The Mercator map or projection is one of the map projections treated in Social Studies of junior high and high school courses. The most important feature is that the projection is conformal meaning that it preserves angles. In this article, we first explain this conformal property elementarily but rigorously in order to propose that the topic is suitable for undergraduates as a STEM topic. Also, we give a parametrization of the plane curve obtained by projecting a great circle on the earth onto the Mercator map. Finally, we examine a rhumb line and calculate the distance along it.