ggmap雄蕊地图精确度故障

Question

ggmap似乎处理来自Google和stamen的地图图块不一致，至少在全球范围内。雄蕊是问题所在。以下代码说明了这一点。

require(ggplot2); require(ggmap)
m_google = get_map(location = c(-.2, 51.5), zoom=3)
m_stamen = get_map(location = c(-.2, 51.5), zoom=3, source = "stamen", maptype = "toner")

ggmap(m_google, extent = 'panel') + geom_point(data=NULL, aes(-.2, 51.5), col='red', size=5)
ggmap(m_stamen, extent = 'panel') + geom_point(data=NULL, aes(-.2, 51.5), col='red', size=5)

毛刺？