Introduction
Conventional level (CL), also known as conventional sea level, is a reference level used to measure the ocean’s height and land elevation. It is based on the average sea level heights over a period of several decades, usually around 19 years, and is determined by the International Hydrographic Organization (IHO). It is an important reference for navigation, cartography, and other oceanographic and geodetic studies, providing a fixed baseline from which to measure changes in sea level.
Background
In the early twentieth century, the concept of conventional sea level was developed in response to the need for a standard reference level for the measurement of ocean depths and land elevation. Before CL, many different local sea level datums were used, but these were difficult to compare, as each was subject to regional variations in gravity, tides, and other factors. CL was designed to provide a consistent and reliable reference for oceanographers and cartographers.
The International Hydrographic Organization (IHO) is responsible for the determination of CL, and their definition states that it is “the average of the hourly heights observed at a station over a period of nineteen years, usually covering the years from 1915 to 1934.” The IHO also states that CL must be used as the reference for surface elevation in all IHO charts.
Measurement
The measurement of CL is based on tidal observations taken near the end of the 19-year period. The IHO uses the average of hourly heights observed at a station over the 19-year period to determine the CL. This is done by first calculating the hourly heights at a specific tide station over the 19 years, and then taking the average of these heights. This provides a baseline sea level that is not influenced by regional variations.
The IHO also states that the CL must be determined from at least eight tide gauges. If less than eight tide gauges are available, the IHO may use other methods, such as harmonic analysis, to determine the CL.
Impacts
CL is an important reference for navigation, cartography, and other oceanographic and geodetic studies. Knowing the height of the ocean, and the elevation of the land, is essential for navigation, as it allows seafarers to determine their position relative to the shore.
CL is also used to measure and monitor changes in sea level. Since CL is based on a fixed reference point, it allows scientists to better understand the impact of climate change, ocean current shifts, and other natural phenomena on sea level.
Conclusion
Conventional level (CL) is a reference level used to measure the ocean’s height and land elevation. It is determined by the International Hydrographic Organization (IHO) and is based on the average sea level heights over a period of several decades. CL is an important reference for navigation, cartography, and other oceanographic and geodetic studies, providing a fixed baseline from which to measure changes in sea level.
References
International Hydrographic Organization. (n.d.). Definition of Conventional Level. Retrieved from http://www.iho.int/mwp/publications/en/iho_pubs/IHO_S-32/S-32_Ed3_1953_EN.pdf
Gouws, E., & Sager, K. (2020). Sea Level Measurements. In Encyclopedia of Marine Geosciences. Retrieved from https://link.springer.com/referenceworkentry/10.1007/978-3-319-63919-2_9-1
National Ocean Service. (2020). Sea Level Rise and Coastal Flooding Impacts. Retrieved from https://oceanservice.noaa.gov/facts/sealevelrise.html