There is a sense of deja vu in reports of a Chinese incursion into Arunachal Pradesh. In 1986, similar reports of Chinese occupation of Sumdorong Chu (chu=river) touched off a major crisis that brought India and China close to war. It is also likely that then, as now; the incident was a result of the fact that Indian intelligence agencies have their own interpretation of the LAC, different from that of the Chinese and the Army. It is well known that the map attached to the treaty was of such a scale that a literal translation of the coordinates map on the ground would still leave a band some four-seven km wide that could be interpreted in different ways, causing confusion as to where the border lay. The problem has really arisen from the IB’s interpretation of the map. Since the 1950s, it has insisted that the border was not the literal McMahon Line as translated on the ground, but the highest crest-line of the Himalayas in the region. The problem is that in this region, north-south streams and rivers have ensured that the highest crest-line is not as distinct as it is in the other parts of the Himalayas. In 1962, the Army insisted that its maps showed this to be the Hathung La (La=pass) ridge, while the IB insisted it was the Thag La across the Namka Chu. The Army was ordered to clear the Chinese off the Thang La ridge, resulting in war.
Source: Manoj Joshi
The Times of India, 28 July, 2003