The map of the canton of Zürich by Hans Conrad Gyger (1599-1674) which he completed in 1667 after 38 years of preparation, is an outstanding masterpiece of cartography. This work of art, unique in size, beauty and accuracy, is the first map of its scale based on trigonometric survey and the very first topographical map drawn in relief. It measures 220 x 230 cm and represents an area 70 x 75 km. It is made of paper and the design executed in an aqueous medium. In 1976 the map was brought to the Swiss Institute for Art Research for examination and conservation treatment. The technique of the map's fabrication, its damages and its treatment are described in detail. The map was backed with Japanese paper and thereafter lined with canvas. The uniqueness of this object, as well as its large size, made its treatment a task requiring the utmost care.