His most significant and celebrated first ascent was of Piz Bernina, at 4,049 m the highest summit in the Eastern Alps, together with two assistants, brothers Jon and Lorenz Ragut Tscharner on 13 September 1850. Their route was via The Labyrinth and the east ridge. Although all three were surveyors, they left their surveying equipment at the foot of the ridge. Coaz later wrote in his diary: