re positive (<code>+</code>) and South and West are negative (<code>-</code>) sign.</p></li>
</ul>
</section>
<section id="geographic-coordinate-systms-gcs-1" class="slide level2">
<h2>Geographic Coordinate Systms (GCS)</h2>
<p>A GCS is defined by 3 mathematical models of the earth’s shape:</p>
<ul>
<li>an <strong>ellipsoid</strong>: earth’s surface that is smooth
<ul>
<li>defined by two radii: semi-major and semi-minor axes</li>
</ul></li>
<li>a <strong>geoid</strong>: earth’s surface that is undulat