title: Merging two data frames, both with coordinates based on the closest location
body: <p>I have one large dataframe (~130000 rows) containing local variables and an other large dataframe (~7000 rows) containing the density of a species. Both have x and y coordinates but these coordinates don't always match. e.g:</p>
  
  <pre><code>df1 &lt;- data.frame(X = c(2,4,1,2,5), Y = c(6,7,8,9,8), V1 = c("A", "B", "C", "D", "E"), V2 = c("G", "H", "I", "J", "K"))
  </code></pre>
  
  <p>And:</p>
  
  <pre><code>df2 &lt;- data.frame(X = c(2,4,6), Y = c(5,9,7), Dens = c(12, 17, 10))
  </code></pre>
  
  <p>I would like to add a column to df1 containing the density (Dens) from df2 if there is a point reasonably close-by. If there is no point close-by I would like it to show up as a NA. e.g:</p>
  
  <pre><code>X Y   V1   V2    Dens
  2 6   A    G      12
  4 7   B    H      NA     
  1 8   C    I      17
  2 9   D    J      NA
  5 8   E    K      10
  </code></pre>