Plotting a matrix on ggmap with geom_tile not working-CodePudding

I'm trying to plot a matrix (mostly random numbers with a few NAs) with longitude/latitude coordinates on a ggmap plot.

This is my code:

library(ggmap)
library(ggplot2)

define the longitude and latitude coordinates

x1=34.2
x2=42.4
y1=37.4
y2=29.4
lon = seq(x1,x2,by=0.1)
lat = seq(y1,y2,by=-0.1)

define a matrix with random numbers with the longitude/latitude dimensions

set.seed(1)
numbers = rnorm(length(lon)*length(lat))
var = matrix(numbers,length(lon),length(lat))

add some NAs to the matrix

var[1:5,1:5] = NA

lat_min <- min(lat)-0.3
lon_min <- min(lon)-0.3
lat_max <- max(lat) 0.3
lon_max <- max(lon) 0.3

construct the ggmap

map_box <- c(left=lon_min,bottom=lat_min,
            right=lon_max,top=lat_max)
total_stmap <- get_stamenmap(bbox=map_box,zoom=5,maptype="toner")
total_ggmap <- ggmap(total_stmap,extent="panel")

make a data.frame to attribute each matrix index to the geographical coordinate

lat_df <- c()
lon_df <- c()
var_df <- c()
for (i in 1:length(lon)) {
  for (j in 1:length(lat)) {
    lon_df <- c(lon_df,lon[i])
    lat_df <- c(lat_df,lat[j])
    var_df = var[i,j]
  }
}
df=data.frame(Longitude=lon_df,Latitude=lat_df,Variable=var_df)

make the plot using ggmap and geom_tile

plot = total_ggmap  
geom_tile(data=df,aes(x=Longitude,y=Latitude,fill=Variable),alpha=1/2,color="black",size=0)  
geom_sf(data = df, inherit.aes = FALSE, fill = NA)

With this code I get the message:

Coordinate system already present. Adding new coordinate system, which will replace the existing one.

...and a blank plot.

CodePudding user response：

There were two problems here. The first is that all of your values in the Variable column are the same, because you are just overwriting var_df with every iteration of your loop. The line

var_df = var[i,j]

Should be

var_df = c(var_df, var[i,j])

Secondly, you should not be using geom_sf if you have a data frame of longitude, latitude and value. geom_sf is used to plot sf objects, which is not what you have.

Instead, you need only do:

plot <- total_ggmap  
   geom_tile(data = df, aes(Longitude, Latitude, fill = Variable), alpha = 1/2, size = 0)

and you get:

plot