The representation of a portion of the earth, which is approximately spherical, onto a flat 2D plane requires a projection. This is what coord_map does. These projections account for the fact that the actual length (in km) of one degree of longitude varies between the equator and the pole. Near the equator, the ratio between the lengths of one degree of latitude and one degree of longitude is approximately 1. Near the pole, it is tends towards infinity because the length of one degree of longitude tends towards 0. For regions that span only a few degrees and are not too close to the poles, setting the aspect ratio of the plot to the appropriate lat/lon ratio approximates the usual mercator projection. This is what coord_quickmap does. With coord_map all elements of the graphic have to be projected which is not the case here. So coord_quickmap has the advantage of being much faster, in particular for complex plots such as those using with geom_tile, at the expense of correctness in the projection. This coordinate system provides the full range of map projections available in the mapproj package. Source.