title: ggmap::get_map doesn't allow the exact specification of my map's corners?
body: <p>I am using R package ggmap.</p>
  
  <p><code>?get_map</code> says:</p>
  
  <blockquote>
    <p>location: an address, longitude/latitude pair (in that order), or
    left/bottom/right/top bounding box</p>
  </blockquote>
  
  <p>My code:</p>
  
  <pre><code>library(ggmap)
  library(mapproj)
  
  lat_bottom = 52.33  # bottom latitude of Berlin
  lat_top    = 52.5   # top latitude of Berlin
  lon_left   = 13.0   # left longitude of Berlin
  lon_rigth  = 13.95  # right longitude of Berlin
  
  mymap &lt;- get_map(location = c(lon_left,lat_bottom,lon_rigth,lat_top),
  source="google")
  ggmap(mymap)
  </code></pre>
  
  <p>Why is it giving me a warning:</p>
  
  <blockquote>
    <p>Warning: bounding box given to google - spatial extent only
    approximate. converting bounding box to center/zoom specification.
    (experimental)</p>
  </blockquote>
  
  <p>Does it mean that there is no way for me to create a map with these
  exact corners?</p>
  
  <p>Based on the advice below I tried this:</p>
  
  <pre><code>lat_bottom = 52.33  # bottom latitude of Berlin
  lat_top    = 52.68   # top latitude of Berlin
  lon_left   = 13.08   # left longitude of Berlin
  lon_rigth  = 13.77  # right longitude of Berlin
  
  mylon = c(lon_left,lon_rigth)
  mylat = c(lat_bottom,lat_top)
  
  mymap &lt;- get_map(location = c(lon = mean(mylon), lat = mean(mylat)),
                 maptype = "roadmap", source = "google", zoom=11) # using zoom
  ggmap(mymap)
  foo&lt;-ggmap(mymap)+
    scale_x_continuous(limits = c(lon_left,lon_right), expand = c(0, 0)) +
    scale_y_continuous(limits = c(lat_bottom,lat_top), expand = c(0, 0))
  foo
  </code></pre>
  
  <p>It looks OK. But when I take other coordinates (those closer to each other), for example, like those below - then the map looks weird - it kinda shifts to the left on the gray background...</p>
  
  <pre><code>lat_bottom = 52.35  # new bottom
  lat_top    = 52.50  # new top
  lon_left   = 13.2   # new left
  lon_rigth  = 13.5   # new right
  </code></pre>