Height is measured from one of two references, the ellipsoid or the geoid. The geoid represents a surface of uniform gravitational potential, representing a putative mean sea level. In the absence of tidal influences, it is the level at which seas would be at rest. Inland, one can think of the geoid as the level of a well connected by a [very, very long] pipe to the ocean. The geoid accounts for irregularities in the distribution of mass in real world. It is not so important that the geoid is at the exact level of the real ocean, as that it is an equipotential surface.
The ellipsoid is an arbitrary figure representing the “average” figure of a geoid, useful for navigation. The most popular ellipsoid these days is WGS-84, which is the basis of GPS navigation. The ellipsoid is defined by its equatorial and polar radii, and the location of the poles. For navigation, a conventional meridian connecting poles is defined, which for WGS-84 is near the old prime meridian of Greenwich.
The article is not at all clear, and the deviation of the geoid from the ellipsoid in the Indian subcontinent is large, in the range of 60-80 below the ellipsoid in the region of the Himalayas.
https://en.wikipedia.org/wiki/Geoid
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