Add base map in the background to .shp layer plotted using geom_sf in R

Question

I am creating a map for sacramento city with a layer indicating the landuse and restricted to utility service area. I need to add a background map to make it more recognizable. Here is the map I am plotting

 ` Parcel_map<-
             ggplot()+
             geom_sf(data = Parcel, aes(fill = Land_Use), color = NA) +
             geom_sf(data = Water_Boundary, fill = NA, color = "blue") +
             scale_fill_viridis_c()+
             labs(title = "Heatmap of Start of Tracking Period (2025 for all others)", fill = "Year_dum") +
             labs(fill=" Meter Installation")+
             theme_minimal()`

I have tried multiple ways to add google map as a background. One the way is following:

 `myLocation <- c(-121.6, 38.4, -121.35, 38.7) 
           myMap <- get_map(location=myLocation, source="google", maptype="terrain", crop=FALSE) 
           ggmap(myMap) 
           `

However the final map is not getting produced using and throwing error.

>            GParcel_map_Strt_dt_dm <-
+              ggmap(myMap)+
+              geom_sf(data = Parcel_Strt_dt, aes(fill = Year_dum), color = NA) +
+              geom_sf(data = Water_Boundary, fill = NA, color = "blue") +
+              scale_fill_viridis_c()+
+              labs(title = "Heatmap of Start of Tracking Period (2025 for all others)", fill = "Year_dum") +
+              labs(fill=" Meter Installation") +
+              coord_sf(datum = NA,
+                       xlim = c(bbox['xmin'], bbox['xmax']),
+                       ylim = c(bbox['ymin'], bbox['ymax'])) +
+              theme_minimal()

Coordinate system already present. Adding new coordinate system, which will replace the existing one. > GParcel_map_Strt_dt_dm Error in geom_sf(): ! Problem while computing aesthetics. ℹ Error occurred in the 4th layer. Caused by error: ! object 'lon' not found Run rlang::last_trace() to see where the error occurred.

I simply need a background map for my current map which look like following.

Answer 1

I've encountered a similar problem before, and if I remember correctly the error can be resolved by adding inherit.aes = FALSE inside each call to geom_sf(). As the warning message indicates, there's already a coordinate system present from myMap. Google returns the basemap as a raster layer in the EPSG: 3857 or "Google PseudoMercator" projection, whereas your other sf objects are likely in a different CRS like EPGS: 4326 or WGS 84 (you can check with sf::st_crs(Parcel_Strt_dt), for instance).

I found this StackOverflow post helpful when encountering this error, you might as well: How to put a geom_sf produced map on top of a ggmap produced raster.

Answer 2

ggmap() is not spatial aware, so as of 2024 it is not the best way to add a base map to a ggplot2. Use as an alternative a combination of maptiles and tidyterra, see How to properly combine a polygon map on top of a raster map in r?.

library(tigris)
library(dplyr)

Parcel <- county_subdivisions(state = "006", county = "067") %>%
  # Mock data
  mutate(Land_Use = NAME)


Water_Boundary <- area_water(state = "006", county = "067")

# The tile (instead using ggmap)
library(maptiles)
library(tidyterra)
basemap <- get_tiles(Parcel, provider = "OpenStreetMap", crop = TRUE)

ggplot() +
  geom_spatraster_rgb(data = basemap) +
  geom_sf(data = Parcel, aes(fill = Land_Use), color = NA) +
  geom_sf(data = Water_Boundary, fill = NA, color = "blue") +
  scale_fill_viridis_d(alpha = 0.5) +
  labs(
    title = "Heatmap of Start of Tracking Period (2025 for all others)",
    fill = "Year_dum"
  )