The mathematical structure of general relativity makes its equations quite remote from a direct understanding of their content. Indeed, the combination of a covariant four-dimensional description of the physical laws and the need to cope with the relativity of the observations makes a physical measurement an elaborate procedure. The latter consists of a few basic steps:
(i) Identify the covariant equations which describe the phenomenon under investigation.
(ii) Identify the observer who makes the measurements.
(iii) Choose a frame adapted to that observer, allowing the space-time to be split into the observer's space and time.
(iv) Decide whether the intended measurement is local or non-local with respect to the background curvature.
(v) Identify the frame components of those quantities that are the observational targets.
(vi) Find a physical interpretation of the above components, following a suitable criterion such as a comparison with what is known from special relativity or from non-relativistic theories.
(vii) Verify the degree of residual ambiguity in the interpretation of the measurements and decide on a strategy to eliminate it.
Clearly, each step of the above procedure relies on the previous one, and the very first step provides the seed of a measurement despite the mathematical complexity.