The terrestrial reference frame and the dynamic Earth

As early as the 15th century Swedes noticed that rocks in their harbors were slowly rising out of the sea [Ekman, 1991]. These local observations were not sufficient to distinguish whether the rocks were rising or the sea level falling. Later, it was realized that Fennoscandia was still rebounding from the last Ice Age. This historical observation is still relevant today. How can you know whether a point on the Earth's surface is slowly moving up, down, or horizontally? One must relate local measurements to a stable and accurate reference frame, one whose scale is much larger than the problem at hand. We remain concerned with sea‐level variations, but present‐day studies recognize that change must be measured from a global point of view and with respect to a globally well‐defined reference frame. Thus, the regional and national geodetic datums developed over the past 200 years are inappropriate for studying global‐level problems.