Sighting also measures angles and alignments. The trick is to hold the pencil perfectly vertical or perfectly horizontal, so you have something steady to compare the object with.
- Angles: hold the pencil level (or upright) at arm's length, in front of a slanted edge such as a roof line. See how far the edge tilts away from the pencil, then draw the same tilt.
- Alignments: hold the pencil straight up and down. It becomes a plumb line, named after a weight on a string that hangs straight down. Look at what lines up along it: which point sits directly above which.
Key idea
Hold the pencil truly vertical or horizontal, never tilted to match the edge. Compare the edge with it to judge the angle, and use it as a plumb line to see what lines up.
Why it works
Our eyes judge angles badly on their own, but they compare two lines side by side very well. A level or upright pencil gives you a fixed reference. Some people also think of a clock face: an edge tilting up to the right like a clock hand pointing at two o'clock rises at about 30 degrees.
Worked example: A roof
You hold your pencil level in front of a roof. The roof edge rises from the level pencil about one third of the way to straight up. About what angle is that?
- Straight up from level is 90 degrees.
- One third of 90 is 30.
Answer: About 30 degrees.
Common mistake
People tilt the pencil to match the edge, which tells them nothing: an edge always matches itself. Keep the pencil truly level or truly upright, and compare.