Why does not the coordinate of the graph match the dataframe?

Question

I drawing the plot of Île-de-France with

library(sf)
library(ggplot2)

df <- read.csv("https://raw.githubusercontent.com/leanhdung1994/Stat_Cons/main/station.csv")[1:3]
df2 <- st_as_sf(df, coords = c('V1','V2'), crs = 2154)

df3 <- ggplot() + 
  geom_sf(data = df2,
          size = 0.1,
          color = "lawngreen",
          fill = "lawngreen") +
  ggtitle("Stations")

plot(df3)

My dataframe is

Simple feature collection with 25338 features and 1 field
geometry type:  POINT
dimension:      XY
bbox:           xmin: 48.66986 ymin: 2.012258 xmax: 49.05865 ymax: 2.782492
projected CRS:  RGF93 / Lambert-93
First 10 features:
    X                  geometry
1   1 POINT (48.85153 2.637754)
2   2 POINT (48.83918 2.638771)
3   3 POINT (48.85163 2.621466)
4   4  POINT (48.9135 2.482264)
5   5 POINT (48.99439 2.523624)
6   6 POINT (48.88744 2.325725)
7   7 POINT (48.85602 2.624391)
8   8  POINT (48.8531 2.281198)
9   9 POINT (49.00259 2.535003)
10 10 POINT (48.84394 2.627361)

My plot is

Could you explain why the coordinate from the plot is somewhere 5.98°S 1.7°W? I expect it would be somewhat 48.85°N 2.63°E.

Answer 1

I think the problem lies in the following code line:

df2 <- st_as_sf(df, coords = c('V1','V2'), crs = 2154)

I notice that the crs is defined as 2154, which is a projected coordinate system in meters (Lambert-93 in RGF93). However, your points are actually geographic coordinates, and you need to tell that to the system. A reprojection can be done later in the process, but at this step, you need to use a geographic crs, such as EPSG:4171 for geographic RGF93, or EPSG:4326 for WGS84. Otherwise the system thinks your coordinates such as (48.85153 2.637754) are meters in Lambert 93, and it lands you in the Atlantic Ocean near Africa.

One more thing, check the axis order on the resulting plot. I can't verify at the moment but it is possible that by default your coords will be interpreted as ('longitude','latitude'), so you may have to swap your two fields here to ('V2','V1').