The convergence angle (CA) is the actual angle on a chart formed by the intersection of the Greenwich meridian and another meridian; the pole serves as the vertex of the angle. CAs, like longitudes, are measured east and west from the Greenwich meridian. Many major online street mapping services (Bing Maps, Google Maps, MapQuest, OpenStreetMap, Yahoo!Maps, and others) use a variant of the Mercator projection for their map images called Web Mercator or Google Web Mercator. Despite its obvious scale variation at small scales, the projection is well-suited as an interactive world map that can be zoomed seamlessly to large-scale (local) maps Define Mercator chart. Mercator chart synonyms, Mercator chart pronunciation, Mercator chart translation, English dictionary definition of Mercator chart. Mercator projection n. A cylindrical map projection in which the meridians and parallels appear as lines crossing at right angles and in which areas appear