Altitude refers to the height of an object or point relative to a specific reference surface. This foundational concept allows for the measurement of elevations, from mountain peaks to the depths of valleys. If a chosen reference point is zero, then any location below it has a negative altitude.
Understanding Altitude Measurement
The standard reference point for measuring geographical altitude is Mean Sea Level (MSL). MSL represents the average height of the ocean’s surface over an extended period, typically 19 years, to account for variations caused by tides, weather, and seasonal changes. This average is established through data collected from tide gauges located at various coastal points and increasingly through satellite altimetry, which measures the distance between satellites and the ocean surface. Geodetic datums, such as the World Geodetic System 1984 (WGS 84), provide a consistent framework for defining MSL and ensuring uniformity in elevation measurements.
Mean Sea Level serves as the universal zero point because the Earth’s oceans are interconnected and tend to seek a common level, making it a globally consistent and practical baseline. While local reference points can exist, MSL offers a stable and widely accepted standard against which the height of landforms and objects can be determined. This consistency is particularly beneficial for cartography, aviation, and various scientific applications, for understanding terrestrial topography.
The Concept of Negative Altitude
Negative altitude signifies a depression or low-lying area relative to the global oceanic baseline. Regions with negative altitudes are formed by geological and geographical processes.
Tectonic depressions, often formed by the movement and interaction of Earth’s crustal plates, can create vast basins that sink below sea level. Additionally, land subsidence, which is the sinking of the ground, can contribute to negative altitudes. This subsidence can result from natural processes like the compaction of unconsolidated sediments or the slow movement of the Earth’s crust. Human activities, such as the withdrawal of groundwater or the draining of wetlands, can also accelerate land subsidence, causing areas to drop below sea level.
Places Below Sea Level
Several notable locations worldwide demonstrate the concept of negative altitude, each shaped by unique geological and historical circumstances. The Dead Sea, situated between Israel, Jordan, and Palestine, is the lowest dry land on Earth. Its shoreline is approximately 413 to 439.78 meters (about 1,355 to 1,443 feet) below sea level. This hypersaline lake lies within a deep valley formed by the transform boundary between the African and Arabian tectonic plates, a process that continues to cause the area to sink.
Death Valley in California, USA, is the lowest point in North America. Its Badwater Basin reaches an elevation of 86 meters (282 feet) below sea level. This extremely arid region is a long, narrow basin bordered by high mountain ranges, and its dry climate prevents it from filling with water, despite its low elevation.
Large portions of the Netherlands also lie below sea level, with approximately 26% to 40% of the country’s land at negative elevations. The lowest point in the European part of the Netherlands is the Zuidplaspolder near Nieuwerkerk aan den IJssel, which is about 6.74 to 6.76 meters (around 22 feet) below Nieuw Amsterdams Peil (N.A.P.), the Dutch equivalent of Mean Sea Level. Much of this land has been reclaimed from the sea over centuries and is protected by an extensive system of dikes and pumps.
New Orleans, Louisiana, is another city with significant areas below sea level. The average elevation of metropolitan New Orleans is around 1.8 meters (approximately 6 feet) below sea level. The city’s low elevation is largely due to its location on unconsolidated river sediments and land subsidence due to factors like groundwater withdrawal and the compacting of drained wetlands. Levees and pumping systems are essential for keeping the city dry.