r - Find closest points (lat / lon) from one data set to a second data set

Question

I have two data sets, A and B, which give locations of different points in the UK as such:

A = data.frame(reference = c(C, D, E), latitude = c(55.32043, 55.59062, 55.60859), longitude = c(-2.3954998, -2.0650243, -2.0650542))

B = data.frame(reference = c(C, D, E), latitude = c(55.15858, 55.60859, 55.59062), longitude = c(-2.4252843, -2.0650542, -2.0650243))

A has 400 rows and B has 1800 rows.
For all the rows in A, I would like to find the shortest distance in kilometers between a point in A and each of the three closest points in B, as well as the reference and coordinates in lat and long of these points in B.

I tried using this post

R - Finding closest neighboring point and number of neighbors within a given radius, coordinates lat-long

However, even when I follow all the instructions, mainly using the command distm from the package geosphere, the distance comes up in a unit that can't possibly be kilometers. I don't see what to change in the code, especially since I am not familiar at all with the geo packages.

Answer 1

I add below a solution using the spatialrisk package. The key functions in this package are written in C++ (Rcpp), and are therefore very fast.

The function spatialrisk::points_in_circle calculates the observations within radius from a center point. Note that distances are calculated using the Haversine formula. Since each element of the output is a data frame, purrr::map_dfr is used to row-bind them together:

purrr::map2_dfr(A$latitude, A$longitude, 
                  ~spatialrisk::points_in_circle(B, .y, .x, 
                                                 lon = longitude, 
                                                 lat = latitude, 
                                                 radius = 1e6)[1:3,], 
                .id = "id_A")

  id_A reference latitude longitude distance_m
1    1         C 55.15858 -2.425284  18115.958
2    1         E 55.59062 -2.065024  36603.447
3    1         D 55.60859 -2.065054  38260.562
4    2         E 55.59062 -2.065024      0.000
5    2         D 55.60859 -2.065054   2000.412
6    2         C 55.15858 -2.425284  53219.597
7    3         D 55.60859 -2.065054      0.000
8    3         E 55.59062 -2.065024   2000.412
9    3         C 55.15858 -2.425284  55031.092