Datums, Coordinate Systems, and Map Projections Khả năng của GPS để xác định vị trí chính xác của một người sử dụng bất cứ nơi nào, trong bất kỳ điều kiện thời tiết, thu hút hàng triệu người dùng trên toàn thế giới từ các lĩnh vực khác nhau và nguồn gốc. Với những tiến bộ trong công nghệ GPS và máy tính, các nhà sản xuất GPS đã có thể để đến với hệ thống thân thiện rất dễ sử dụng. Tuy nhiên, một vấn đề phổ biến mà nhiều người mới đến để đối mặt với GPS là. | 4 Datums Coordinate Systems and Map Projections The ability of GPS to determine the precise location of a user anywhere under any weather conditions attracted millions of users worldwide from various fields and backgrounds. With advances in GPS and computer technologies GPS manufacturers were able to come up with very user-friendly systems. However one common problem that many newcomers to the GPS face is the issue of datums and coordinate systems which require some geodetic background. This chapter tackles the problem of datums and coordinate systems in detail. As in the previous chapters complex mathematical formulas are avoided. As many users are interested in the horizontal component of the GPS position the issue of map projections is also introduced. For the sake of completeness the height systems are introduced as well at the end of this chapter. 47 48 Introduction to GPS What is a datum The fact that the topographic surface of the Earth is highly irregular makes it difficult for the geodetic calculations for example the determination of the user s location to be performed. To overcome this problem geodesists adopted a smooth mathematical surface called the reference surface to approximate the irregular shape of the earth more precisely to approximate the global mean sea level the geoid 1 2 . One such mathematical surface is the sphere which has been widely used for low-accuracy positioning. For high-accuracy positioning such as GPS positioning however the best mathematical surface to approximate the Earth and at the same time keep the calculations as simple as possible was found to be the biaxial ellipsoid see Figure . The biaxial reference ellipsoid or simply the reference ellipsoid is obtained by rotating an ellipse around its minor axis b 2 . Similar to the ellipse the biaxial reference ellipsoid can be defined by the semiminor and semimajor axes a b or the semimajor axis and the flattening a f where f 1 - b a . An appropriately positioned .
