The Matterhorn has two distinct summits, situated at either end of a exposed rocky crest which forms the Italian/Swiss border. In August 1792, the Genevan geologist and explorer Horace Bénédict de Saussure made the first measurement of the Matterhorn's height, using a sextant and a chain spread out on the Theodul glacier. He calculated its height as . In 1868 the Italian engineer Felice Giordano measured a height of by means of a mercury barometer, which he had taken to the summit. The Dufour map, which was afterwards followed by the Italian surveyors, gave as the height of the Swiss summit.: