title: Using proj4 and EPSG code to transform into different coordinate system
body: <p>I have coordinates in  <code>EPSG:25833</code>, which is <code>UTM</code>, zone 32N.
  I want to transform them into <code>EPSG:3857</code>, WebMercator. The <a href="https://github.com/proj4js/proj4js" rel="nofollow noreferrer">documentation</a> says</p>
  
  <pre><code>var firstProjection = 'PROJCS["NAD83 / Massachusetts Mainland",GEOGCS["NAD83",DATUM["North_American_Datum_1983",SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],AUTHORITY["EPSG","6269"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4269"]],UNIT["metre",1,AUTHORITY["EPSG","9001"]],PROJECTION["Lambert_Conformal_Conic_2SP"],PARAMETER["standard_parallel_1",42.68333333333333],PARAMETER["standard_parallel_2",41.71666666666667],PARAMETER["latitude_of_origin",41],PARAMETER["central_meridian",-71.5],PARAMETER["false_easting",200000],PARAMETER["false_northing",750000],AUTHORITY["EPSG","26986"],AXIS["X",EAST],AXIS["Y",NORTH]]';
  var secondProjection = "+proj=gnom +lat_0=90 +lon_0=0 +x_0=6300000 +y_0=6300000 +ellps=WGS84 +datum=WGS84 +units=m +no_defs";
  
  proj4(firstProjection,secondProjection,[2,5]);
  </code></pre>
  
  <p>Is it possible to pass only a EPSG string as <code>firstProjection</code> and <code>secondProjection</code>? Passing i.e. for <code>EPSG:25833</code> which is <code>UTM 32N</code> contains already all the needed information. Why is there a need to pass to do this</p>
  
  <pre><code>Proj4js.defs["EPSG:25833"] = "+proj=utm +zone=33 +ellps=GRS80 +units=m +no_defs"; 
  </code></pre>
  
  <p>Using this definition restricts me too. What if <code>EPSG:25833</code> changes to another <code>EPSG</code>? Then I have to update this <code>+proj=utm +zone=32 +ellps=WGS84 +datum=WGS84 +units=m +no_defs</code> too...</p>