A function for snapping points to a grid. The grid doesn't even have to be regular!

get.nearest.neighbour.R

library(raster)
library(tidyverse)

get.nearest.neighbour <- function(point_x,
                                  point_y,
                                  grid){
  dists <- pointDistance(
    matrix(c(point_x,point_y), nrow = 1),
    as.matrix(grid[, c('longitude','latitude')]),
    lonlat = F)
  
  idx <- which.min(dists)
  
  return(idx)
  
}

# make points
my.points <- data.frame(x=runif(n=100),
                        y=runif(n=100))
# make a grid
my.grid <- expand.grid(longitude=seq(0, 1, by=0.1),
                      latitude=seq(0, 1, by=0.1)) %>%
  mutate(idx=1:nrow(.))

# find nearest
my.points %<>%
  rowwise %>%
  mutate(idx = get.nearest.neighbour(x, y, my.grid)) %>%
  ungroup %>%
  inner_join(my.grid)

head(my.points)

# visualise
ggplot(data=my.grid, aes(x=longitude, 
                         y=latitude)) +
  theme_void() +
  geom_tile(fill=NA, color="grey90") +
  geom_text(aes(label=idx), color="grey90") +
  geom_text(data=my.points,
          aes(label=idx, x=x, y=y))  +
  coord_equal()