The Everest Spheroid

The Indian government insists on using the Everest Spheroid as the reference datum for its maps, citing greater accuracy and military significance. However, with the development of the World Geodetic System, which uses the Earth’s center of mass, the government’s insistence on surface reference mapping and restrictions on map exportation appears outdated and unnecessary.

This article was first published in The Mint. You can read the original at this link.

I have always been perplexed by the vigour with which the Indian government (and in particular its defence establishment) insists that map data should not leave the country. Whenever I’ve spoken with people in the government about this, they have argued that the Government of India (GoI) maps are significantly more locationally accurate than those derived from satellite imagery because they have been prepared using the Everest Spheroid as the reference datum. They insist that it is only those maps of India that have been prepared using this unique framework that are capable of pinpointing with any degree of useful accuracy the location of a given point in India in relation to any other. This is why the GoI still takes pains to ensure that GOI maps don’t fall into the hands of enemy nations and why it has imposed a blanket ban on the movement of maps and all sorts of map data out of the country.

In order to appreciate the significance of the Everest Spheriod as a reference datum, we have to understand that, despite everything we’ve been taught in school, the Earth is neither perfectly round nor an oblate spheroid. Instead, the shape that the Earth most closely resembles is that of a lumpy potato. There is, therefore, a significant difference in the distance between two points calculated on the assumption that they are both located on a perfect spheroid as compared with the actual distance between them —which may be compressed or elongated, as the case may be, by the kinks and ridges on the physical surface of the earth.

Surveyors have always known this fact and have taken pains to improve the accuracy of their maps by first plotting a reference surface that most closely approximates the shape of the Earth they are mapping. The distance between two locations on a map drawn against this sort of reference datum is far more accurate than that between the same two points represented on a more globally approximate spheroid.

In the 1830s, Sir George Everest, India’s first Surveyor General, mapped out the geodetic reference datum for India. This datum, called the Everest Spheroid in his honour, has since been used as the basis for all government-issued maps of India. Given the tactical military significance of knowing the exact distance between one point and another on the earth’s surface, it is easy to understand why the government is at such pains to ensure that these maps don’t fall into enemy hands.

The problem with using reference datums is that, while they work in the context of relatively small portions of the Earth’s surface (such as the territory of India in the context of the Everest Spheroid) they are usually misaligned with the Earth’s geographic centre. Consequently, while they are useful for local navigation, these reference maps are useless for higher military and scientific applications that require cross-border information.

This fact came into sharp focus during World War II when troops missed their military targets because the geodectic datum of their country was unconnected to that of their enemies. This was even more marked during the Cold War when, despite the fact that both the Soviet Union and the US were armed with intercontinental missiles, neither country could be assured of positioning accuracy given that the maps of both countries were drawn on reference datum unconnected with each other.

In order to address these military issues, the US government built an entirely new system that used the Earth’s centre of mass as its reference point. Earth-centred systems do not need an anchor point on the surface from which the reference datum is built. So long as they are based on the precise geodetic centre of the earth, they can be free of the constraints of national reference spheroids and, are guaranteed to be just as accurate (if not more so) as traditional surface anchor-based maps.

The World Geodetic System (WGS) was developed in 1960 and was significantly overhauled in 1984. It is maintained by the US National Geospatial-Intelligence Agency that invests billions of dollars into ensuring that it continues to most accurately depict the Earth geoid. Its calculation of the location of Earth’s centre is now accurate to within the width of a postage stamp. WGS-84 is currently used by all GPS applications and is, conclusively, the most accurate measure of the distance between any two points on the surface of the Earth.

Seen in this context, the Indian government’s continued insistence on preventing the export of maps is inexplicable. Surface reference mapping was the proxy we were compelled to rely on when we lacked the technological prowess to accurately pinpoint the centre of the earth’s mass. Now that we can do that, insisting on enforcing regulations that protect surface reference maps just makes us look silly.