title: SRID meaning in postgis
body: <p>I would like to find out what is the pragmatic meaning of SRID (spatial reference id) in postgis.</p>
  
  <p>I really do not understand what it is for. Can anyone throw some light on the matter?</p>
  
  <p>For instance I noticed that the postigs function <code>ST_GeomFromText(text WKT, integer srid)</code> accept such an (optional) param as second argument. Why would I need to pass it in the get postigs to turn the text representation into a binary one? What is the value it adds?</p>
  
  <p>Thanks</p>