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To measure the rockets altitude, an inclinometer will be needed. The inclinometer simply measures the angle from the horizon to the rockets apogee.
Above is a basic protractor, you can print this and tape it onto your inclinometer.
Here is Bill holding the inclinometer. We used a small fishing weight on a string as an indicator on the scale. The string needs to be attached to the inclinometer at the upper right hand corner of the protractor (where the little "o" is located). With this inclinometer we used a 1/4" thick plywood handle, the sight is a wood dowel slotted for the handle to fit into. You could use a hollow tube with cross hairs for a really accurate sight. Ours sights more like a rifle sight.
To calculate altitude you need two measurements, the distance you are from the launch pad, and the angle of the rocket at apogee. Within reason, the farther you are from the launch pad with the inclinometer the more accurate your measurements will be. For small rockets 100' would be ok, for higher flights I'd go 200' or more.
The above graphic represents a simulated flight. In this case the person with the inclinometer is 100' from the launch pad. The rockets flight is followed to apogee with the inclinometer, taking care to keep the inclinometer at the angle of apogee, it helps to have another person there to read the angle from the inclinometer. In our example above, we had a reading of 30 degrees on the inclinometer.
To calculate the altitude, simply find the tangent of the angle. (Use a scientific calculator or read it from the graph below.) Then multiply the tangent of the angle by the distance from the launch pad. Tangent of angle x distance from pad. Our tangent of angle is (.577) times our distance from pad (100') = 57.7 feet. That's probably a water rocket type of launch, but you get the idea.
A couple of things about this method of altitude calculation. First, it assumes a perfectly vertical flight, and that rarely occurs. Second, it assumes the person using the inclinometer handles it properly. That takes some practice too. So the actual altitude measurements are at best, an approximation. If the rocket travels at any angle other than more or less perfectly vertical, this method has a large margin of error.
| Angle | Tangent |
| 30 | .577 |
| 31 | .600 |
| 32 | .625 |
| 33 | .649 |
| 34 | .675 |
| 35 | .700 |
| 36 | .727 |
| 37 | .754 |
| 38 | .781 |
| 39 | .810 |
| 40 | .839 |
| 41 | .869 |
| 42 | .900 |
| 43 | .932 |
| 44 | .966 |
| 45 | 1 |
| 46 | 1.036 |
| 47 | 1.072 |
| 48 | 1.111 |
| 49 | 1.1504 |
| 50 | 1.192 |
| 51 | 1.235 |
| 52 | 1.280 |
| 53 | 1.327 |
| 54 | 1.376 |
| 55 | 1.428 |
| 56 | 1.483 |
| 57 | 1.540 |
| 58 | 1.600 |
| 59 | 1.664 |
| 60 | 1.732 |
| 61 | 1.804 |
| 62 | 1.881 |
| 63 | 1.963 |
| 64 | 2.050 |
| 65 | 2.145 |
| 66 | 2.246 |
| 67 | 2.356 |
| 68 | 2.475 |
| 69 | 2.605 |
| 70 | 2.747 |
| 71 | 2.904 |
| 72 | 3.078 |
| 73 | 3.271 |
| 74 | 3.487 |
| 75 | 3.732 |
| 76 | 4.011 |
| 77 | 4.331 |
| 78 | 4.704 |
| 79 | 5.145 |
| 80 | 5.671 |