Mean sea level
Also called: MSL · Topic: Topographical Maps: Relief Representation and Interpretation · NCERT: Class 11, Ch 5 "Topographical Maps"
Meaning
Mean sea level (MSL) is the average height of the sea surface between high tide and low tide, measured over many years. It is the zero datum, meaning the starting level from which all land heights are measured. All contours, spot heights and bench marks on a map show heights above MSL. Using one common zero lets heights from different places be compared fairly.
Example
The heights of Himalayan peaks such as Kanchenjunga in Sikkim are all measured from mean sea level. The same is true for a village in the Ganga plain near Varanasi. That is why the two heights can be compared directly.
Don't confuse with
- WGS-84 ellipsoid height: GPS gives heights above the WGS-84 ellipsoid, which is a mathematical model of the earth's shape. These heights must be converted to heights above sea level using a geoid model before they match map heights.