I can use a protractor to measure a 90° angle from the middle of the horizontal beam.
Once I know where 90° is, I can use a ruler to measure the distance from the center of the horizontal beam to the peak.
In this drawing, I can measure the height of the triangle, x. It is 3 in.
The side that is 16ft. in the diagram is 4in.
Using a proportion, the 3-in. side would be 12 ft. in real life.