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Later OS maps include information about landform, augmenting spot heights with contours. These connect points of equal elevation. Before leaving this chapter it is worth recounting how an early use of contours was employed during the ‘Schiehallion experiment’. After Newton’s discourse on the universal theory of gravitation, there was much scientific debate about the shape and mass of the earth. Mountains could be used to test contrasting theories. Deflections of a weighted plumb line from the vertical could be measured and used by extrapolation to find the mass and volume of the earth. Schiehallion or Sìdh Chailleann - the Fairy Hill of the Caledonians was deemed ideal for the experiment because of its isolation from other peaks. These would exert a gravitational pull, and the peak’s apparent symmetry meant that any declination from the vertical could be considered commensurate on all sides.

Accordingly, Charles Mason, later to be replaced by Nevil Maskelyne, the Astronomer Royal, embarked upon an expedition to the mountain. They spent four months on the summit. A mathematician and surveyor called Charles Hutton realised that the numerous readings and measurements of deflections could be organised according to common values by plotting them along a line circling the mountain. Hutton did not discover the contour or isopleth. Other thinkers of the time had developed similar ideas, but he did apply the concept to the experiment.