I bought this inclinometer because I needed an optical device I could fit in my tool pouch to measure heights in a refinery. This type of environment requires equipment be intrinsically-safe, i.e., unable to provide enough current to cause a fire. This pretty much rules out GPS, etc. Besides, as I explained to a friend once, GPS tells you where you are not how high something is in front of you.
From my field tests I've found this instrument to be accurate to within +/- 1-ft. This is not as good as you would get with a theodolite (transit), which involves a tripod. A single measurement this way would be highly accurate, perhaps to an inch or so but would take a half-hour or so to set up, if you knew what you were doing. If you can live with the accuracy the PM-5 is great for fast measurements.
The Clinometer works best on days when it is partially cloudy. Frequent use, say more than an hour, in bright sunshine, may cause modest headache and eyestrain.
Here's how it work:
1. Measure the horizontal distance first (D). Keep the tape straight.
2. Measure the height of the instrument as you hold it (Ho).
3. Hold the instrument by the brass handle and find the black eyepiece.
4. Choose the best eye that sees close up.
5. Close the other eye.
6. Look for the horizontal hairline.
7. To the right, is % slope (or height in ft at 100-ft); to the left is degrees.
8. Zero with the horizon.
9. Move the instrument to the desired elevation.
10. Open the second eye using it to see the object for which you want the elevation.
11. An optical illusion will bring the hairline in focus in front of the second eye.
12. Adjust the instrument elevation and read the measurement.
13. The best accuracy is with the degrees.
14. Do the math: H = Ho + DTan(angle).
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