as we all know the dilution of precision. there are four components which are used in calculation of positional accuracy of DOP like standard deviation of north, east, upper and in time. is there any specific reason to use east and north direction only? why not West and South direction?
Y not? you can use cartesian coordinates to represent your object in space. for more clarifications look at the following link.
https://en.wikipedia.org/wiki/Euclidean_space
For the first, it is very import to find magnetic declination up to the north, because that always shows for some epoch. Every coordinates mast be in connections with datum and ellipsoid, universal datum (WGS 84) must sometimes be with the different projection, especially in old maps. Another way is to find cartesian coordinates, or polar if you want to know exact precision on the Globe.
Actually, when one calculates the DOP of an estimated position solution, the quality of the geometry refers to the coordinate frame in which the geometry matrix (let's call it G) is. In principle, G is given in ECEF frame but of course the DOP, as well as the position itself, expressed in the ECEF frame is far from intuitive.
Thus, people often express the solution, the geometry matrix and also the DOP in a more friendly coordinate frame. The NED (North-East-Down) and the ENU (East-North-Up) frames are used just for an easier way to visualize data, and the reason behind North and East is generally link to other sensors (like the magnetometer or the compass) giving their data with respect to those frames and also because most of the most-well known Bayesian filters are generally found in the literature following this convention.
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