The LineOfSightOperator uses elevation data to compute the regions that have a free line of sight to a given position.
Choose the distance of the view and the sensor height below and then click on a position on the map to place an observer and the area of free visibility will be displayed.The colors yellow to orange mean, that the observer is not able to see these parts.
Distance:
Sensor height:
Map data (c) USGS, Lake Tahoe Clearinghouse, public domain data