 He was greatly assisted in this work by Chinese books of geography, where he found a mass of descriptive information, the distances between important places, and even maps which however were very crude, the distances having been measured with little exactitude.These imperfect data he supplemented and completed by astronomical observations made in the chief towns by himself and his associates; hence the positions of his Atlas are remarkably accurate.The favour enjoyed by the missionaries with the Kangxi Emperor (1662–1722) made it possible for them to improve on this.