Plotting Heatmap with geom_raster in ggmap

I have a data frame containing longitude, latitude and a intensity variable (var1.pred).

I would like to plot a smooth filled contour plot on a ggmap map. I have done it using a geom_tile plot but it doesn't look smooth. I have decreased the tile sizes but the plot takes up to much storage. What I want to do is use geom_raster to plot a nice clean contour plot over the map object. Using the bellow code I have done it using ggplot:

raster <- ggplot(idw.output, aes(x = lon, y = lat, z = var1.pred))+
geom_raster(aes(fill = var1.pred), alpha = 0.5) +
scale_fill_gradient(low = "white", high = "blue")+
geom_contour(colour = "white", binwidth = 1) +
labs(fill = "Frequency", title = "Frequency per area", x = 'Longitude', y = 'Latitude') 
raster  

This returns the plot:

However I cannot work out how to combine this with a ggmap object. I have tried various things such as:

ggmap(fr) + #fr is a map from get_map
ggplot(idw.output, aes(x = lon, y = lat, z = var1.pred))+
geom_raster(aes(fill = var1.pred), alpha = 0.5) +
scale_fill_gradient(low = "white", high = "blue")+
geom_contour(colour = "white", binwidth = 1) +
labs(fill = "Frequency", title = "Frequency per area", x = 'Longitude', y = 'Latitude')

But I get the error:

Error: Don't know how to add o to a plot

I know the issue has something to do with combining a ggplot and ggmap object but I cannot work out how to get it to work. Any help would be appreciated.

Thanks,

Robin

Without a reproducible example, it is impossible to test your code, but it is likely that you can move the ggplot call to the base_layer argument so that you can keep adding geoms.

ggmap(fr, base_layer = ggplot(idw.output, aes(x = lon, y = lat, z = var1.pred))) + 
    geom_raster(aes(fill = var1.pred), alpha = 0.5) +
    scale_fill_gradient(low = "white", high = "blue")+
    geom_contour(colour = "white", binwidth = 1) +
    labs(fill = "Frequency", title = "Frequency per area", x = 'Longitude', y = 'Latitude')