google map from dismo::gmap() and ggplot2
<p>I obtained a map with the function <code>dismo::gmap()</code> and want to plot it with ggplot2 because I want to add different feautures using geom_point and other ggplot functions. I prefer to use <code>dismo::gmap</code> instead of <code>ggmap::get_map()</code> to download the google map layer. This is because <code>dismo::gmap()</code>, unlike <code>ggmap::get_map()</code>, returns a raster layer from package raster including complete CRS information and therefore is should be possible to modify the projection of the layer.</p>

<pre><code>&gt; head(data_info$latitude, 20)
#[1] 49.11306 49.39333 48.78083 51.85000 53.57361 50.67806 52.69083 52.21389 53.46361 50.99917 53.99750 53.54528 53.61417 48.00556 48.01306 53.45000
#[17] 51.93667 54.53083 51.95500 54.29639
&gt; head(data_info$longitude, 20)
#[1] 13.134722 12.323056 13.803889 12.177778 14.143611 13.175833 12.649444 13.454167 11.629722 10.906111 11.415556  8.426944  7.160000 11.123889 10.786111
#[16] 12.766667 11.987222 13.091389 10.967500 13.684167


   e = extent(-14 , 58 , 28 , 64)
mapImageData2 &lt;- gmap(e, type = c("terrain"), lonlat = TRUE, 
                      path = "&amp;style=feature:all|element:labels|visibility:off&amp;style=feature:administrative.country|element:geometry.stroke|visibility:off")

mapImageData2_proj &lt;- projectExtent(mapImageData2, crs = "+proj=utm +zone=31 +datum=WGS84")

# plot the points on the map 
ggplot(mapImageData2_proj, extent = "device") + 
  geom_point(inherit.aes = FALSE, aes(x = data_info$longitude, y = data_info$latitude),
             data = gps, colour = "red", size = 1, pch = 20)
</code></pre>

<p>After trying this, I get the following error:</p>

<blockquote>
  <p>Error: ggplot2 doesn't know how to deal with data of class
  RasterLayer</p>
</blockquote>

<p>If I try this</p>

<pre><code>plot(mapImageData2_proj)
</code></pre>

<blockquote>
  <p>Error in .plotraster2(x, col = col, maxpixels = maxpixels, add = add, 
  :    no values associated with this RasterLayer</p>
</blockquote>