Getting an impression on coverage near Airports

Hi all,

We are in contact with people wanting to improve coverage near some airports, so getting low altitude data.

To give those a good advice whether or not they should consider running a new receiver, I want to know how the coverage in that area is.

I checked opensky-network.org/network/facts but couldn't quite understand the legend of it (there is none). The page states darker colors indicate better coverage, and everywhere I expect good coverage (FRA/EDDF e.g.) there are no colors, just basemap visible. But the basemap is also visible in regions where I expect no coverage at all (central Australia, central africa).
To add to the confusion, the colors also change when changing zoom levels of the map.

I already found out that you segment the map into buckets and assign aircraft positions. I assume this is based on some threshold (undocumented?)

Can one of you give me an explanation on what the colors actually mean?
What is the altitude threshold below you add a position to the bucket?

Thank you and best regards
/j

ah okay, I'll do so using impala then. Anyway I'm curious what the '3D map' is good for then and what the represents actually. Can you give me a hint?