Question

这不是我的问题的关键，但这是我的情节示例，我想在其上添加比例尺。

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")

NC map

Answer 1

要实现这一目标，您需要做一些事情。

首先是将您的数据放入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"),
                        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")

现在我们开始计算出比例。首先，我们需要一个函数给出两点之间的距离，基于纬度/经度。为此，我们使用了Floris在Calculate distance in (x, y) between two GPS-Points描述的Haversine公式：

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
}

下一步是找出定义比例尺的点。在这个例子中，我使用我们已经想到的边界框在图的左下角放了一些东西：

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：

好处是所有绘图都使用ggplot2()，因此您可以使用http://ggplot2.org处的文档来了解您的需求。

Answer 2

我已经重新编写了@Andy Clifton的代码来添加更精确的距离测量，并允许比例尺具有所需的长度，而不是取决于杆

Andy的代码让我99％的代价，但他的代码中使用的Haversine公式未经其他来源的结果验证，尽管我自己无法找到错误。

第一部分是从Andy Clifton的回答中复制的，只是为了完整代码：

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"))
map.base <- get_map(location = c(lon = mean(sites.data$lon),
                                   lat = mean(sites.data$lat)),
                      zoom = 10)
bb <- attr(map.base,"bb")
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)))

接下来的两个步骤是不同的：

首先使用distVincentyEllipsoid包中的geosphere函数来计算比Haversine公式更精确的距离：

sbar$distance <- geosphere::distVincentyEllipsoid(c(sbar$lon.start,sbar$lat.start),
 c(sbar$lon.end,sbar$lat.end))

然后校正比例尺，使其成为标准长度 - 具体取决于地图的比例。在这个例子中，20km似乎是一个不错的合理选择，即20,000米：

scalebar.length <- 20
sbar$lon.end <- sbar$lon.start +
((sbar$lon.end-sbar$lon.start)/sbar$distance)*scalebar.length*1000

再次使用Andy的代码，我只将箭头添加到geom_segment，因为我认为它看起来更好

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),
               arrow=arrow(angle = 90, length = unit(0.1, "cm"),
                           ends = "both", type = "open")) +
  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(scalebar.length),
                              '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))  

# 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"))

有没有办法为ggmap添加比例尺（线性距离）？

2 个答案: