A nice Unesco cover from Estonia, depicting a miniature sheet issued in 2011 on topic "Struve Geodetic Arc".
The Struve Arc is a chain of survey triangulations stretching from Hammerfest in Norway to the Black sea, through 10 countries (including Moldova, of course) and over 2,820 km. These are points of a survey, carried out between 1816 and 1855 by the astronomer Friedrich Georg Wilhelm Struve, which represented the first accurate measuring of a long segment of a meridian. This helped to establish the exact size and shape of the planet and marked an important step in the development of earth sciences and topographic mapping. It is an extraordinary exemple of scientific collaboration among scientists from different countries and of collaboration betweeen monarchs for a scientific cause.