Johnson was a surveyor and took part in the Great Trigonometric Survey which was a half a century spanning project which aimed to measure the Indian subcontinent with Scientific Precision. In 1865, as he was conducting survey operations in the northern limits of the Maharajah’s dominions he received an Invitation from a certain Habibullah, the Khan of Khotan in Illichi, Turkestan, whose dominions lay beyond the Kunlun Mountains, allowing him to perform this remarkable geographical exploit.
The British Raj whose understanding of this region was hazy, was hungry for topographical, geological and political information on these areas. The Khan who had overthrown the hated Chinese yoke over his lands was eager to receive British help. There was also interest in creating a trade link between the British India and Turkestan.
Johnson himself confirms that the Maharaj’s dominion extended up to the Kunlun Mountains and the mountain passes leading into Turkestan and that he levied transit duties on trade through them. The idea behind the journey was to look for an eastern route, bypassing the established trade routes through the Karakoram Pass. Information that Johnson had gleamed suggested that such a route was possible through the Changchengmo valley.
He started his journey in July 1865 from Leh, hemming closely to the Indus River by Tikse then turning westward to Tanksi and to the region of the Pangong Lake. He then travelled in a north easterly direction over the Masimik Pass, then through the Changchengmo valley to Hot Spring, also called Kiam where he halted. This is roughly the stretch that the new road will traverse. He then marched northwards onto the Linzi Thang Plains of what is now occupied Aksai Chin. At the Northern end of these plains he arrived at the Karakash River. Crossing it and after a hard march, he arrived at his destination in Illichi where he stayed for sixteen days. He returned to Leh via the Karakoram Pass.
P.S
The current Indian territorial claim in the region is based on the line proposed by Johnson, demarcating the borders between Ladakh-Aksai Chin and Tibet.