title: Add bathymetry lines to ggplot using marmap package and getNOAA.bathy
body: <p>I am wanting to add bathymetry lines to a map plot I am looking at. I am plotting points off the coast and we are interested as to how close to the continental shelf they are. I have seen a package called Marmap - but now I am using ggplot as it gives a higher resolution. </p>
  
  <p>The code I've seen for getting bathymetry lines is this:</p>
  
  <pre><code>library(marmap)
  
  Peru.bath &lt;- getNOAA.bathy (lon1 = -90, lon2 = -70, lat1 = -20, 
                              lat2 = -2, resolution = 10) 
  
  plot(Peru.bath)
  </code></pre>
  
  <p>The code I'm using which I want to add the bathymetry lines to is below:</p>
  
  <pre><code>coast_map &lt;- fortify(map("worldHires", fill=TRUE, plot=FALSE))
  gg &lt;- ggplot()
  gg &lt;- gg + geom_map(data=coast_map, map=coast_map,
                      aes(x=long, y=lat, map_id=region),
                      fill="white", color="black") +
    theme(panel.background = element_blank()) + 
    theme(panel.grid.major = element_blank()) + 
    theme(panel.grid.minor = element_blank()) +
    theme(axis.text.x = element_blank(), axis.text.y = element_blank(), axis.ticks = element_blank())
  gg &lt;- gg + xlab("") + ylab("")
  gg &lt;- gg + geom_map(data=data.frame(region="Peru"), map=coast_map,
                      aes(map_id=region), fill="gray") 
  gg &lt;- gg + xlim(-90,-70) + ylim(-20,-2)
  gg &lt;- gg + coord_map()
  gg
  </code></pre>
  
  <p>Therefore I assumed it would be </p>
  
  <pre><code>gg &lt;- gg + Peru.bath
  </code></pre>
  
  <p>However I am getting 'Error: Don't know how to add Peru.bath to a plot'</p>
  
  <p>NB Just to make it clear, I do not have bathymetry data, I just wish to plot known shelf lines onto a map I have created, if that is possible.</p>