title: Writing an R package: needing a package I don't explicitly call
body: <p>I'm working on an R package that uses the <code>spTransform</code> function in the <code>sp</code> package. The rub is that this function need <code>rgdal</code> loaded to work, or I get an error message:</p>
  
  <pre><code>Error in eval(expr, envir, enclos) : load package rgdal for spTransform methods
  </code></pre>
  
  <p>My <code>Imports</code> statement in the <code>DESCRIPTION</code> file includes the following:</p>
  
  <pre><code>Imports: sp,
      rgdal
  </code></pre>
  
  <p>But I'm still getting the error. <em>However</em>, if I explcitly load rgdal (using <code>library(rgdal)</code>) before using the package, everything works fine. I'm guessing that when my package is loaded rgdal is not attached because none of my code uses it explcitly via <code>::</code> etc. </p>
  
  <p>So I think my question is: How can I make my package attach a package that I am not explicitly using?</p>