VDatum Tides Modeling for Hawaii and Pacific Islands

A vertical datum is a reference surface or level used to measure elevations and heights above or below a specific point on the Earth's surface. This reference point provides a crucial foundation for accurate measurements, planning, development, research, and a wide range of applications. These applications include engineering and construction, navigation and surveying, infrastructure management, and floodplain management, all of which rely on consistent and standardized elevation references. The vertical datum plays a pivotal role in ensuring that data collected across diverse locations and disciplines are consistent, compatible and meaningful.

VDatum is a software tool developed by the National Oceanic and Atmospheric Administration (NOAA) National Ocean Service (NOS) that facilitates the transformation of geospatial data among 48 ellipsoidal, orthometric, and tidal datums in coastal and marine areas. VDatum assists in maintaining accuracy and consistency when working with different vertical reference systems.

Currently, there is an ongoing effort to develop a VDatum tool that covers a substantial geographical region that encompasses the Hawaii Islands, American Samoa, Guam, Northern Mariana Islands, Federated States of Micronesia, Marshall Islands, and Palau. VDatum tool is highly valuable for Hawaii and the Pacific Islands due to its wide-ranging applications in coastal management, Navigation and Marine Transportation, disaster preparedness, marine resource utilization, and environmental conservation. In addition, it assists in accurately defining maritime boundaries and Exclusive Economic Zones (EEZs). These references are crucial for ensuring compliance with international maritime laws and agreements. By offering standardized and accurate elevation data, these tools contribute to informed decision-making and sustainable development in this unique and ecologically diverse region.

This paper presents NOAA’s efforts to integrate a new 3D (density layered) model for deriving the local Mean Sea Level in VDatum, and the associated tidal datum fields, including mean lower low water (MLLW), mean low water (MLW), mean high water (MHW), and mean higher high water (MHHW) using the water level time series which is obtained through numerical simulations of circulation models. The new circulation model to be evaluated into the VDatum workflow is Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), which utilizes an unstructured grid framework. SCHISM is a cutting-edge, community-driven, open-source modeling system that employs a unique approach. It employs a combination of triangular and quadrangular unstructured meshing for horizontal dimensions, along with a highly adaptable vertical coordinate system. This innovative design allows for accurate simulations of 3D baroclinic circulation, spanning from small waterways to ocean-wide patterns, all within a single grid. Consequently, this methodology eliminates the need for intricate grid nesting or the application of bathymetric data smoothing techniques. Precise high-resolution coastal data and NOAA bathymetry were utilized to construct a detailed unstructured grid optimized for the model's use.

The tidal datums predicted by SCHISM are subsequently cross-referenced with datums derived from observations collected by NOAA's Center for Operational Oceanographic Products and Services (CO-OPS) tide stations.

To address discrepancies between the modeled and observed data using spatial interpolation techniques. The refined tidal datum products are subsequently integrated into the VDatum software as structured marine grids. NOAA's National Geodetic Survey (NGS) employs these results to compute the conversion rate between the predicted tidal datums and the North American Vertical Datum of 1988 (NAVD 88). Additionally, NGS offers the required transformations for converting between orthometric and ellipsoidal references, which are integral to the functionality of the VDatum software.