title: Combining geographic layers with different projections in R
body: <p>EDIT:  I have reworded the title question slightly, and adjusted the text to respond to the comment by @DWin.</p>
  
  <p>Combining geographic layers that are projected and not projected can be challenging.  Often, it seems, some transformation is necessary, as geographic layers come from different products and publishers. </p>
  
  <p>I am aware that R has several tools to perform geographic transformations.  For example:</p>
  
  <ol>
  <li>For objects of class <code>Spatial*</code> in the <code>sp</code> package, the <code>spTransform()</code> function in the <code>rgdal</code> package can be used; and,</li>
  <li>For objects of class <code>Raster*</code> in the <code>raster</code> package, the <code>projectRaster()</code> function can be used.</li>
  </ol>
  
  <p>Here is a specific task that I would like to accomplish in R:  Transform to UTM grid Zone 15N (Datum: NAD83) a polygons layer describing lakes in a UTM grid Zone 15N (Datum: NAD27) projection (this is in an ESRI shapefile format).</p>