Does seeing ADSB tower symbols on a chart indicate reception, or that there is just one located there?
I'm almost positive it's towers that are currently being rec'd. It seems to match up with the displayed active tower count, and I believe the ADS-B transmission includes the lat/lon.
We don't actually database the towers. The icon indicates we have received a signal from that tower. It will flash blue/red when it is actively receiving data from that tower.