R Spatial: Mismatch of latitude / longitude between 2 shapefiles with same CRS

Question

my goal is to count the frequency of extreme weather events within subnational African regions. To do so, I have set up a shapefile containing African provinces, using mostly GADM data and the new geocoded EMDAT GDIS dataset for point-data on weather events.

This is how the region shapefile looks like:

    library(sf)

    st_geometry(africa_map)
    Geometry set for 796 features 
    Geometry type: GEOMETRY
    Dimension:     XY
    Bounding box:  xmin: -25.36042 ymin: -46.96575 xmax: 63.49391 ymax: 37.3452
    Geodetic CRS:  WGS 84
    First 5 geometries:
    MULTIPOLYGON (((-4.821226 24.99475, -4.821355 2...
    MULTIPOLYGON (((1.853562 35.8605, 1.8424 35.865...
    MULTIPOLYGON (((-1.361976 35.3199, -1.358957 35...
    MULTIPOLYGON (((2.984874 36.81497, 3.014171 36....
    MULTIPOLYGON (((7.262677 37.076, 7.266449 37.07..

And the GDIS dataset after converting longitude and latitude to WGS 84:

    gdis_africa_sf <- st_as_sf(x = gdis_africa, 
                    coords = c("longitude", "latitude"),
                    crs = "+proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0")

    st_geometry(gdis_africa_sf)
    Geometry set for 5171 features 
    Geometry type: POINT
    Dimension:     XY
    Bounding box:  xmin: -34.04233 ymin: -25.19619 xmax: 37.08849 ymax: 63.4228
    CRS:           +proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0
    First 5 geometries:
    POINT (-17.09348 15.66576)
    POINT (-16.53153 15.77399)
    POINT (-16.20006 15.84419)
    POINT (-17.09348 15.66576)
    POINT (-16.53153 15.77399)

By now, you can already tell that something's off because the bounding boxes do not correspond at all, even though the projections seem to fit.

    st_crs(africa_map)==st_crs(gdis_africa_sf)
[1] TRUE

When plotting the two next to each other, the issue becomes clearer, no matter if I use the new shapefile or just apply longitude and latitude of the data frame.

    ggplot() +
      geom_sf(data = africa_map) +
      geom_sf(data = gdis_africa_sf)

Plot 1

    ggplot(data = africa_map) +
      geom_sf() +
      geom_point(data = gdis_africa, aes(x = longitude, y = latitude),
                 color = "red",
                 alpha = 0.3,
                 size = 2, 
                 shape = 1)

Plot 2

It seems like the weather event coordinates are shifted some thousand kilometers to the North West - but what's the source? And how can I fix the issue and make my two geographical data compatible? Any hints would be much appreciated.

Miff Miff · Accepted Answer · 2021-05-09T12:59:25

It seems like you've mis-diagnosed the problem, it's not a translation, but a reflection in a diagonal line - you've swapped latitude and longitude in your station points. Try:

 gdis_africa_sf <- st_as_sf(x = gdis_africa, 
                    coords = c("latitude", "longitude"),
                    crs = "+proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0")

R Spatial: Mismatch of latitude / longitude between 2 shapefiles with same CRS

1 Answers