section>
<section id="so" class="slide level2">
<h2>So …</h2>
<ul>
<li><p>They can be <strong>geographic</strong> (3D, angular units)</p>
<ul>
<li><p>Ellipsoid (squished sphere model)</p></li>
<li><p>geoid (mathematical model of gravitational field)</p></li>
<li><p>datum marks the relationship between the ellipsoid and geoid</p>
<ul>
<li><p>can be local (NAD27)</p></li>
<li><p>can be global (WGS84, NAD83)</p></li>
</ul></li>
</ul></li>
</ul>
</section>
<section id="so-1" class="slide level2">
<h2>So …</