What is the “Simplest” way to add a scale to a map

问题:

For real, I searched everywhere, and coding a scale in a map is so hard... Adding scale bar to ggplot map

Is there a way to add a scale bar (for linear distances) to ggmap?

Could it be possible to make a simple line that scale differently to the zoom preset that we select in the function?

I have this simple map:

library(ggmap)
pngMAP_df2 = get_map(location = c(-90.5, -0.5), source = "google", zoom = 8,color = "bw")
s = ggmap(pngMAP_df2)
s

I wanted to add as well GPS coordinate in this graph:

myGPS = data.frame(lat=c( -0.6850556,-0.6854722,  -0.6857778  ),lon=c(-90.22275,-90.22261,  -90.22272)) 

Is it easy to implement?

I just want to add something realllllllllly simple. Like a line with always a round number that give an indication of the zoom in the map.

Also, is it possible with this code to make the map look even simpler. Like seeing the water in white and the contour of the land in black?

Thanks,

回答1:

Something like:

library(rgdal)
library(rgeos)
library(ggplot2)
library(ggthemes)
library(ggsn)

URL <- "https://osm2.cartodb.com/api/v2/sql?filename=public.galapagos_islands&q=select+*+from+public.galapagos_islands&format=geojson&bounds=&api_key="
fil <- "gal.json"
if (!file.exists(fil)) download.file(URL, fil)

gal <- readOGR(fil, "OGRGeoJSON")

# sample some points BEFORE we convert gal
rand_pts <- SpatialPointsDataFrame(spsample(gal, 100, type="random"), data=data.frame(id=1:100))

gal <- gSimplify(gUnaryUnion(spTransform(gal, CRS("+init=epsg:31983")), id=NULL), tol=0.001)

gal_map <- fortify(gal)

# now convert our points to the new CRS
rand_pts <- spTransform(rand_pts, CRS("+init=epsg:31983"))

# and make it something ggplot can use
rand_pts_df <- as.data.frame(rand_pts)

gg <- ggplot()
gg <- gg + geom_map(map=gal_map, data=gal_map,
                    aes(x=long, y=lat, map_id=id),
                    color="black", fill="#7f7f7f", size=0.25)
gg <- gg + geom_point(data=rand_pts_df, aes(x=x, y=y), color="steelblue")
gg <- gg + coord_equal()
gg <- gg + scalebar(gal_map, dist=100, location="topright", st.size=2)
gg <- gg + theme_map()
gg

回答2:

This would be the complete answer with specific points on the map.

library(rgdal)
library(rgeos)
library(ggplot2)
library(ggthemes)
library(ggsn)
myGPS = data.frame(lat=c( -0.6850556,-0.6854722,  -0.6857778  ),lon=c(-90.22275,-90.22261,  -90.22272)) 
coord.deg = myGPS

class(coord.deg)
## "data.frame"
coordinates(coord.deg)<-~lon+lat
class(coord.deg)
## "SpatialPointsDataFrame"
## attr(,"package")
## "sp"

# does it have a projection/coordinate system assigned?
proj4string(coord.deg) # nope
## NA

# Manually tell R what the coordinate system is
proj4string(coord.deg)<-CRS("+proj=longlat +ellps=WGS84 +datum=WGS84")

# now we can use the spTransform function to project. We will project
# the mapdata and for coordinate reference system (CRS) we will
# assign the projection from counties

coord.deg<-spTransform(coord.deg, CRS(proj4string(gal)))
# double check that they match
identical(proj4string(coord.deg),proj4string(gal))
## [1] TRUE
my_pts <- SpatialPointsDataFrame(coords = coord.deg, data=data.frame(id=1:length(coord.deg)))

URL <- "https://osm2.cartodb.com/api/v2/sql?filename=public.galapagos_islands&q=select+*+from+public.galapagos_islands&format=geojson&bounds=&api_key="
fil <- "gal.json"
if (!file.exists(fil)) download.file(URL, fil)
gal <- readOGR(fil, "OGRGeoJSON")
gal <- gSimplify(gUnaryUnion(spTransform(gal, CRS("+init=epsg:31983")), id=NULL), tol=0.001)
gal_map <- fortify(gal)
rand_pts <- spTransform(my_pts, CRS("+init=epsg:31983"))

# ggplot can't deal with a SpatialPointsDataFrame so we can convert back to a data.frame
my_pts <- data.frame(my_pts)
my_pts.final = my_pts[,2:3]
# we're not dealing with lat/long but with x/y
# this is not necessary but for clarity change variable names
names(my_pts.final)[names(my_pts.final)=="lat"]<-"y"
names(my_pts.final)[names(my_pts.final)=="lon"]<-"x"

gg <- ggplot()
gg <- gg + geom_map(map=gal_map, data=gal_map,
                    aes(x=long, y=lat, map_id=id),
                    color="black", fill="#FFFFFF", size=.5)
gg <- gg + coord_equal()
gg <- gg + ggsn:::scalebar(gal_map, dist=50, location="bottomleft", st.size=5)
gg <- gg + theme_map()
gg <- gg + geom_point(data=my_pts.final, aes(x=x, y=y), color="red")
gg