Wikipedia defines the equator as “the imaginary line on the spheroid (Earth), equidistant from its poles, dividing it into northern and southern hemispheres.” This provides the zero reference for north-south coordinates. The most fundamental reference for coordinate systems is the equator. All must have a reference (actually three or more), and every other point on the planet’s surface is located in relation to those references. So it is with geographic coordinate systems.

But even then, we’re actually measuring the voltage of one side of the resister in relation to the voltage on the other side. When we measure somewhere other than from ground (for example, across a resistive device in the middle of the circuit), we must be careful to state that perhaps by saying that we have a “voltage drop of x volts across that resister”. We define that “ground” level to be zero and measure every other point as a positive or negative amount away from that reference. Voltage is always measured in relation to a reference level usually some point in the circuit we call “ground”. We all learned while studying for our first ham radio license that there is no absolute voltage level. In our Further Information section below are links to pages with more information on these two as well as other systems. This page will focus on two of the most popular. Each has its own advantages and disadvantages. There are numerous geographic coordinate systems available, each taking its own approach to locating a specific point.

Ghana national grid mapping system