Sea Level. This hypothetical point in space plays an important role in some measurements that have real world applications. Atmospheric Pressure, important for aviation and forecasting the weather. Geological altitude such as the height of Mt. Everest is another example. As we all know sea levels are rising. Does this fact have an impact on the altitude measurement systems? I have never heard of Mountains getting "shorter" because of sea level rise. Is "Sea Level" considered a fixed point in space or is it in flux? If so then when was it considered fixed and what was the year it was determined the "hight of sea level"? How much higher is the actual "sea level" now compared to its "agreed on point in space"? If it is not fixed then why do mountains stay at the same height? Should that difference have an impact on predicting the weather?