The most common and convenient datum which is internationally accepted is mean sea level. Countries take their nearby sea levels as datum planes for calculations of Reduced levels. For example, Pakistan takes sea near Karachi as its datum while India takes sea near Mumbai as its datum for calculation of Reduced levels of different places in their respective countries. The term Reduced Level is denoted shortly by ' RL '. National survey departments of each country determines RL 's of significantly important locations or points. These points are called as permanent benchmarks and this survey process is known as Great Trigonometrical Surveying (GTS). The permanent bench marks act as reference points for determining RL 's of other locations in a particular country.
the datum adopted in india is the mean sea level at
Mumbai