N.A. volcanoes and country populations overlay exercise
Exercise 1: generate a world map, marking North American volcanoes with circles.
Layer 1: Create world map. Add circles to denote volcanoes, reading coordinates from txt file. Color code circles red, orange and green (70% fill in grey circle) based on height of volcanoe (heights from txt file). Show height of each volcanoe in popup over each circle.
Layer 2: Overlay countries on map according to population size, read from JSON file, denoted by colour fill in red, orange, green.
Create layer Control Panel to allow switching on/off of layers 1 & 2 above