Chart datum

Abstract: Satellite Derived Bathymetry (SDB) is being adopted as a cheaper and more spatially extensive method for bathymetric mapping than traditional acoustic surveys, with research being conducted by the Canadian Hydrographic Service under a Government Related Initiatives Program (GRIP) of the Canadian Space Agency. Established SDB methods involve either an empirical approach, where a regression between known depths and various colour indexes is developed; or a physics-based Radiative Transfer Model (RTM) approach, where light interactions through the water column are simulated. Both methods have achieved vertical accuracies of around 1 m. However, the empirical approach is limited to areas with existing in-situ depth data, and has limited applicability in heterogeneous benthic environments, while the physics-based approach requires precise atmospheric correction. This paper proposes a through-water photogrammetric approach which avoids these limitations, in heterogeneous seafloor environments, by using feature extraction and image geometry rather than spectral radiance to estimate bathymetry. The method is demonstrated in Coral Harbour, Nunavut, Canada using a WorldView-2 stereo pair. A standard photogrammetric extraction was performed on the stereo pair, including a blunder removal and noise reduction. Apparent depths were then calculated by referencing under-water points to the extracted elevation of the water-line. Actual in-image depths were calculated from apparent depths by applying a correction factor to account for the effects of refraction at the air-water boundary. A tidal reduction brought depths to local chart datum, allowing for validation with Canadian Hydrographic Service survey data showing a mean error of 0.031 m and an RMSE of 1.178 m. The method has a similar accuracy to the two established SDB methods, allowing for its use for bathymetric mapping in circumstances where the established methods are not applicable due to their inherent limitations.

Abstract: National hydrographic offices need a better means of assessing the adequacy of existing nautical charts in order to plan and prioritize future hydrographic surveys. The ability to derive bathymetry from multispectral satellite imagery is a topic that has received considerable attention in scientific literature. However, published studies have not addressed the ability of satellite-derived bathymetry to meet specific hydrographic survey requirements. Specifically, the bathymetry needs to be referenced to a chart datum and statistical uncertainty estimates of the bathymetry should be provided. Ideally, the procedure should be based on readily-available, low-cost software, tools, and data. This paper describes the development and testing of a procedure using publicly-available, multispectral satellite imagery to map and portray shallow-water bathymetry in a GIS environment for three study sites: Northeast United States, Nigeria, and Belize. Landsat imagery and published algorithms were used to derive estimat...

Abstract: A Global Vertical Reference Frame (GVRF) has been realized by means of several regional and local vertical datums (LVD) distributed world-wide: the North American Vertical Datum 1988 (NAVD 88), Australian Height Datum 1971 (AHD 71), LVD France, Institute Geographique National 1969 (IGN 69) and Brazilian Height Datum 1957 (BHD 57). The vertical shifts of the above LVD origins have been related to the adopted reference geopotential value W 0 = (62 636 856.0 ± 0.5) m2s−2 and they were determined at the 5 cm level. However, the W 0 reference value can be chosen arbitrarily, the methodology, which was developed here, does not require that the above value be adopted.

Abstract: The Vertical Offshore Reference Frames (VORF) project is described, with a summary of the methodology and an explanation of the data sets used. The latter include satellite altimetry, tidal and geoid models, long and short term tide gauge data, and specially undertaken GNSS observations. This paper goes on to describe the theoretical basis for deriving spatially variant error estimates that respond to the varying quality of the input data. The paper then describes the testing programme undertaken by the United Kingdom Hydrographic Office, which has included 245 checks on datum connections at mostly coastal points, 63 comparisons between VORF-corrected tidal levels observed with GNSS and tide gauge data, and six specially commissioned offshore tide gauge deployments. It is shown that across the vast majority of the domain of applicability the VORF surfaces meet their target accuracies of 0·10 m inshore and 0·15 m offshore (both 1σ values) and the formal uncertainties are a fair reflection of the er...