This is not a question about relativity per se, but relates to it sufficiently I hope. Standard values for many scientific constants are defined, and ideally in the most universal way possible. For example, the standard length unit, a meter, is defined with respect to the distance light travels in certain amount of time, that being defined by a certain number of oscillations of an atomic "clock". By relativity, the speed of light is constant in a vacuum, however distance and time are affected by relative movement and gravitational fields. So my question is, is the standard meter defined with respect to a particular location on earth, with the understanding that it will vary according to relative position of another location either moving or under different gravitational influences (like further away from the center of the earth, or on the moon)? Or are the necessary corrections beyond the requirements of most practical scientific or engineering applications?