Dynamic Heights for the International Great Lakes Datum of 2020
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Dynamic Heights for the International Great Lakes Datum of 2020

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    The Great Lakes in North America serve as a valuable resource of freshwater, hydropower and a means of navigation from interior North America to international markets. As such, maintaining a consistent reference system has been essential to both Canada and the United States. The border between the two countries is roughly 8900 km and extends through the middle of many of the Lakes. Treaties also emphasize equal access to the waters in the Lakes. Hence, adoption of a common reference system is essential. The International Great Lakes Datum of 1985 (IGLD 85) has served for this for over 30 years. Due to changes from Global Isostatic Adjustment and improvements in positioning technology, IGLD 85 must be updated. The International Great Lakes Datum of 2020 (IGLD 2020) will replace IGLD 85 in about 2025. IGLD 2020 will use the same geopotential model as the North American-Pacific Geopotential Datum of 2022 (NAPGD 2022). NAPGD2022 is being realized as both a geoid height model and a gravity field model at one arcminute. These models will be combined with GNSS observations of mean water surfaces throughout the Great Lakes to determine dynamic heights. Comparisons will be made on each Lake to estimate the potential for a permanent water topography that would indicate the need for hydraulic correctors (HCs). In IGLD 85, there was a need for HC’s to account for over 30 cm of residual tilt across the Lakes that were believed to be due to a datum defect in NAVD 88. In this paper, we provide a preliminary analysis of relative water levels at gauges across the Great Lakes. We identify, estimate, and correct for variations due to relative motion, elastic deformation associated with changes in water loading, and wind effects between gauges. The analysis demonstrates that, after these corrections are made, annual changes in water levels between gauges on each Lake agree at the cm level. As such, this method provides a tool for removing biases due to relative motion between gauges and ensures that these biases will not be a significant source of error in the final evaluation of absolute dynamic heights for Lake water levels. However, the analysis also demonstrates that dynamic topography due to the prevailing winds is present and may create enough of a persistent topography to require hydraulic correctors. The absolute extent of the wind effects will be evaluated in the future once GNSS and leveling data is finalized and absolute dynamic heights can be calculated.
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