SEACOOS CDL v2.0 December 9, 2004 8 (temperature, salinity, velocity, etc) as a function of time and location(s) in the same coordinate system, we are able to merge data from different sources. The coordinate system used by SEACOOS CDL is the Cartesian coordinate system. The three spatial dimensions are longitude (E-W), latitude (N-S), and depth (or height); denoted as x, y and z, respectively. Usually a right-handed coordinate system is used where x is positive towards the east, y is positive towards the north and z is positive upwards; but sometimes in the ocean z is taken to be positive downward and a left-handed coordinate system is used.    Time (t) and the three coordinates (x, y and z) are the independent variables.  The field or measurement is the dependent variable.   Six categories emerge by viewing each measurement, for example temperature (T) and velocity components (u,v) as a function of  the independent variables. These six categories are fixed- point, fixed-profiler, fixed-map, moving-point-2D, moving-point-3D, and moving- profiler. (A moving-map category is omitted. Not many oceanographic sensors fit this category.) These are summarized in Table 5.1.