有沒有辦法為ggmap新增比例尺(線性距離)？

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,以實現這一點.

首先將資料放入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"))

現在我們可以使用gg_map包獲取該地區的地圖：

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"

我們需要該影象的範圍：

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

現在我們開始瞭解規模.首先,我們需要一個函式給我們兩點之間的距離,基於lat / long.為此,我們使用Haversine公式,由Floris在 ofollow,noindex" target="_blank">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
}

下一步是找出將定義我們的比例尺的點.對於這個例子,我把東西放在情節的左下角,使用我們已經想到的邊框：

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))

最後,我們可以用比例繪製地圖.

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))

然後我們儲存它

# 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"))

這給你這樣的東西：

好的是,所有的繪圖使用ggplot2(),所以你可以使用 http://ggplot2.org 的文件,使你看起來如何需要.