after having visualized the boundaries of the continents in the form of a map, I would like to create isolines within this map which identify and connect points with the same characteristics to each other. example: considering Naples as the center, I would like to create a line that connects the points that are located at a distance of 100 km from Naples, but only those on the continents not on the sea. an example would be lines 1-2-3, etc. in this figure
how can I do?