In this paper, the conception of Geo-Spatial Information Science (Geomatics) and Digital Earth are introduced. The formation, basic theory and technical system of Geomatics are discussed, and the key techniques and applications of Digital Earth are also discussed. After the relationship between them is analysed, we present that National Spatial Data Infrastructure (NSDI) is the fundamental construction of Digital Earth. The development of Digital Earth brings opportunities and challenges for development of traditional surveying and mapping.