may be used in measurement of angles. If more than on
9、e instrument is used, hand signals or radios should be employed to ensure that angles are measured simultaneously. 7.2 Sextants A sextant may be used from a boat to determine the angle be- tween two points on shore. Care should be taken to ensure that the sextant is held carefully so that a horizont
10、al angle is measured. Observations may be made to a precision of 20” of arc although under field conditions the uncertainty in a single observation is of the order of 3 of arc. The measured angles should be larger than 25O and the shore base line should be as long as possible, certainly not less tha
11、n 50 % of the distance to the boat, in order to avoid significant positioning errors. 7.3 Electronic angle measuring equipment Electronic angle measuring equipment is now available. These devices are, in effect, an electronic version of the sextant and have about the same accuracy. As they are usual
12、ly combined with electronic distance measuring devices, details are given in 8.4. 8 Equipment that measures distance 8.1 Tag lines The tag line is used most frequently for width measurements when measuring by boat or wading. A typical tag line consists of a marked corrosion-resistant steel cable of
13、diameter 2 to 3 mm. The diameter of the tag line depends on the width of the channel, the velocity of the water and whether or not the same tag line is used for holding the boat and for determining its position. In the latter case, larger diameter tag lines may be needed. Long tag lines are usually
14、wound on a drum having a diameter of at least 0,3 m and equipped with a means of braking. Tag lines are commonly used on channels up to 300 m in width and in special circumstances even wider; however, the accuracy of the tag line measurement depends on cable ten- sion. Under favourable conditions, t
15、he error may be less than 0,l % of the distance from the initial point to the measured point. 8.2 Rangefinders 8.2.1 The simple optical rangefinder is a device that can be used to measure distances in a range of about 30 to 500 m. When measuring the distance to an object, the point of interest is vi
16、ewed through one eyepiece and a smaller, inverted image brought into coincidence with the main image using a vernier adjustment. The distance to the object may then be read directly through a second eyepiece. Typical uncertainties using a rangefinder having a 0,5 m baseline are about 0,2 % at 30 m,
17、I,0 % at 250 m and 2,5 % at 500 m. 8.2.2 Ranging instruments are available that use infra-red or laser radiation. They reduce to an insignificant level the prob- lem of the degradation of accuracy with increasing distance. 8.3 Stadia rods By using a stadia rod in the boat and an engineer s level or
18、other stadia reading instruments on shore, the distance to the boat may be measured. If half a stadia interval is used, distances up to about 600 m may be measured without ex- ceeding a reasonable stadia rod length. The accuracy of the distance measured is not good due to the difficulty of holding t
19、he stadia rod vertical in a boat and to the problem of reading the stadia rod correctly especially if the boat has some vertical movement as a result of rough water. The uncertainty of the measurement of the stadia interval is likely to be about 0,l m. 8.4 Electronic distance measuring equipment 8.4
20、.1 General Electronic distance measuring devices can use electromagnetic waves in the frequency band from the optical region to the super-high radio frequency region. Instruments using radio waves operate on the principle that if a carrier is frequency modulated, it will exhibit a phase shift that i
21、s proportional to the distance travelled and to the modulating frequency. By using a number of modulating frequencies and comparing the phase shifts of a signal that has travelled between a master unit and a remote unit to a reference signal, it is possible to determine distance to a high degree of
22、accuracy over a wide range of distances. Instruments that use various carrier frequencies, both optical and radio, have been developed. Typical operating frequencies are about 3 or 9 GHz. 8.4.2 Instrument performance criteria The functional requirements for a position fixing system will vary accordi
23、ng to user needs. The criteria that shall be used in evaluating various manufacturers products are as follows : a) System configuration : The system should provide a series of simultaneous distance measurements from a 3 Copyright International Organization for Standardization Provided by IHS under l
24、icense with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/22/2007 22:48:22 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 8420-1984 (E) mobile boat (velocities up to 15 m/s) to one or two shore- based locations. The unit carried on the boat shal
25、l have a visual display of distance and shall have provision for exter- nally triggered digital data outputs. b) Distance determination : The distances measured shall be an independent, direct determination of the distance be- tween the master and remote units. The system should automatically recove
26、r from any length of temporary signal loss due to interference. c) System coverage : The field-of-view from the master unit should be 360” horizontally and at least loo vertically. Remote shore stations should have at least a 90” horizontal and loo vertical field-of-view. d) System range : The syste
27、m should measure distances ranging from 100 to 50 000 m. It should also be possible to use the system in a static mode to measure base line distances to the required accuracy. e1 Packaging : The system should be as small and as light as possible to permit ease of transportation and installation. The
28、 master unit should preferably be capable of being mounted in readily available electronic equipment racks and the remote units on readily available survey instrument tripods. No single piece of the system should have a mass exceeding 25 kg. f) Antennae : The antennae should be simple and light in w
29、eight and should be capable of being remotely located up to 5 m away using coaxial cable rather than wave guide. g) Power supply : The system should operate using con- ventional batteries. Power consumption should be suffi- ciently low to ensure not less than 8 h continuous operation of the master a
30、nd remote units without battery charging. h) Environmental : The system should be capable of operation during any ambient temperature and humidity that is likely to be encountered, typically -20 to + 50 OC, and a relative humidity of 100 %, without condensation. Also, the system should operate witho
31、ut additional protec- tion during periods of heavy rain or splash. j) Voice communication : A voice communication system between the master and remote units is desirable. k) Resolution and accuracy : The system shall provide distance measurements to a resolution of 0,l m and have an accuracy that is
32、 of the order of I,0 m ptus a percentage (for example 0,001 %I of the distance measured. 9 Selection of position fixing equipment 9.1 General The selection of position fixing equipment for a given task will depend on the accuracy requirements and the resources available, both personnel and equipment
33、. The final system selection will involve a compromise between these factors, 9.2 Accuracy requirements When position fixing equipment is used in stream gauging, the uncertainty in the distances from an initial point should be of the order of * 1 % of the total width of the measuring section or + 1
34、m, whichever is greater. Care should be taken to ensure that the boat is on the section line, not upstream or downstream. In morphological surveys, uncertainty should be as little as possible - normally f 1 m is sufficient. Clause 11 describes the errors that can arise in the various methods of posi
35、tion fixing. 9.3 Resource requirements Ideally a boat operator should be able to use the position fixing equipment to position a craft without the assistance of other persons. In actual practice, some position fixing techniques re- quire the presence of one or two persons on shore and others in the
36、boat. Where feasible, the use of intersecting target lines or electronic distance measuring equipment will enable a boat operator to work independently, while one or more persons operate gauging or other equipment. Conventional surveying equipment is usually readily available and is frequentty used.
37、 As the complexity of the equipment in- creases, it is less likely to be available: however, some types of position fixing equipment may be obtained on a rental basis. In considering equipment, it should be kept in mind that some types may provide greater accuracy and may reduce personnel or data co
38、mputation requirements. IO Operating instructions manual A comprehensive manual shall be supplied, where necessary, with each piece of equipment. The manual shall provide full operating instructions complete with illustrations and accurate circuit diagrams, where applicable. The manual shall also co
39、n- tain maintenance and fault finding information, and a list of recommended spare parts. If particular safety precautions are required, these shall be stated. 11 Errors 11 .l General Many methods of position fixing do not lend themselves to rigorous analyses of errors since they involve the use of
40、the human eye in, for example, aligning targets under dynamic conditions. The accuracy of such observations depends on an individual s visual acuity plus the atmospheric and lighting con- ditions at the time of the observation. As observations for posi- tion fixing frequently involve sighting on a m
41、oving target, repetition of an observation does not always improve the ac- curacy of the observation. The preferred procedure is to use a number of simultaneous and independent observations. The errors that occur in position fixing tend to be random in nature; systematic errors become a problem only
42、 when in- struments are improperly adjusted or calibrated. The calibration of an instrument should be verified prior to use by measuring, for example, known angles or distances. Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical St
43、andards 1/9972545001 Not for Resale, 04/22/2007 22:48:22 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 8420-1984 (El 11.2 Errors in position fixing for stream gauging Position fixing for stream gauging may involve the use of intersecting target lines, in which case th
44、e uncertainty in the fix is unpredictable although it will tend to be inversely propor- tional to the length of base line, or may involve the use of a target line which establishes the section line plus measurement of a distance or an angle. In this case, results are more pre- dictable. If a fix is
45、made when a boat is upstream or down- stream of the section line, the result is a translation error along the section line away from the reference point or line. These translation errors tend to be small in comparison to in- strument error even for significant deviations upstream or downstream of th
46、e section line. For example, a translation error of + 0,l m is produced when a boat is 5 m off the section line at a distance of 100 m. Translation errors can be compensated for to some extent if the true distance to the far shore is known. The uncertainty in the fix is therefore dependent mainly on
47、 the instrument used in making the fix. If it is assumed that a boat is exactly on the section line and position is determined by measuring one angle, the uncertainty in the fix will depend on the magnitude of the angle measured. For example, assume a 100 m base tine at right angles to the section l
48、ine and a sextant reading accuracy of 21 3 of arc. The uncertainty in the distance along the section for various angles (see figure 6) becomes : 80 - f 2,90 m 60 - * 0,35 m 40 - f 0,15 m 20 - f 0,lO m When distance measuring equipment is used, the uncertainty generally increases with distance measur
49、ed. The figures given in clause 8 are typical. 11.3 Errors in position fixing for morphological surveys In the case of morphological surveys, an uncertainty ellipse is set up around the measured boat position. When trilateration is used, the ellipse is smallest when the boat position is at right angles to the base line and increases in size as the boat moves away from this position. When angular m