S identifies locations on the <em>curved</em> surface of the earth.</p></li>
<li><p>Locations are measured in <strong>angular</strong> units from the center of the earth relative to the plane defined by the equator and the plane defined by the prime meridian.</p></li>
<li><p>The vertical angle describes the <em>latitude</em> and the horizontal angle the <em>longitude</em></p></li>
<li><p>In most coordinate systems, the North-South and East-West directions are encoded as +/-.</p></li>
<li><p>North and East a