Field methods in geology

What’s the true height of a mountain?

  • For a long time, a place’s height has been measured with reference to the surrounding sea.
  • Mount Everest, for instance, is 8,848 metres when measured from the surface of the sea but may have a different value if measured from the ocean floor. Maps, with their MSL (mean sea level) markings, bear the stamp of this tradition.
  • In the age of satellites, measurements are no longer made from below but above.
  • An expedition, in 1999, that measured Mount Everest using a GPS receiver found that it was 10 metres higher. Not only does that make your climb higher but several measurements, of dams and skyscrapers are affected depending on whether they are computed by traditional surveying or satellites.
  • To fix this, surveyors need to account for the so-called geoid.

What is a geoid?

  • Were a giant pump to miraculously suck the oceans dry, the earth would look grotesquely lumpy, like an evolving dumpling.
  • The Himalayas and the Andes jut out. Many places cave in.
  • A large spot, for instance, spanning a quarter of South India, Sri Lanka and deep into the Indian Ocean makes that portion of the globe gnawed out.
  • An uneven surface also means varying levels of gravity across the earth. A geoid model of the earth tries to account for the all the undulation and imagines a new kind of earth ‘surface’ where the oceans were smoothened out and gravity the same everywhere.
  • Map-making authorities employ a mathematical calculation to ‘correct for the geoid’ and thus, the true height of a structure or landform.

Who is responsible for this in India?

  • The Survey of India, a 250-year-old organisation that’s still the official map maker of the country, developed a system called INDGEOID Version 1 that will automatically correct for the error in GPS and sea-level measurement of structures in India.
  • The most immediate and notable beneficiary of this would be Mount Everest.

How so?

  • Mount Everest, it’s been claimed, has lost a few metres due to the Nepal earthquake of 2015 that killed thousands. To re-ascertain this, the Survey of India will conduct a new GPS-based measurement — in a joint mission with Nepal — of the mountain peak.
  • This will incorporate the new INDGEOID measurement. This will hopefully popularise use and be widely adopted by surveyors, infrastructure companies and basically anybody who uses maps and measurements.

Source: The Hindu

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