Editing Parallax (section)

== Metrology ==
[[File:Parallax pointer error.PNG|thumb|The correct line of sight is required to avoid parallax error where a different line of sight gives a different reading from a measurement instrument. ]]

Measurements made by viewing the position of some markers relative to something to be measured are subject to an error caused by parallax, if the markers are some distance away from the object under measurement and not viewed from the correct position or angle. An example is reading the position of a pointer against a scale in an instrument such as an analog [[multimeter]] as shown in the right figure. The same effect alters the speed read on a car's speedometer by a driver in front of it and a passenger off to the side, values read from a [[Oscilloscope#Graticule|graticule]], not in actual contact with the display on an [[oscilloscope]], etc.

To help the user avoid this problem, the scale is sometimes printed above a narrow strip of [[mirror]], and the user's eye is positioned so that the pointer obscures its reflection, guaranteeing that the user's line of sight is perpendicular to the mirror and therefore to the scale.