The keraunic level (Nk) corresponds to the strikes number and, more precisely, the number of thunders heard in a defined area.
The lightning density (Ng) is the number of lightning impacts per km² and per year.
The density of the touching points of the lightning onto the ground (Nsg) is the average number of lightning strikes onto the ground per km² and per year.
It is estimated that lightning strikes about 1 time out on 10 thunders heard. So Nk (Keraunic level) =10Ng
Below is the French map of the density of the touching points of the lightning onto the ground (Nsg) from the NF C 17-102 (2011) standard according to the data supplied by Météorage :
Below is a world map showing the lightning density (Ng) measured by the NASA :
