title: Postgis ST_Transform
body: <p>I have a point (x, y) with srid 900913. I transform it to srid 2180 and then again to srid 900913. Imo I should have the same point, but it differs. Why?</p>
  
  <pre><code>SELECT ST_X (ST_Transform(ST_Transform(ST_GeomFromText('POINT(21.01233628836129 52.23044648850736)', 900913), 2180), 900913)),
  ST_Y(ST_Transform(ST_Transform(ST_GeomFromText('POINT(21.01233628836129 52.23044648850736)', 900913), 2180), 900913));
  </code></pre>