有没有办法为ggmap添加比例尺(线性距离)？

ggmap(get_map(location="Kinston, NC", zoom=12, maptype='hybrid'))+
      geom_point(x=-77.61198,y=35.227792, colour="red", size=5)+
      geom_point(x=-77.57306, y=35.30288, colour="blue", size=3)+
      geom_point(x=-77.543, y=35.196, colour="blue",size=3)+
      geom_text(x=-77.575, y=35.297, label="CRONOS Data")+
      geom_text(x=-77.54, y=35.19, label="NOAA")+
      geom_text(x=-77.61, y=35.22, label="PP Site")

有一些事情你需要做,以实现这一点.

首先将数据放入data.frame()中：

sites.data = data.frame(lon = c(-77.61198, -77.57306, -77.543),
                        lat = c(35.227792, 35.30288, 35.196),
                        label = c("PP Site","NOAA", "CRONOS Data"),
                        colour = c("red","blue","blue"))

现在我们可以使用gg_map包获取该地区的地图：

require(gg_map)
map.base <- get_map(location = c(lon = mean(sites.data$lon),
                                 lat = mean(sites.data$lat)),
                    zoom = 10) # could also use zoom = "auto"

我们需要该图像的范围：

bb <- attr(map.base,"bb")

现在我们开始了解规模.首先,我们需要一个函数给我们两点之间的距离,基于lat / long.为此,我们使用Haversine公式,由Floris在 Calculate distance in (x, y) between two GPS-Points 描述：

distHaversine <- function(long, lat){

  long <- long*pi/180
  lat <- lat*pi/180  
  dlong = (long[2] - long[1])
  dlat  = (lat[2] - lat[1])

  # Haversine formula:
  R = 6371;
  a = sin(dlat/2)*sin(dlat/2) + cos(lat[1])*cos(lat[2])*sin(dlong/2)*sin(dlong/2)
  c = 2 * atan2( sqrt(a), sqrt(1-a) )
  d = R * c
  return(d) # in km
}

下一步是找出将定义我们的比例尺的点.对于这个例子,我把东西放在情节的左下角,使用我们已经想到的边框：

sbar <- data.frame(lon.start = c(bb$ll.lon + 0.1*(bb$ur.lon - bb$ll.lon)),
                   lon.end = c(bb$ll.lon + 0.25*(bb$ur.lon - bb$ll.lon)),
                   lat.start = c(bb$ll.lat + 0.1*(bb$ur.lat - bb$ll.lat)),
                   lat.end = c(bb$ll.lat + 0.1*(bb$ur.lat - bb$ll.lat)))

sbar$distance = distHaversine(long = c(sbar$lon.start,sbar$lon.end),
                              lat = c(sbar$lat.start,sbar$lat.end))

最后,我们可以用比例绘制地图.

ptspermm <- 2.83464567  # need this because geom_text uses mm, and themes use pts. Urgh.

map.scale <- ggmap(map.base,
                   extent = "normal", 
                   maprange = FALSE) %+% sites.data +
  geom_point(aes(x = lon,
                 y = lat,
                 colour = colour)) +
  geom_text(aes(x = lon,
                y = lat,
                label = label),
            hjust = 0,
            vjust = 0.5,
            size = 8/ptspermm) +    
  geom_segment(data = sbar,
               aes(x = lon.start,
                   xend = lon.end,
                   y = lat.start,
                   yend = lat.end)) +
  geom_text(data = sbar,
            aes(x = (lon.start + lon.end)/2,
           y = lat.start + 0.025*(bb$ur.lat - bb$ll.lat),
           label = paste(format(distance, 
                                digits = 4,
                                nsmall = 2),
                         'km')),
           hjust = 0.5,
           vjust = 0,
           size = 8/ptspermm)  +
  coord_map(projection="mercator",
            xlim=c(bb$ll.lon, bb$ur.lon),
            ylim=c(bb$ll.lat, bb$ur.lat))

然后我们保存它

# Fix presentation ----
map.out <- map.scale +  
  theme_bw(base_size = 8) +
  theme(legend.justification=c(1,1), 
        legend.position = c(1,1)) 

ggsave(filename ="map.png", 
       plot = map.out,
       dpi = 300,
       width = 4, 
       height = 3,
       units = c("in"))

这给你这样的东西：

好的是,所有的绘图使用ggplot2(),所以你可以使用 http://ggplot2.org 的文档,使你看起来如何需要.