title: Building a proj4 string for readShapePoly in R
body: <p>I have read in a shapefile of lodgepole pine seed planning units in R with <code>readShapePoly</code>, like so</p>
  
  <pre><code>spu &lt;- readShapePoly("spus.shp")
  </code></pre>
  
  <p>When I map the spus alone, they show up.</p>
  
  <pre><code>plot(spu, border=TRUE)
  </code></pre>
  
  <p>When I try to add the shapefile to a map of Canada, where I know the spus should go, they do not show up. </p>
  
  <pre><code>map("worldHires","Canada", xlim=c(-141,-110), ylim=c(45,65), col="gray90", fill=TRUE)
  plot(spu, add=TRUE, border=TRUE)
  </code></pre>
  
  <p>I think this is because the shapefile is missing the projection information contained in the .prj file:</p>
  
  <pre><code>PROJCS["NAD_1983_Albers",GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137.0,298.257222101]],
  PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Albers"],PARAMETER["False_Easting",1000000.0],
  PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",-126.0],PARAMETER["Standard_Parallel_1",50.0],
  PARAMETER["Standard_Parallel_2",58.5],PARAMETER["Latitude_Of_Origin",45.0],UNIT["Meter",1.0]]
  </code></pre>
  
  <p>The partial output of </p>
  
  <pre><code>summary(spu)
  
  Object of class SpatialPolygonsDataFrame
  Coordinates:
       min     max
  x 642162.8 1870556
  y 457057.2 1421478
  Is projected: NA 
  proj4string : [NA]
  Data attributes:
  </code></pre>
  
  <p>shows that no projection information is associated with the spu object in R. I think I can fix this by using the correct <code>proj4string</code> with <code>readShapePoly,</code>but could not find how to translate the information in the .prj file into a proj4 string. How can I do this/is this the correct approach?</p>