Quantifying sidescan sonar positioning accuracy, with special reference to measurement in the intertidal zone

Accuracy testing has been described vis-à-vis shallow-water hydrographic surveying by phase-measuring sidescan sonar [M. Borrelli et al., Estuaries and Coasts, 2021; doi 10.1007/s12237-021-00979-2]. This is rigorous but sufficiently complicated, hence time-consuming, to justify use of a simplified protocol in the case of short-duration sonar surveys of diverse kinds in or proximate to shallow water. This is done by a particular combination of position measurements on bottom targets in the intertidal zone. Reference measurements are made by RTK-GPS on selected parts of targets at low tide, when the intertidal zone is dry. Sonar measurements are made on the same targets at or near the following high tide, when the intertidal zone is flooded. Repetition of the RTK-GPS measurements at the next low tide can confirm the consistency of the reference measurements, to exclude, among other errors, those due to possible movement of targets due to tidal or surface-wave effects. An example is presented based on multiple targets. The agreement of sidescan sonar measurements, made at about 550 kHz, of target position in the horizontal plane with RTK-GPS measurements of the same is quantified by a mean offset of order 50-200 mm. This may be compared with the intrinsic RTK-GPS uncertainty in positioning, which is of order 20-30 mm. The context for this is a maximum single-side echo registration range of 20 m and a survey area of 1 hectare. These dimensions reflect the primary purpose of the measurements, for a survey, from which the present study was derived. Thus, the dimensions have not been optimized. While the present work presumes proximity of an intertidal zone, the simplified protocol also applies to the case of deeper water when the horizontal position of one or more recognizable bottom objects is known by suitable surveying.