title: How to put a geom_sf produced map on top of a ggmap produced raster
body: <p>I tried the following code:</p>
  
  <pre><code>library(ggplot2)
  library(ggmap)
  library(sf)
  nc &lt;- st_read(system.file("shape/nc.shp", package = "sf"))
  str(nc)
  
   Classes ‘sf’ and 'data.frame':  100 obs. of  15 variables:
   $ AREA     : num  0.114 0.061 0.143 0.07 0.153 0.097 0.062 0.091 0.118 0.124 ...
   $ PERIMETER: num  1.44 1.23 1.63 2.97 2.21 ...
   $ CNTY_    : num  1825 1827 1828 1831 1832 ...
   $ CNTY_ID  : num  1825 1827 1828 1831 1832 ...
   $ NAME     : Factor w/ 100 levels "Alamance","Alexander",..: 5 3 86 27 66 46 15 37 93 85 ...
   $ FIPS     : Factor w/ 100 levels "37001","37003",..: 5 3 86 27 66 46 15 37 93 85 ...
   $ FIPSNO   : num  37009 37005 37171 37053 37131 ...
   $ CRESS_ID : int  5 3 86 27 66 46 15 37 93 85 ...
   $ BIR74    : num  1091 487 3188 508 1421 ...
   $ SID74    : num  1 0 5 1 9 7 0 0 4 1 ...
   $ NWBIR74  : num  10 10 208 123 1066 ...
   $ BIR79    : num  1364 542 3616 830 1606 ...
   $ SID79    : num  0 3 6 2 3 5 2 2 2 5 ...
   $ NWBIR79  : num  19 12 260 145 1197 ...
   $ geometry :sfc_MULTIPOLYGON of length 100; first list element: List of 1
   ..$ :List of 1
   .. ..$ : num [1:27, 1:2] -81.5 -81.5 -81.6 -81.6 -81.7 ...
     ..- attr(*, "class")= chr  "XY" "MULTIPOLYGON" "sfg"
     - attr(*, "sf_column")= chr "geometry"
     - attr(*, "agr")= Factor w/ 3 levels "constant","aggregate",..: NA NA NA NA NA NA NA NA NA NA ...
     ..- attr(*, "names")= chr  "AREA" "PERIMETER" "CNTY_" "CNTY_ID" ...
  
  map &lt;- get_map("north carolina", maptype = "satellite", zoom = 6, source = "google")
  a &lt;- unlist(attr(map,"bb")[1, ])
  bb &lt;- st_bbox(nc)
  ggplot() + 
    annotation_raster(map, xmin = a[2], xmax = a[4], ymin = a[1], ymax = a[3]) + 
    xlim(c(bb[1], bb[3])) + ylim(c(bb[2], bb[4])) + 
    geom_sf(data = nc, aes(fill = AREA))
  </code></pre>
  
  <p>The two layers do not overlap properly; I tried changing projection with <code>coord_sf()</code> but I did not have success.</p>
  
  <p>any suggestion?
  thanks</p>