Now we zoom in on a certain position.

We usually have latitude/longitude coordinates as input (WGS 1984, projection name ""EPSG:4326") but need to transfer these to the spherical mercator projection (projection name "EPSG:900913") used by the OpenStreetMap and GoogleMaps map tiles.

Don't wonder about these projection names too much, the two used here will probably be the only ones you'll ever need.