Instruments

Entry 06.3MappingEntry

Three tools, correctly read, were enough to fix a position and an altitude anywhere on the plateau.

01The Instruments and What They Measured

A sextant finds latitude by measuring the angle between a celestial body and the horizon.

In clear mountain air, where the stars are hard and the atmosphere thin, it is remarkably precise. The Survey of India, working out of Dehradun in the foothills of Uttarakhand, issued each trained surveyor a pocket sextant small enough to conceal: for the pundits — the Indian surveyors sent covertly across the plateau from the 1860s onward — concealment was as important as accuracy. Nain Singh Rawat carried his sextant hidden in a false-bottomed travelling chest and took his star sights at night, when he was less likely to be observed.

A magnetic compass fixed direction and, used with a known pace count, allowed dead-reckoning between sextant observations.

Nain Singh counted his paces on a Buddhist rosary that had been shortened from the standard 108 beads to an even 100, each bead marking a hundred paces, each circuit of the rosary marking ten thousand. The pace itself was calibrated: he had walked measured distances before departure so that his stride, on flat ground, was known to within an acceptable tolerance. On this method — one instrument measuring angle, one measuring direction, the body itself measuring distance — he produced the first accurate survey of the road from Nepal to Lhasa, completed in 1866 and accepted without significant revision.

02Reading Altitude from Boiling Water

A sextant and a compass, together, give position in two dimensions.

The third dimension — altitude — demanded a different instrument altogether. Barometric pressure drops with elevation, and water's boiling point drops with pressure, falling by roughly one degree Celsius for every three hundred metres gained. A calibrated boiling-point thermometer therefore becomes an altimeter: measure the temperature at which water boils, consult a prepared table, and altitude follows.

The instrument used on the plateau was a hypsometer — a small closed vessel that brought water to a steady boil over a spirit lamp, with a thermometer immersed in the steam.

Thomas George Montgomerie, the Royal Engineers officer who conceived the pundit programme at the Survey of India, had his surveyors carry hypsometers alongside their sextants. The readings were not instantaneous: a spirit lamp had to be lit, water brought to a stable boil, and the thermometer read at the moment equilibrium was reached. Wind disrupted the flame; rain disrupted the water; patience was the operative skill. On the Chang Tang, the high northern plateau where wind rarely stops and water for boiling had sometimes to be melted from ice, a reliable reading could take the better part of an hour.

The resulting altitudes were not perfect — calibration drifted, and local barometric anomalies could introduce error — but they were far better than anything available before. Montgomerie's trigonometrical methods, applied from fixed baselines in the plains, could not reach the interior of Tibet; the hypsometer could go wherever its carrier could walk.

03Accuracy and Its Limits

The pundit surveys were checked, corrected and compiled at Dehradun, where Survey of India draughtsmen turned raw notebooks into publishable maps.

The Royal Geographical Society in London received and published results, and the maps that emerged from the programme placed hundreds of lakes, rivers, passes and settlements — including Lake Namtso and Lake Manasarovar — at positions that modern GPS confirms as close. Kishen Singh's traverse of the plateau in the early 1880s covered thousands of miles and produced altitude readings for much of the northern plateau using exactly these instruments.

What the three-instrument method could not do was resolve fine detail.

A sextant observation accurate to within a few miles is good enough to fix a lake but not a narrow gorge; a pace count drifts over broken ground; a hypsometer gives altitude at points, not along profiles. The instruments were enough — precisely enough, for the purpose — and they were everything the plateau allowed. Precision beyond that would wait for the twentieth century and instruments the pundits never carried.