Elevation of railway station is measured.

With respect to which point the mean sea level of any railway station is measured ?

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  • Suppose you want to measure the MSL of a hilly town, B near railway station A. The reason could be anything from mentioning it in a tourist brochure to check the coverage of radio. 

    If A's height is 100m above MSL, B's height is (the height calculated + 100)m above MSL. Now, you can do either of these steps to calculate the height above MSL: 

    1.  You measure the barometric pressure at station A. Then, you measure the  barometric pressure at town B. Approximately, the pressure at sea  level is about 1013 millibars (mb) and it decreases by about 10 mb for  every 85 metres of increased height. Thus, from the difference in pressures, you can calculate the difference in height between A and B. This is the least accurate  measurement method.

    2. You take two survey points, A and A1, and measure the vertical angle between them(Using a theodolite). Thus, the difference in height can be calculated from basic trigonometry.  You continue doing this by repeating the process between A1 and A2, A2 and A3 and so on until you reach point B. Of course, you need to include corrections like Earth's curvature, etc. 

    3. You take two survey points, A and A1, and measure the difference in their height using an optical level. Typically, A and A1 should be no more than 50 metres distant from each olther. The principle of an Optical level is that, it just project horizontal lines, forwards and backwards. Thus, a vertical rod is kept at the other end and the point where the horizontal line touches the rod is noted. This process is continued and at each step, the difference in height between the two points is calculated. This is the toughest, yet the most accurate method.
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