Originally Posted by
mklotz
Most of the commercial ones, of which I have two, have a full 360 degree protractor scale. This allows one to measure angles overhead. (Imagine measuring the pitch angle of a roof from inside the attic.)
Since Doug's has only a 180 degree protractor one might expect that it can't make such measurements, but they would be wrong. The trick is to put a right angle plate against the "roof" and measure the angle relative to vertical of the part that extends outward from the "roof". This angle is the complement of the pitch angle so simple arithmetic will yield the desired measurement.
It should be obvious that this trick is used to measure any angle that is greater than ninety degrees.
In reality, Doug's 180 degree setup is a feature, making the tool more compact and thus easier to store.
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