[OpenLayers-Users] SphericalMercator and EPSG aliases

[Steve.Toutant at inspq.qc.ca]

Hi,
We are maiking the modification in order to use epsg:3857 instead of 
900913.
We read this section: SphericalMercator and EPSG aliases in
http://trac.osgeo.org/openlayers/wiki/SphericalMercator

We don't understand this part
// set transformation functions from/to alias projection 
OpenLayers.Projection.addTransform("EPSG:4326", "EPSG:3857", 
OpenLayers.Layer.SphericalMercator.projectForward); 
OpenLayers.Projection.addTransform("EPSG:3857", "EPSG:4326", 
OpenLayers.Layer.SphericalMercator.projectInverse); 

Why is this for and when should we use it? Do we need to do that for 
several epsg code?

thanks for your help,
Steve
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