Nov. 21, 2019: Improved map resolution while mouse not moving.
Horizontal mouse drag moves the map in east-west direction, and vertical mouse drag moves the map in north-south direction regardless of mouse position or current rotate angle.
Internet Explorer, Google Chrome, or Safari is recommended. Firefox is slow because of its low performance in canvas 2D path processing.(Mar. 29, 2021: Firefox performance seems improved and not much difference of browsers is observed. Thus, whole sentence is removed)