Design of a Trusted Community Bathymetry System

TitleDesign of a Trusted Community Bathymetry System
Publication TypeConference Proceedings
Year2018
AuthorsCalder, BR
EditorDijkstra, SJ, Hoy, S, Himschoot, K, Schofield, A
Conference Name2018 Canadian Hydrographic Conference
Conference DatesMarch 27 - 29
PublisherCanadian Hydrographic Association
Conference LocationVictoria, BC, Canada
KeywordsCrowd-sourced Bathymetry, Ellipsoidal Survey Methods, Post-processed GNSS Solution, Trusted Community Bathymetry, Volunteered Geographic Information

Crowd-sourced bathymetry (CSB), meaning volunteered geo-spatial information collected from whatever GNSS and sonar system is available on the volunteer's vessel, has received a significant amount of attention in recent years. Although increasing amounts of data are being collected, attributed with metadata (to different degrees), and archived, finding a route to the nautical chart has so far been problematic. Partially, this is to do with a lack of a formal and robust means to represent data quality on the chart (paper, raster, or vector), but mostly it is to do with a lack of qualifying metadata associated with data collected in this manner. CSB efforts generally suffer from a lack of calibration, and are therefore limited by uncontrolled vertical offsets with respect to the waterline that are not necessarily constant over time. Indeed, even applying appropriate tidal correctors can be difficult. Assumptions that these issues can be resolved by having a sufficient number of independent observations (the "wisdom of crowds" argument) are often frustrated by  basic physical limitations: the ocean is big, and ships are (relatively) small. Except in limited circumstances, or specific areas, the chances of having any repeated measurements are vanishingly small.

 

As an alternative to the collection of unqualified data which then needs to be corrected and/or qualified, we propose the use of a data collection system which, by construction, can provide sufficient guarantees of data quality as to allow the measurements to be considered for hydrographic use. We call this method Trusted Community Bathymetry (TCB).

 

A TCB system resolves many of the issues associated with CSB data by providing for significantly improved positioning accuracy in the vertical. High-accuracy, high-precision post-processed 3D GNSS solutions allow for the estimation of offsets between GNSS antenna and echosounder so that appropriate calibration of the system can be done autonomously. This then allows for reference of depths to a suitable ellipsoid, obviating the need to apply tidal corrections to the data.  Given a known offset between antenna and sonar, the same techniques can be used to autonomously establish calibration sites. TCB systems also have the potential to act as a force multiplier (through cross-calibration) for other CSB efforts where calibration is lacking.

 

We demonstrate these ideas using a prototype TCB system developed by SeaID Ltd., which combines a NMEA data logger with a GNSS system capable of being post-processed for high-precision solutions. By comparison with survey-grade GNSS and INS systems, we demonstrate how to establish the vertical offset calibration in a system, and the construction of a calibration site. We also qualify the performance of the prototype system.