6.07 Limited precision due the absence of unique reference points.

When we take a measurement such as length we imagine that we have in two clearly defined reference points to measure between. In practice this may not always be the case. Imagine being asked to measure the depth of a pond where the ground beneath the pond is uneven and ripples are present on the surface of the water. In this case the magnitude of the uncertainty expressed in the measurement must be large enough to cover these variations.