<p>z=20 does not seem to be in an RFC but basically all of the major map services respect it:</p>

<ul>
<li>geo:34.99393,-106.61568?z=11</li>
<li><a href="http://osmand.net/go?lat=34&lon=-106&z=11">http://osmand.net/go?lat=34&lon=-106&z=11</a></li>
<li><a href="https://www.google.com/maps/?q=loc:34.99393,-106.61568&z=11">https://www.google.com/maps/?q=loc:34.99393,-106.61568&z=11</a></li>
<li><a href="http://maps.yandex.ru/?ll=34.99393,-106.61568&z=11">http://maps.yandex.ru/?ll=34.99393,-106.61568&z=11</a></li>
<li><a href="http://maps.apple.com/?ll=34.99393,-106.61568&z=11">http://maps.apple.com/?ll=34.99393,-106.61568&z=11</a></li>
</ul>

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