ISO 23435:2022

Air quality — Test methods for snow depth sensors

This document provides requirements for the evaluation and use of test method for snow depth sensors. This document is applicable to the following types of automatic snow depth sensors which employ different ranging technologies by which the sensors measure the distance from the snow surface to the sensor: a) Ultrasonic type, also known as sonic ranging depth sensors; b) Optical laser snow depth sensors including single point and multipoint snow depth sensors; c) Other snow depth sensors. This document mainly covers two major tests: a laboratory(indoor) test and a field (outdoor) test. The laboratory test includes the basic performance test and other tests under various environmental changes. The field test is proposed to ensure the performance of the snow depth sensors in field measurement conditions. For the field test, both the natural ground and artificial target surface such as snow plates are considered for the procedures defined in this document.

Qualité de l'air — Méthodes d'essai des capteurs de hauteur de neige

General Information

• Status: Published
• Publication Date: 20-Apr-2022
• ICS: 07.060 - Geology. Meteorology. Hydrology
• Technical Committee: ISO/TC 146/SC 5 - Meteorology
• Drafting Committee: ISO/TC 146/SC 5/WG 10 - Snow depth sensors
• Current Stage: 6060 - International Standard published
• Start Date: 21-Apr-2022
• Due Date: 13-Sep-2022
• Completion Date: 21-Apr-2022

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 23435
First edition
2022-04
Air quality — Test methods for snow
depth sensors
Qualité de l'air — Méthodes d'essai des capteurs de hauteur de neige
Reference number
ISO 23435:2022(E)
© ISO 2022

---------------------- Page: 1 ----------------------
ISO 23435:2022(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
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---------------------- Page: 2 ----------------------
ISO 23435:2022(E)
Contents Page
Foreword .v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Fundamentals of snow depth sensors . 2
4.1 Overview . 2
4.2 Observation methods . 2
4.3 Points to note . 2
5 Test criteria and summary of methods . 2
5.1 Test criteria and considerations . 2
5.1.1 Measurement performance . 2
5.1.2 Installation-related . 3
5.1.3 Environmental/operational . 3
5.2 Summary of test methods. 3
6 Manufacturer design specifications check . 4
6.1 Purpose . 4
6.2 Requirements to list . 4
7 Basic functional test .4
7.1 Purpose . 4
7.2 Calibration of the sensor prior to testing. 4
7.2.1 Calibration using a reference object . 4
7.2.2 Calibration by moving the target surface . 5
7.2.3 Plumb procedure (only applicable to sensors using visible laser signals) . 6
7.3 Basic functional test procedure . 7
7.3.1 Test setup . 7
7.3.2 Running test . 7
7.3.3 Evaluation of the results . 8
7.3.4 Consideration . 8
8 Temperature chamber test (optional) . 8
8.1 Purpose . 8
8.2 Test chamber . 8
8.3 Procedure . 8
8.4 Evaluation . 9
9 Calibration (ground level adjust) test . 9
9.1 Purpose . 9
9.2 Procedures . 9
10 Field tests . 9
10.1 Purpose . 9
10.2 Duration . 9
10.3 Siting . 10
10.4 Climate . 10
10.5 Installation . 11
10.5.1 Mounting and installation of ultrasonic snow depth sensors . 11
10.5.2 Mounting and installation of laser-based sensors . 11
10.6 Field site equipment . .12
10.6.1 Target surface (snow plate) .12
10.6.2 Reference measurements for comparison .12
10.6.3 Auxiliary environmental sensors . 13
10.7 Evaluation . 13
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ISO 23435:2022(E)
10.7.1 Malfunctions . 13
10.7.2 Automatic quality control . 13
10.7.3 Evaluation of differences to reference . 13
10.7.4 Errors caused by field inhomogeneity or other unexpected conditions .13
Bibliography .14
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ISO 23435:2022(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO 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.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 5,
Meteorology.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
v
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ISO 23435:2022(E)
Introduction
Solid precipitation is one of the more complex parameters to be observed and measured by automatic
sensors. The measurement of precipitation has been the subject of a multitude of studies, but there has
been limited information regarding the procedures and performance criteria describing the ability and
[13]
reliability of automatic sensors to accurately measure solid precipitation .
Recently, an increasing percentage of precipitation data used in a variety of applications have been
obtained using automatic instruments and stations including the measurement of snow depth, and many
[13]
new applications have emerged .Also, the modern data processing capabilities, data management,
and data assimilation techniques provide the means for better assessment and error analysis.
For the past years, various automatic snow depth measurement systems or snow depth sensors
have been deployed and tested at different places to take advantages of their efficiency and get more
[6]
objective measurement results .
An ultrasonic snow depth sensor measures the time interval between transmission and reception of
ultrasonic pulses reflected from a target surface. This measurement is used to determine the distance
between the sensor and the surface. The performance of the acoustic snow depth measurement
technique depends on air temperature. Therefore, the ultrasonic sensor requires correction for
variations in the speed of sound in air due to temperature. The measurement uncertainty of sonic
rangers (distance meters) is 0,5 % to 1 % of the distance, which leads under typical conditions to a
[2]
measurement uncertainty for snow depth in the order of 1 cm .
Laser sensors for snow depth measurement were introduced a few years ago and have already been
[11][14][18]
under test and in operational use in various places . A laser snow depth sensor uses an
optoelectronic distance measurement principle to measure the distance between the sensor and the
surface of the snow. Most of the laser snow sensors today employ a single laser distance meter, and, this
results in an important drawback of this type of snow sensors, the lack of spatial representativeness. To
resolve this issue, there have been a few trials and products with multipoint measurements, including a
fixed 3 points sensor and scanning laser snow depth sensors which scan multiple points along a circular
path or a segment of line. Apart from the laser distance sensors, there are other optical techniques
[2]
capable of measurement of the state of ground and snow depth .
In spite of some of the drawbacks and difficulties, automated snow depth measurement techniques are
evolving to offer more objective results which can be made available continuously and in near real-time.
The procedures presented in this document define methods for performance test of snow depth sensors
to be used for snow depth measurements. Minimum requirements for conformance with this document
include successful completion of the basic functional test (see Clause 7), the temperature chamber test
(see Clause 8), and the field test (see Clause 10).
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INTERNATIONAL STANDARD ISO 23435:2022(E)
Air quality — Test methods for snow depth sensors
1 Scope
This document provides requirements for the evaluation and use of test method for snow depth sensors.
This document is applicable to the following types of automatic snow depth sensors which employ
different ranging technologies by which the sensors measure the distance from the snow surface to the
sensor:
a) Ultrasonic type, also known as sonic ranging depth sensors;
b) Optical laser snow depth sensors including single point and multipoint snow depth sensors;
c) Other snow depth sensors.
This document mainly covers two major tests: a laboratory(indoor) test and a field (outdoor) test.
The laboratory test includes the basic performance test and other tests under various environmental
changes. The field test is proposed to ensure the performance of the snow depth sensors in field
measurement conditions. For the field test, both the natural ground and artificial target surface such as
snow plates are considered for the procedures defined in this document.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 5725 (all parts), Accuracy (trueness and precision) of measurement methods and results
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
mean
mean value over the (selected) averaging interval of the sonic
3.2
dead zone
area that cannot be measured near the sensor
3.3
half-power beam width
beam angle width that the transmitted acoustic power decreases by half
3.4
beam angle clearance
angular range where obstacles should be excluded to prevent interference due to acoustic reflection
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ISO 23435:2022(E)
4 Fundamentals of snow depth sensors
4.1 Overview
The term “snow” should also include ice pellets, glaze, hail, and sheet ice formed directly or indirectly
from precipitation. Snow depth usually means the total depth of snow on the ground at the time of
observation. Depth measurements of snow cover on the ground had been taken mainly with snow
rulers until a couple of decades ago. The development of practical ultrasonic and laser ranging devices
to provide reliable snow depth measurements at automatic stations has provided feasible alternatives
to the standard observation. Most of these sensors are capable of an uncertainty within ±1,0 cm
(Reference [17]). In addition, these sensors can be utilized to control the quality of automatic recording
gauge measurements by providing additional details on the type, amount and timing of precipitation.
4.2 Observation methods
The ultrasonic snow depth sensor has an ultrasonic wave transmitter/receiver installed downward on
the upper part of the observation pole, emits an ultrasonic pulse, and measures the time it takes for
the ultrasonic pulse to be reflected and returned. The depth of snow is calculated by converting the
propagation time and the speed of sound into distance.
The laser snow depth sensor uses laser light instead of the ultrasonic waves. Laser light is emitted
obliquely downward from the upper part of the observation pole. The laser light has a spot shape on
the irradiated snow surface, and there are single-point laser sensors and multi-point laser sensors that
measure a single point and multiple points on the snow surface respectively.
4.3 Points to note
There are a few points to note. Firstly, measurements should be taken at the representative observing
point without slope and with no obstructions around measurement points, since the snow drifts and is
redistributed under the effects of the wind.
Secondly, since the ultrasonic snow depth sensor has a wide beam, it requires a wide irradiation surface.
Thirdly, a single-point laser sensor requires attention to be paid to the snow surface representativeness
of the measurement point.
5 Test criteria and summary of methods
5.1 Test criteria and considerations
5.1.1 Measurement performance
1) Resolution: the minimum measurement unit in 0,1 cm. (typical e.g. 0,1 cm)
2) Measurement accuracy: deviation of the measurement from the real depth in 0,1 cm. (typical e.g.
0,5 cm)
3) Dead zone: area that cannot be measured near the sensor. (typical e. g. 50 cm from the centre of the
target area)
4) Measurement height range: maximum measurable snow height considering dead zone in cm.
(typical e.g. 300 cm).
5) Maximum measurable distance from the ground and/or "dead-zone" in cm. (typical e.g. 500 cm)
2
6) Measurement area (in cm ): the size of the target area (typical e.g. 100 cm in radius (approx.
2
7 850 cm ). For ultrasonic sensors, the measurement area is limited by the “half-power beam
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ISO 23435:2022(E)
width” not the "beam angle clearance. The former is usually within 10 deg., on the other hand the
latter is about 30 deg.
7) Measurement pattern: the shape of the scanning measurement (e.g. a single point, a triangle, a
rectangle, a circle, a line etc.).
8) Measurement speed: minimum measurement period or data output interval. (typical e.g. 1 min)
5.1.2 Installation-related
1) Allowed installation angle: the maximum angle between the vertical pole or wall and the pointing
direction of the snow depth sensor.
2) Influence of shadows: the influence of shadows generated by obstacles such as cables, tree branches,
poles, and other snow depth meters.
3) Max height: the maximum height where the snow sensor should perform measurement with the
proclaimed accuracy and resolution; it is to determine if the maximum measurable height is bigger
than the maximum possible snow depth at the site.
4) Target surface: determine if the target surface, either natural ground or a snow plate is structured
for optimal measurement of snow depth.
5) Calibration procedure: determine if there is a straight forward procedure to calibrate the sensor.
5.1.3 Environmental/operational
1) Snow measuring temperature: temperature range where the snow sensor should perform
measurement with the proclaimed accuracy and resolution. (e.g. -40 ~ 30 °C).
2) Operating temperature and humidity where the snow sensor can be operated without being
damaged or malfunctions. (e.g. -40 ~ 50 °C, 0 ~ 99 %).
3) Wind (ultra-sonic sensors only): (e.g. 0 ~ 20 m/s speed, apply the manufacturer’s specification).
4) Visibility (laser sensor only): effect of snowstorm, fog and dirt on the snow surface. (apply the
manufacturer’s specification).
5) Conditions of testing and calibration: conditions that have impacts on the performance of the
sensor. Although testing in extreme environments should be encouraged, one should not infer
results from these tests to the performance that can be expected in less extreme conditions.
5.2 Summary of test methods
1) Manufacturer design specifications check: the sensor should be examined for damage and
conformance with manufacturer design specifications prior to testing. The accuracy of all
measurements and results shall be ascertained and reported in accordance with ISO 5725 (all parts).
2) Basic functional test: the instrument’s basic performance in terms of resolution, accuracy, and
measurement range are tested and determined.
3) Temperature chamber test: The deviation of the measured is determined over the operational
temperature range.
4) Calibration (ground level adjustment) or manual configuration test: the offset of the measured
distance is determined over the operational temperature range.
5) Field test: addresses the response to potentially adverse environmental conditions, which are
difficult to simulate in the laboratory.
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ISO 23435:2022(E)
6 Manufacturer design specifications check
6.1 Purpose
Check if the measurement performance and specifications proclaimed by the manufacturer meets the
intended use.
6.2 Requirements to list
The list includes the sensor’s measurement performance, installation related issues including zero
adjustment and calibration procedures, and environmental/operational conditions.
7 Basic functional test
7.1 Purpose
The purpose of the basic functional test is to verify the basic performance of the test subject. This clause
defines procedures of calibration of the sensors, test setup, and running tests.
7.2 Calibration of the sensor prior to testing
Before any use or test of the sensors, they need to be properly calibrated.
There are different ways to calibrate the snow sensors depending on the measurement types of the
sensors. Below is a list of proposed calibration methods:
1) Calibration using a reference object;
2) Calibration using a moving target surface (or changing the installation height);
3) Plumb procedure.
7.2.1 Calibration using a reference object
1) In this calibration setup, measurements are done with and without a reference object. The
measurement without the reference object is supposed to be same as the measurement for the
ground level.
2) The installation angle (α) can be derived easily by the formula as shown in Figure 1.
3) Adjust the scale, the offset, and other factors so that the resulting output matches the height of the
reference object, c.
4) In this scheme, if the height of the reference object c is not well determined, the corresponding
error in the installation angle, α leads to 1:1 error in the measured distance between the sensor
and the surface.
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ISO 23435:2022(E)
Key
A calibration setup and Measurement without the reference object
B measurement with reference object
C calculation of the installation angle
α installation angle
c reference object
x distance from the ground level
1
x distance from the top of the reference object c
2
Figure 1 — Calibration using a reference object
7.2.2 Calibration by moving the target surface
1) In this scheme, the direction of the laser beam doesn’t have to perpendicular to the target surface.
2) This scheme shown in Figure 2 is virtually equivalent to the one using a reference object in 7.2.1,
but, the distance between the sensor and the target surface is changed by moving either the sensor
or the target surface instead of inserting a reference object in 7.2.1.
3) First measure the distance m (equivalent to x in Figure 1 of 7.2.1) as the ground level.
1 1
4) Second measure the distance m (equivalent to x in Figure 1 of 7.2.1) as a reference height.
2 2
5) Compare (m – m ) and (d – d ).
1 2 1 2
Adjust the scale, the offset, and other factors that the calculated output from (m – m ) matches the
1 2
distanced moved, (d – d ). When the sensor’s beam is perpendicular to the target surface or the
1 2
ground (as with ultrasonic snow depth sensors) as shown in Figure 3, the process becomes much
simpler. In this scheme (m – m ) is equal to (d – d ).
1 2 1 2
Rail distance d or d corresponds to the height from the ground. Calibration is carried out
1 2
according to 7.3.2 Running test
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ISO 23435:2022(E)
Key
m laser distance reading at the distance d
1 1
m laser distance reading at the distance d
2 2
d , d rail distance
1 2
Figure 2 — Calibration by moving the target surface (laser)
Figure 3 — Calibration by moving the target surface (ultrasonic)
Key
d , d rail distance
1 2
7.2.3 Plumb procedure (only applicable to sensors using visible laser signals)
1) Establish a setup as shown in Figure 4.
2) Measure and record the distance x from the sensor to the laser spot (A) at an angle α.
3) Measure the distance (y) from A to the shortest point on the ground (A).
4) Use y to calculate the actual height d using α = arcsin (y/x) and d = y/tan(α) or d = x cos(α).
5) Adjust the scale, the offset, and other factors so that the resulting output from the sensor matches
the calculated height d.
6) In this scheme, it requires attention to precisely locate point B where the vertical line from the
sensor to the ground perfectly perpendicular to the target s
...

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 23435
ISO/TC 146/SC 5
Air quality — Test methods for snow
Secretariat: DIN
depth sensors
Voting begins on:
2021-12-01
Voting terminates on:
2022-01-26
RECIPIENTS OF THIS DRAFT ARE INVITED TO
SUBMIT, WITH THEIR COMMENTS, NOTIFICATION
OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/FDIS 23435:2021(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN-
DARDS TO WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS. © ISO 2021

---------------------- Page: 1 ----------------------
ISO/FDIS 23435:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
  © ISO 2021 – All rights reserved

---------------------- Page: 2 ----------------------
ISO/FDIS 23435:2021(E)
Contents Page
Foreword .v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Fundamentals of snow depth sensors . 2
4.1 Overview . 2
4.2 Observation methods . 2
4.3 Points to note . 2
5 Test criteria and summary of methods . 2
5.1 Test criteria and considerations . 2
5.1.1 Measurement performance . 2
5.1.2 Installation-related . 3
5.1.3 Environmental/operational . 3
5.2 Summary of test methods. 3
6 Manufacturer design specifications check . 4
6.1 Purpose . 4
6.2 Requirements to list . 4
7 Basic functional test .4
7.1 Purpose . 4
7.2 Calibration of the sensor prior to testing. 4
7.2.1 Calibration using a reference object . 4
7.2.2 Calibration by moving the target surface . 5
7.2.3 Plumb procedure (only applicable to sensors using visible laser signals) . 6
7.3 Basic functional test procedure . 7
7.3.1 Test setup . 7
7.3.2 Running test . 7
7.3.3 Evaluation of the results . 8
7.3.4 Consideration . 8
8 Temperature chamber test (optional) . 8
8.1 Purpose . 8
8.2 Test chamber . 8
8.3 Procedure . 8
8.4 Evaluation . 9
9 Calibration (ground level adjust) test . 9
9.1 Purpose . 9
9.2 Procedures . 9
10 Field tests . 9
10.1 Purpose . 9
10.2 Duration . 9
10.3 Siting . 10
10.3.1 Climate . 10
10.4 Installation . 11
10.4.1 Mounting and installation of ultrasonic snow depth sensors . 11
10.4.2 Mounting and installation of laser-based sensors . 11
10.5 Field site equipment . .12
10.5.1 Target surface (snow plate) .12
10.5.2 Reference measurements for comparison .12
10.5.3 Auxiliary environmental sensors . 13
10.6 Evaluation .13
iii
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ISO/FDIS 23435:2021(E)
10.6.1 Malfunctions . 13
10.6.2 Automatic quality control . 13
10.6.3 Evaluation of differences to reference . 13
10.6.4 Errors caused by field inhomogeneity or other unexpected conditions .13
Bibliography .14
iv
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---------------------- Page: 4 ----------------------
ISO/FDIS 23435:2021(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO 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.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 5,
Meteorology.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
v
© ISO 2021 – All rights reserved

---------------------- Page: 5 ----------------------
ISO/FDIS 23435:2021(E)
Introduction
Solid precipitation is one of the more complex parameters to be observed and measured by automatic
sensors. The measurement of precipitation has been the subject of a multitude of studies, but there has
been limited information regarding the procedures and performance criteria describing the ability and
[13]
reliability of automatic sensors to accurately measure solid precipitation .
Recently, an increasing percentage of precipitation data used in a variety of applications have been
obtained using automatic instruments and stations including the measurement of snow depth, and many
[13]
new applications have emerged .Also, the modern data processing capabilities, data management,
and data assimilation techniques provide the means for better assessment and error analysis.
For the past years, various automatic snow depth measurement systems or snow depth sensors
have been deployed and tested at different places to take advantages of their efficiency and get more
[6]
objective measurement results .
An ultrasonic snow depth sensor measures the time interval between transmission and reception of
ultrasonic pulses reflected from a target surface. This measurement is used to determine the distance
between the sensor and the surface. The performance of the acoustic snow depth measurement
technique depends on air temperature, and the ultrasonic sensor requires to be corrected for variations
of the speed of sound in air. The measurement uncertainty of sonic rangers (distance meters) is 0,5 % to
1 % of the distance, which leads under typical conditions to a measurement uncertainty for snow depth
[2]
in the order of 1 cm .
Laser sensors for snow depth measurement were introduced a few years ago and have already been
[11][14][18]
under test and in operational use in various places . A laser snow depth sensor uses an
optoelectronic distance measurement principle to measure the distance between the sensor and the
surface of the snow. Most of the laser snow sensors today employ a single laser distance meter, and, this
result in an important drawback of this type of snow sensors, the lack of spatial representativeness. To
resolve this issue, there have been a few trials and products with multipoint measurements, including a
fixed 3 points sensor and scanning laser snow depth sensors which scan multiple points along a circular
path or a segment of line. Apart from the laser distance sensors, there are other optical techniques
[2]
capable of measurement of the state of ground and snow depth .
In spite of some of the drawbacks and difficulties, automated snow depth measurement techniques are
evolving to offer more objective results which can be made available continuously and in near real-time.
The procedures presented in this document define methods for performance test of snow depth sensors
to be used for snow depth measurements. Minimum requirements for conformance with this document
include successful completion of the basic functional test (see Clause 7), the temperature chamber test
(see Clause 8), and the field test (see Clause 10).
vi
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---------------------- Page: 6 ----------------------
FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 23435:2021(E)
Air quality — Test methods for snow depth sensors
1 Scope
This document provides requirements for the evaluation and use of test method for snow depth sensors.
This document is applicable to the following types of automatic snow depth sensors which employ
different ranging technologies by which the sensors measure the distance from the snow surface to the
sensor:
a) Ultrasonic type, also known as sonic ranging depth sensors;
b) Optical laser snow depth sensors including single point and multipoint snow depth sensors;
c) Other snow depth sensors.
This document mainly covers two major tests: a laboratory(indoor) test and a field (outdoor) test.
The laboratory test includes the basic performance test and other tests under various environmental
changes. The field test is proposed to ensure the performance of the snow depth sensors in field
measurement conditions. For the field test, both the natural ground and artificial target surface such as
snow plates are considered for the procedures defined in this document.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 5725 (all parts), Accuracy (trueness and precision) of measurement methods and results
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
mean
mean value over the (selected) averaging interval of the sonic
3.2
dead zone
area that cannot be measured near the sensor
3.3
half-power beam width
beam angle width that the transmitted acoustic power decreases by half
3.4
beam angle clearance
angular range where obstacles should be excluded to prevent interference due to acoustic reflection
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4 Fundamentals of snow depth sensors
4.1 Overview
The term “snow” should also include ice pellets, glaze, hail, and sheet ice formed directly or indirectly
from precipitation. Snow depth usually means the total depth of snow on the ground at the time of
observation. Depth measurements of snow cover on the ground had been taken mainly with snow
rulers until a couple of decades ago. The development of practical ultrasonic and laser ranging devices
to provide reliable snow depth measurements at automatic stations has provided feasible alternatives
to the standard observation. Most of these sensors are capable of an uncertainty within ±1,0 cm (WMO-
No.8 the CIMO Guide, 2018). In addition, these sensors can be utilized to control the quality of automatic
recording gauge measurements by providing additional details on the type, amount and timing of
precipitation.
4.2 Observation methods
The ultrasonic snow depth sensor has an ultrasonic wave transmitter/receiver installed downward on
the upper part of the observation pole, emits an ultrasonic pulse, and measures the time it takes for
the ultrasonic pulse to be reflected and returned. The depth of snow is calculated by converting the
propagation time and the speed of sound into distance.
The laser snow depth sensor uses laser light instead of the ultrasonic waves. Laser light is emitted
obliquely downward from the upper part of the observation pole. The laser light has a spot shape on
the irradiated snow surface, and there are single-point laser sensors and multi-point laser sensors that
measure a single point and multiple points on the snow surface respectively.
4.3 Points to note
There are a few points to note. Firstly, measurements should be taken at the representative observing
point without slope and with no obstructions around measurement points, since the snow drifts and is
redistributed under the effects of the wind.
Secondly, since the ultrasonic snow depth sensor has a wide beam, it requires a wide irradiation surface.
Thirdly, a single-point laser sensor requires attention to be paid to the snow surface representativeness
of the measurement point.
5 Test criteria and summary of methods
5.1 Test criteria and considerations
5.1.1 Measurement performance
1) Resolution: the minimum measurement unit in 0,1 cm. (typical e.g. 0,1 cm)
2) Measurement accuracy: deviation of the measurement from the real depth in 0,1 cm. (typical e.g.
0,5 cm)
3) Dead zone: area that cannot be measured near the sensor. (typical e. g. 50 cm from the centre of the
target area)
4) Measurement height range: maximum measurable snow height considering dead zone in cm.
(typical e.g. 300 cm).
5) Maximum measurable distance from the ground and/or "dead-zone" in cm. (typical e.g. 500 cm)
2
6) Measurement area (in cm ): the size of the target area (typical e.g. 100 cm in radius (approx.
2
7 850 cm ). For ultrasonic sensors, the measurement area is limited by the “half-power beam
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ISO/FDIS 23435:2021(E)
width” not the "beam angle clearance. The former is usually within 10 deg., on the other hand the
latter is about 30 deg.
7) Measurement pattern: the shape of the scanning measurement (e.g. a single point, a triangle, a
rectangle, a circle, a line etc.).
8) Measurement speed: minimum measurement period or data output interval. (typical e.g. 1 min)
5.1.2 Installation-related
1) Allowed installation angle: the maximum angle between the vertical pole or wall and the pointing
direction of the snow depth sensor.
2) Influence of shadows: the influence of shadows generated by obstacles such as cables, tree branches,
poles, and other snow depth meters.
3) Max height: the maximum height where the snow sensor should perform measurement with the
proclaimed accuracy and resolution; it is to determine if the maximum measurable height is bigger
than the maximum possible snow depth at the site.
4) Target surface: determine if the target surface, either natural ground or a snow plate is structured
for optimal measurement of snow depth.
5) Calibration procedure: determine if there is a straight forward procedure to calibrate the sensor.
5.1.3 Environmental/operational
1) Snow measuring temperature: temperature range where the snow sensor should perform
measurement with the proclaimed accuracy and resolution. (e.g. -40 ~ 30 °C).
2) Operating temperature and humidity where the snow sensor can be operated without being
damaged or malfunctions. (e.g. -40 ~ 50 °C, 0 ~ 99 %).
3) Wind (ultra-sonic sensors only): (e.g. 0 ~ 20 m/s speed, apply the manufacturer’s specification).
4) Visibility (laser sensor only): effect of snowstorm, fog and dirt on the snow surface. (apply the
manufacturer’s specification).
5) Conditions of testing and calibration: conditions that have impacts on the performance of the
sensor. Although testing in extreme environments should be encouraged, one should not infer
results from these tests to the performance that can be expected in less extreme conditions.
5.2 Summary of test methods
1) Manufacturer design specifications check: the sensor should be examined for damage and
conformance with manufacturer design specifications prior to testing. The accuracy of all
measurements and results shall be ascertained and reported in accordance with ISO 5725 (all parts).
2) Basic functional test: the instrument’s basic performance in terms of resolution, accuracy, and
measurement range are tested and determined.
3) Temperature chamber test: The deviation of the measured is determined over the operational
temperature range.
4) Calibration (ground level adjustment) or manual configuration test: the offset of the measured
distance is determined over the operational temperature range.
5) Field test: addresses the response to potentially adverse environmental conditions, which are
difficult to simulate in the laboratory.
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6 Manufacturer design specifications check
6.1 Purpose
Check if the measurement performance and specifications proclaimed by the manufacturer meets the
intended use.
6.2 Requirements to list
The list includes the sensor’s measurement performance, installation related issues including zero
adjustment and calibration procedures, and environmental/operational conditions.
7 Basic functional test
7.1 Purpose
The purpose of the basic functional test is to verify the basic performance of the test subject. This clause
defines procedures of calibration of the sensors, test setup, and running tests.
7.2 Calibration of the sensor prior to testing
Before any use or test of the sensors, they need to be properly calibrated.
There are different ways to calibrate the snow sensors depending on the measurement types of the
sensors. Below is a list of proposed calibration methods:
1) Calibration using a reference object;
2) Calibration using a moving target surface (or changing the installation height);
3) Plumb procedure.
7.2.1 Calibration using a reference object
1) In this calibration setup, measurements are done with and without a reference object. The
measurement without the reference object is supposed to be same as the measurement for the
ground level.
2) The installation angle (α) can be derived easily by the formula as shown in Figure 1.
3) Adjust the scale, the offset, and other factors so that the resulting output matches the height of the
reference object, c.
4) In this scheme, if the height of the reference object c is not well determined, the corresponding
error in the installation angle, α leads to 1:1 error in the measured distance between the sensor
and the surface.
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ISO/FDIS 23435:2021(E)
Key
A calibration setup and Measurement without the reference object
B measurement with reference object
C calculation of the installation angle
α installation angle
c reference object
x distance from the ground level
1
x distance from the top of the reference object c
2
Figure 1 — Calibration using a reference object
7.2.2 Calibration by moving the target surface
1) In this scheme, the direction of the laser beam doesn’t have to perpendicular to the target surface.
2) This scheme shown in Figure 2 is virtually equivalent to the one using a reference object in 7.2.1,
but, the distance between the sensor and the target surface is changed by moving either the sensor
or the target surface instead of inserting a reference object in 7.2.1.
3) First measure the distance m (equivalent to x in Figure 1 of 7.2.1) as the ground level.
1 1
4) Second measure the distance m (equivalent to x in Figure 1 of 7.2.1) as a reference height.
2 2
5) Compare (m – m ) and (d – d ).
1 2 1 2
Adjust the scale, the offset, and other factors that the calculated output from (m – m ) matches the
1 2
distanced moved, (d – d ). When the sensor’s beam is perpendicular to the target surface or the
1 2
ground (as with ultrasonic snow depth sensors) as shown in Figure 3, the process becomes much
simpler. In this scheme (m – m ) is equal to (d – d ).
1 2 1 2
Rail distance d or d corresponds to the height from the ground. Calibration is carried out
1 2
according to 7.3.2 Running test
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ISO/FDIS 23435:2021(E)
Key
m laser distance reading at the distance d
1 1
m laser distance reading at the distance d
2 2
d /d rail distance
1 2
Figure 2 — Calibration by moving the target surface (laser)
Figure 3 — Calibration by moving the target surface (ultrasonic)
7.2.3 Plumb procedure (only applicable to sensors using visible laser signals)
1) Establish a setup as shown in Figure 4.
2) Measure and record the distanc
...