Here is a picture of the gap between the ruoler and the scale on the wall. This gap can cause parallax error. I need to make sure that the top of the ruler is lined up with the bottom of the scale on the wall. The gap causes a problem - how do I know I am looking at it from the correct angle?
I got my special bendy ruler and set the angle to 90 degrees.
I set it at 90 degrees to the top of the ruler and across to the wall. Then I could make sure they were in line. I'd have used a set square if I had on.
If the top of the ruler was accidentally 1cm below the bottom of the scale on the wall, every single reading would be 1cm too low. The zero on the wall scale would be in the wrong place - called a ZERO ERROR. Every reading would now be wrong for the same reason by the same amount - this is called a SYSTEMATIC ERROR.
