[SOLVED] Convert coordinates from British National Grid (Easting, Northing) to WGS84 Lat Lon

Convert coordinates from British National Grid (Easting, Northing) to WGS84 Lat Lon

I am struggling to convert with R coordinates from British National Grid (BNG) to WGS84 Lat Lon.

Here an example of data:

df = read.table(text = 'Easting Northing 
 320875 116975     
 320975 116975     
 320975 116925     
 321175 116925    
 321175 116875     
 321275 116875', header = TRUE)

How can I convert the Easting and Northing to WGS84 Lat Lon?

There is a function called spTransform from rgdal package but the documentation is very confusing.

Any suggestion?

Solution

Here is a way to do it with the sf package in R. We take the table and convert to point geometries, specifying that these values are in the BNG coordinate reference system. Then we transform to WGS84, extract the coordinates as a matrix, and return a data frame.

I believe from a quick google that the British National Grid has EPSG code 27700, though if this is not the right projection then you can modify the crs = argument in st_as_sf. The points as given appear to be in some fields in the Blackdown Hills AONB south of Taunton; I would check the georeferencing yourself.

df = read.table(text = 'Easting Northing 
 320875 116975     
                320975 116975     
                320975 116925     
                321175 116925    
                321175 116875     
                321275 116875', header = TRUE)

library(tidyverse)
library(sf)
#> Linking to GEOS 3.6.1, GDAL 2.2.3, proj.4 4.9.3
df %>%
  st_as_sf(coords = c("Easting", "Northing"), crs = 27700) %>%
  st_transform(4326) %>%
  st_coordinates() %>%
  as_tibble()
#> # A tibble: 6 x 2
#>       X     Y
#>   <dbl> <dbl>
#> 1 -3.13  50.9
#> 2 -3.13  50.9
#> 3 -3.13  50.9
#> 4 -3.12  50.9
#> 5 -3.12  50.9
#> 6 -3.12  50.9

Created on 2018-05-11 by the reprex package (v0.2.0).