title: Determining shortest distance between Isobaths
body: <p>I'm interested in finding the shortest distance between two 120m isobaths to determine the interaction of species during the Pleistocene.</p>
  
  <p>Thus far I have found methods to plot the isobaths with package <code>marmap</code>. But have not been able to find functions to determine the shortest distance between isobaths without having to manually measure them as there will be quite a substantial number of measurements involved which would not be efficient to do so manually.</p>
  
  <p>Would appreciate any advice on this, thanks!</p>