Maybe you can manage it easily with
sp::point.in.polygon() function. The idea is to test if the point(s) are in the polygon made by your coordinates.
library(sp) # first you've to get one column for the long, and one column for the lat new_df <- data.frame(long = c(long_1,long_2), lat = c(lat_1, lat_2)) # then you can put the points to test in a vector, like these, and use the function point.in.polygon(c(-85.927,-84.2),c(18.7,18.5),new_df$long,new_df$lat)  1 0
One is in, one is out.
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