Alignment is a process of measuring something, comparing to a reference or standard, making adjustments, then remeasuring. The key element of this process is measurements. This chapter describes the instruments and methods for obtaining those quantities.

There is no limit to the measurement of alignment accuracy, except the noise level of the instruments used. ^[1] Alignment is a feature that has continuity to a value of zero and does not lose physical existence when approaching zero. Compare this to weight, which loses its property of the measured quantity as it gets smaller. Zero weight is not measurable because there are no forces left to deflect a spring, regardless of how much resolution the spring scale has. Zero weight is judged by the absence of any spring deflection. Alignment, by contrast, retains its capacity to be observed as it gets ever smaller, because it is a differential rather than an absolute quantity. Eventually, our instruments will record no deviation, but that is a problem with the noise level of the instruments, not the property of alignment. We could invent more sensitive instruments and continue measuring and adjusting to an atomic scale. The futility of this scenario will be clarified in Chapter 9, "Alignment Tolerances." For now, we need to better understand some metrology and geometry if we are to avoid chasing a crooked line.

^[1]"Noise level" refers, in electronic measurements, to the lowest detectable levels where background randomness is just as large as the quantity being measured.