Measuring the angle of a sloping surface, like a roof or a sloping riverside, is simple.
Attach a pendulum to a stick, about 1 to 3 meters long. The longer the pendulum and the stick, the more accurate your measurement will be. In case of emergency situation, simply attach a stone to a string, then connect it to a stick.
I took a roof for an example. Get to the top of the roof. Place the stick on the surface. Gently move the stick forward or backward until the stone touches the sloping surface of the roof. Make sure that the joint between the stick and the string forms a perfect 90 degrees elbow. Mark the part of the stick which touches the edge of the rooftop, then measure A and B using a measure tape. Finally, you can measure alpha using equation below.
