CODEWITHSUNDEEP
rggplot2r-sf
hierb
hierb

Reputation: 59

geom_point does not always show up on map

I try to add points with custom coordinates to a map. In general, this seems to work with the NC map provided by the sf package. However, it does not work in a shapefile I downloaded from https://gdz.bkg.bund.de/index.php/default/open-data/verwaltungsgebiete-1-2-500-000-stand-31-12-vg2500-12-31.html - as long as I plot this new map only. If I plot both shapefiles at the same time, the points on the second shapefile suddenly appear. How would I add the custom points to the Germany-map only?

library("ggplot")    
library("sf")

nc <- st_read(system.file("shape/nc.shp", package="sf"))
ger_shape <- st_read("data/shapefiles/VG2500_LAN.shp")
ger_shape <- ger_shape[which(ger_shape$GF == 4),]

city <- data.frame(name = "1", lat = 35.7721, lng = -78.63861)
city2 <- data.frame(name = c("2","3"), lat = c(53,50), lng = c(10.2,10.2))

ggplot() +
  geom_sf(data = nc) + 
  coord_sf(lims_method = "geometry_bbox") +
  geom_point(data = city, aes(x = lng, y = lat), color = 'red')

NC map with red point

ggplot() +
  geom_sf(data = ger_shape) + 
  coord_sf(lims_method = "geometry_bbox") +
  geom_point(data = city2, aes(x = lng, y = lat), color = 'red')

map without points

ggplot() +
  geom_sf(data = nc) + 
  geom_sf(data = ger_shape) + 
  coord_sf(lims_method = "geometry_bbox") +
  geom_point(data = city, aes(x = lng, y = lat), color = 'red') +
  geom_point(data = city2, aes(x = lng, y = lat), color = 'red')

both maps with both sets of points

Upvotes: 2

Views: 535

Answers (2)

stefan
stefan

Reputation: 123783

The issue is that the coordinate reference system (crs) used by the shapefiles for Germany provided by the BKG is Gauss-Kruger zone 3 (check st_crs(ger_shape)) which does not specify locations in degrees whereas you specified the city locations in degrees. As a result the coordinates for the cities are far away from Germany which you could see be dropping the coord_sf line:

library("ggplot2")    
library("sf")

nc <- st_read(system.file("shape/nc.shp", package="sf"))
ger_shape <- st_read("shp/VG250_LAN.shp")
ger_shape <- ger_shape[which(ger_shape$GF == 4),]

city2 <- data.frame(name = c("2","3"), lat = c(53,50), lng = c(10.2,10.2))

ggplot() +
  geom_sf(data = ger_shape) + 
  geom_point(data = city2, aes(x = lng, y = lat), color = 'red')

enter image description here

Hence, to add your points to the map of Germany you have to specify the coordinates in the crs of the shape file or transform the crs to e.g. WGS84 using st_transform which specifies locations in degrees:

ger_shape <- sf::st_transform(ger_shape, "WGS84")

ggplot() +
  geom_sf(data = ger_shape) + 
  coord_sf(lims_method = "geometry_bbox", crs = st_crs(ger_shape)) +
  geom_point(data = city2, aes(x = lng, y = lat), color = 'red')

enter image description here

Upvotes: 4

AndS.
AndS.

Reputation: 8110

Your problem is probably just an issue with your coordinate system (projected instead of geographic). I am just guessing since I don't want to download the shapefile from the link provided. Here is an example:

library(tidyverse)    
library(sf)


nc <- st_read(system.file("shape/nc.shp", package="sf"))
city <- data.frame(name = "1", lat = 35.7721, lng = -78.63861)

#projected, no point
ggplot() +
  geom_sf(data = st_transform(nc, crs = st_crs("EPSG:32616"))) + 
  coord_sf(lims_method = "geometry_bbox") +
  geom_point(data = city, aes(x = lng, y = lat), color = 'red') 


#geographic, point
ggplot() +
  geom_sf(data = nc) + 
  coord_sf(lims_method = "geometry_bbox") +
  geom_point(data = city, aes(x = lng, y = lat), color = 'red')

Your problem will likely be fixed by setting the correct projection to use lat long coordinates:

ggplot() +
  geom_sf(data = st_transform(ger_shape, crs = st_crs("NAD27"))) + 
  coord_sf(lims_method = "geometry_bbox") +
  geom_point(data = city2, aes(x = lng, y = lat), color = 'red')

Upvotes: 2

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