有没有办法给ggmap添加比例尺（用于线性距离）？ [英] Is there a way to add a scale bar (for linear distances) to ggmap?

问题描述

不是这对我的问题至关重要，但这里是我的情节示例，我想在其中添加一个比例尺。

  ggmap（get_map（location =Kinston，NC，zoom = 12，maptype ='hybrid'））+ 
 geom_point（x = -77.61198，y = 35.227792，color =red ，size = 5）+ 
 geom_point（x = -77.57306，y = 35.30288，color =blue，size = 3）+ 
 geom_point（x = -77.543，y = 35.196，color =蓝色，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）
  

解决方案

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你需要做一些事情来做到这一点。

首先将您的数据放入 data.frame（）：

  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），
 color = c（red，blue，blue））
  

现在我们可以使用 gg_map 包：

  require（gg_map）
 map.base<  -  get_map（location = c（lon = mean （sites.data $ lon），
 lat = mean（sites.data $ lat）），
 zoom = 10）＃也可以使用zoom =auto
  
 
 
 
我们需要该图片的范围： 
 
 
  bb < -  attr（map.base，bb）
  
现在我们开始计算出规模。首先，我们需要一个函数根据经纬度给我们两点之间的距离。为此，我们使用Floris在 
 
 
 
好的是所有的绘图使用 ggplot2（），因此您可以使用 http://ggplot2.org 让你看起来如何。  
Not that it's critical to my question, but here is my plot example, on top of which I'd like to add a scale bar.
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")
解决方案
There are a few things you need to do to make this happen. 

First is to put your data into a data.frame():
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"))
Now we can get the map for this region using the gg_map package:
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"
We'll need the extents of that image:
bb <- attr(map.base,"bb")
Now we start figuring out the scale. First, we need a function give us the distance between two points, based on lat/long. For that, we use the Haversine formula, described by Floris at Calculate distance in (x, y) between two GPS-Points: 
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
}
The next step is to work out the points that will define our scale bar. For this example, I put something in the lower left of the plot, using the bounding box that we've already figured out:
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))
Finally, we can draw the map with the scale.
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))  
Then we save it...
# 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"))
Which gives you something like this:



The nice thing is that all of the plotting uses ggplot2(), so you can use the documentation at http://ggplot2.org to make this look how you need. 

                        
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