One observer who is measuring an angle and the second observer who is running right under the plane.
The one running under the plane simple stays under the plane until the other observer shouts out the angle of measurement. At that point the runner stops and the distance between the two observers is measured.
You can use the mathematical tangent to calculate the height indirectly.
The Squirrel-O-Meter can be used to measure the angle more accurately than an ordinary protractor. You can print it, cut it out and glue parts together.
You can use your Squirrel-O-Meter to measure other objects such as trees, buildings and perhaps even the height of a cloud!