title: r heatmap on ggmap from kriging data
body: <p>I want to plot a heatmap on a ggmap.</p>
  
  <pre><code>  library(ggmap)
    turku&lt;-get_map('turku', zoom=13)
    turkumap&lt;-ggmap(turku, extent="device", legend="topleft")
    turkumap
    turkumap+geom_density2d(mapping=aes(x = lon, y = lat),data = test, )
  </code></pre>
  
  <p>We have made a measurement campaign, so I have 4460 geo-referentiated points. </p>
  
  <p>To have a prediction map, I created a grid 400*400, for a total of 160000 points in which I calculate the prediction with kriging. 
  To have a full picture of the phenomenon in my 3 km *3 km i think some kind of heat map is necassary because plotting only the points, if I am not
  doing anything wrong plots a filled square on the map. (See <a href="http://www.mediafire.com/?6bd51x6xkp5e3h7" rel="nofollow noreferrer">file</a>)
  The code I use to plot the points is:</p>
  
  <pre><code>turkumap &lt;- turkumap + geom_point(data=temp, aes(x=lon, y=lat),size=var1.pred)
  </code></pre>
  
  <p>So basically the problem is that the points are along roads, so a heat map is problematic 
  (see <a href="https://stackoverflow.com/questions/18888048/heatmap-on-ggmap-error-and-best-practice">here</a>) and there are too many to plot them singularly.
  Any suggestions?</p>
  
  <p>The other related question is that my data are in the classical format of gstat, SpatialPointsDataFrame, this means that I have to coerce them to be dataframes to use
  ggplot, is there a better practice?</p>