There is more than one possible
frame of reference. Classically, there is a coordinate system with a designated origin (a 'fixed point' in Space-Time).
The nature of the observer is a dilemma only because not everything is known about the observer. Somethings are known about the observer. Perhaps sufficient things are known about the observer to make measurements.
Units of measurement themselves are an interesting problem also. In modern times, our units of measurements tend to be defined in terms of their relation to what we believe are physical constants (such as the speed of light in vacuum).
Let's say you were living in the ancient world and you wanted to count out ten feet. You might use your feet because your feet are available for the task. Once you start to examine the nature of the observer, you start to realize that not everyone's foot is the same size. Now you make a standard foot for which an individual person's foot is an approximation.
As to whether or not measurements are clear, science acknowledges that no measurement is taken without error. Do you disagree?
