DOI: 10.5061/dryad.dfn2z358w
title: The accuracy of length measurements made using imaging SONAR is inversely proportional to the beam width

description: Multibeam imaging SONARs have been used for a range of measurement
 applications, such as measurements of fish lengths. This study aimed to
 quantify the accuracy of imaging SONAR systems, that varied in frequency
 and beam geometry, to measure the length of synthetic targets positioned
 perpendicularly. Blueprint Oculus imaging SONAR systems, with four
 different (centre) frequencies (750 kHz, 1.2 MHz, 2.1 MHz, and 3 MHz),
 were used to measure the length of three targets of nominal lengths: 10
 cm, 20 cm, and at ranges between 1 m and 15.5 m. The effect of beam
 geometry on measurement error was then examined using regression analysis.
 This study found that there was an overestimation of the actual length of
 the target for all measurements that was inversely proportional to the
 horizontal beam width. The measurement error can be reduced by normalising
 for beam width. However, the variation of measurements (i.e. the
 precision), was found to also increase with range, which was attributed to
 the increasing beam separation. It is important that the effect of beam
 width on the accuracy of target length measurements, by imaging SONARs is
 acknowledged in future studies. One approach to mitigating this problem,
 is to limit the range at which length measurements are made to a beam
 width that produces an estimated level of error that is acceptable for
 that study. Keywords: acoustic camera, imaging SONAR, underwater
 acoustics, underwater measurements.
description: <strong>Data collection</strong>
 To investigate measurement accuracy, three different Blueprint
 Oculus imaging SONAR systems were used in this study, models: M750d,
 M1200d, and M3000d. Between them, they operated at four different (centre)
 frequencies: 750 kHz, 1.2 MHz, 2.1 MHz, and 3 MHz. These imaging SONAR
 systems were used to measure the length of three rectangular targets of
 nominal lengths: 10 cm, 20 cm, and 1 m. The targets were designed to
 present consistent, uniform acoustic scattering. The measurements were
 carried out in a (freshwater) swimming pool on 16th September 2021. The
 targets were created from layers of hollow polycarbonate sheets, with the
 edges sealed to maintain the airspaces and backed with a 1 mm thick
 aluminium sheet. The targets were completely wrapped in black PVC duct
 tape (Figure 3). The actual final length of each nominal target size was 1
 m = 1005 mm, 20 cm = 195 mm, 10 cm = 105 mm, but are referred to by their
 nominal lengths. The height of each target was 1 m = 255 mm, 20 cm = 115
 mm, 10 cm = 55 mm. For the 1 m and the 20 cm targets, the thickness was 33
 mm, and 25 mm for the 10 cm target. The targets were attached to fishing
 line and orientated so that the length was perpendicular to the face of
 the imaging SONAR (Figure 3). Surfactant was applied to the fishing line
 immediately prior to measurements, to mitigate any contribution bubbles on
 the line could have to “elongating the target” as per standard underwater
 acoustic protocols.  The Blueprint Oculus imaging SONAR
 systems were connected through a RJE Oceanbotics™ SRV-8 ROV and recorded
 using Oculus Viewpoint software. The sound velocity of the freshwater pool
 was calculated through a temperature probe integrated into the Oculus
 systems, and was also independently verified. Imaging SONAR data of the
 targets were collected at ranges between 1 m and 15.5 m, by moving the
 targets either closer to or further away from the ROV, which remained at a
 fixed location. At each range step (nominally 1 m steps), the ROV would
 pan horizontally to collect measurements at different bearings over the
 full swath width. The ROV was paused when necessary to allow targets to be
 perpendicular to the SONAR. This was repeated for the four different
 system frequencies. The swimming pool depth was approximately 1.8 m, and
 there was minimal reverberation noise. 
 <strong>Length measurements</strong>
 The imaging SONAR data were analyzed by someone who was not
 present during the data collection and did not know the length of the
 targets, so as not to introduce bias. The measurements were made using the
 measurement tool in Oculus Viewpoint software, and the co-registered video
 from the ROV was used to confirm the targets were present. For each length
 measurement, the frequency, range setting, and measured range of the
 target were recorded. For the 750 kHz and 1.2 MHz systems, the bearing of
 the target was recorded if it was measured at greater than +/-32.5o, as
 beyond this, the beam width was more than 20% of that of the centre beam
 for those systems and the effects of larger beam widths on the imagery
 could be seen at those bearings. These bearings were displayed on the
 software, making it easy for the user to distinguish this area. The
 measurements were repeated for consistency, and the mean length of the
 repeated measurements was used in the analysis. 
description: # The accuracy of length measurements made using imaging SONAR is
 inversely proportional to the beam width Iain M. Parnum1*, Benjamin J.
 Saunders 2, Melanie Stott2, Travis S Elsdon3,2, Michael J Marnane3, Euan
 S. Harvey2 1 Centre for Marine Science and Technology, Curtin University,
 Bentley, 6102, Western Australia, Australia 2 School of Molecular and Life
 Sciences, Curtin University, Bentley, 6102, Western Australia, Australia 3
 Chevron Technical Centre, 250 St Georges Tce, Perth, 6000, Western
 Australia Citation for this dataset:
 https://doi.org/10.5061/dryad.dfn2z358w ## **Abstract** In this study,
 Blueprint Oculus imaging SONAR systems, with four different (centre)
 frequencies (750 kHz, 1.2 MHz, 2.1 MHz, and 3 MHz), were used to measure
 underwater the length of three targets: 10.5 cm, 19.5 cm, and 100.5 cm, at
 ranges between 1 m and 15.5 m. This was to investigate the effect of beam
 geometry on measurement error. ## **Description of the data and file
 structure** The Blueprint Oculus imaging SONAR systems were connected
 through a RJE Oceanbotics™ SRV-8 Remotely Operated Vehicle (ROV) and
 recorded using Oculus Viewpoint software. The sound velocity of the
 freshwater pool was calculated through a temperature probe integrated into
 the Oculus systems, and was also independently verified. Imaging SONAR
 data of the targets were collected at ranges between 1 m and 15.5 m, by
 moving the targets either closer to or further away from the ROV, which
 remained at a fixed location. At each range step (nominally 1 m steps),
 the ROV would pan horizontally to collect measurements at different
 bearings over the full swath width. The ROV was paused when necessary to
 allow targets to be perpendicular to the SONAR. This was repeated for the
 four different system frequencies. Inside the zipped folder “Data from the
 Parnum et al 2024.zip”, is A zipped up folder called Oculus, which
 contains the raw Oculus SONAR files (extension .oculus) and the Oculus
 Viewpoint software installer (Oculus_Setup_v1.20.272.exe) that was used to
 review and make measurements of the data. A zipped up folder called
 “SR8.zip” of raw SRV8 video files (extension .srv8) and the software
 SubNav installer (SubNav_01240_29100_setup.exe) that was used to review
 the video to confirm the orientation of the targets. An Excel spreadsheet
 with the measurements made for each file. ## **How to view the data** To
 view the data unzip the zipped folder, and zipped subfolders. ### *Target
 Measurements* To view the measurements taken, open the file:
 “OculusPoolMeasurements_16thSept2021_LengthMeasurements.xlsx” in Excel. In
 this file, in the sheet named “measurements”, it lists the measurements
 made of the different targets from the different imaging SONAR records and
 the corresponding filenames of the imaging SONAR. In the sheet named
 “Metadata”, it describes what the different table headings represent. ###
 *Imaging SONAR data* To view the imaging SONAR data, install the Oculus
 Viewpoint software using the installer file: Oculus_Setup_v1.20.272.exe
 Once installed, open the Oculus Viewpoint software and click on the folder
 icon to open a file, navigate to where the unzipped (.oculus) files are
 and select the file you wish to view. Once opened, click the play icon in
 the bottom left to start the file, then pause when you wish to make a
 measurement. To take a length measurement, click the rule icon in the
 bottom right, and click and hold at one end of the target and drag to the
 other end of the target, the length measurement is displayed by the
 cursor. ### Underwater video files To view the underwater video taken by
 the ROV taken at the same time as the imaging SONAR, install the RJE
 SubNav software using the installer file: SubNav_01240_29100_setup.exe
 Once installed, open the RJE SubNav software and click the “Review” button
 on the home page, then click “Select project” and navigate to the
 (unzipped) folder where the .srv8 files are located. Click on the file you
 wish to review I the file list in the middle of the screen, and then click
 “Review Divelog”. Once in the Divelog review screen, you can play, pause
 and fast forward through to the time you wish to view.