OpenCellID heatmap
opencellid.org, Original uploader was <a href="https://en.wikipedia.org/wiki/User:Markus591" class="extiw" title="wikipedia:User:Markus591">Markus591</a> at <a href="https://en.wikipedia.org/wiki/" class="extiw" title="wikipedia:">English Wikipedia</a>
CC BY-SA 3.0
<p>Screenshot from <a href="https://en.wikipedia.org/wiki/User:Markus591" class="extiw" title="en:User:Markus591">Markus591</a><br></p>
<b>Published:</b> opencellid.org (OpenCellID database: CC-BY-SA 3.0, Map data: OpenStreetMap, ODbL 1.0, Tiles CC-BY-SA 2.0)