Polylines are the representations of Earth's features on the Earth, but sometimes, they are not enough to show the mobility of Earth features. For example, a river can be shown with the help of polylines, but the flow direction of the river can't be demonstrated by the polylines alone. Animating polylines can be a solution to show the mobility of Earth features. The flow direction of a river can be shown with the help of animated polylines.
Symbol class, or you can also use the predefined symbols that are accessed from the