有没有办法为ggmap添加比例尺(线性距离)？
并不是说这对我的问题至关重要,但这是我的情节示例,我想在其上添加一个比例尺.
ggmap(get_map(location = "Kinston, NC", zoom = 12, maptype = 'hybrid')) +
geom_point(x = -77.61198, y = 35.227792, 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")
And*_*ton 14
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包来获取该区域的地图:
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规模.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,我们使用由Floris描述的Haversine公式计算两个GPS点之间(x,y)的距离:
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
}
下一步是确定定义我们比例尺的点.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$ur.lat))
然后我们保存它......
# Fix presentation ----
map.out <- map.scale +
theme_bw(base_size = 8) +
theme(legend.justification=c(1,1),
legend.position = c(1,1))
ggsave(filename ="map.png",
plot = map.out,
dpi = 300,
width = 4,
height = 3,
units = c("in"))
这给你这样的东西:
好处是所有的绘图都使用ggplot2(),因此您可以使用http://ggplot2.org上的文档来查看您的需求.
我已经重新设计了@Andy Clifton的代码,以增加更精确的距离测量,并允许比例尺具有所需的长度,而不是取决于杆的位置.
Andy的代码让我获得了99%的代码,但是他的代码中使用的Haversine公式没有通过其他来源的结果验证,尽管我自己找不到错误.
第一部分是从Andy Clifton的答案复制而来,只是为了完整的代码:
sites.data = data.frame(lon = c(-77.61198, -77.57306, -77.543),
lat = c(35.227792, 35.30288, 35.196),
label = c("PP Site","NOAA", "CRONOS Data"),
colour = c("red","blue","blue"))
map.base <- get_map(location = c(lon = mean(sites.data$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$ll.lat)))
接下来的两个步骤是不同的:
首先使用包中的distVincentyEllipsoid函数geosphere计算距离比Haversine公式更精确的距离:
sbar$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$distance)*scalebar.length*1000
再次使用Andy的代码,我只添加了箭头,geom_segment因为我认为它看起来更好
ptspermm <- 2.83464567 # need this because geom_text uses mm, and themes use pts. Urgh.
map.scale <- ggmap(map.base,
extent = "normal",
maprange = FALSE) %+% sites.data +
geom_point(aes(x = lon,
y = lat,
colour = colour)) +
geom_text(aes(x = lon,
y = lat,
label = label),
hjust = 0,
vjust = 0.5,
size = 8/ptspermm) +
geom_segment(data = sbar,
aes(x = lon.start,
xend = lon.end,
y = lat.start,
yend = lat.end),
arrow=arrow(angle = 90, length = unit(0.1, "cm"),
ends = "both", type = "open")) +
geom_text(data = sbar,
aes(x = (lon.start + lon.end)/2,
y = lat.start + 0.025*(bb$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"))
