What is the difference between projection and displayprojection for a Map in OpenLayers?

Question

I am familiarizing myself with Openlayers, and am having trouble with understanding the projection and displayprojection properties for a Map.

When the map is initialized with the following code:

map = new OpenLayers.Map('map', {           
        numZoomLevels: 20,
        projection: new OpenLayers.Projection("EPSG:900913"),
        displayProjection: new OpenLayers.Projection("EPSG: 4326")
        });

What exactly is happening?

The documentation of the OpenLayers.Map says that

projection -Set in the map options to override the default projection string [in] this map - also set maxExtent, maxResolution, and units if appropriate. Default is “EPSG:4326”.

while displayProjection is the Projection used by several controls to display data to user. If this property is set, it will be set on any control which has a null displayProjection property at the time the control is added to the map.

This does not make it clear.

Could someone explain what exactly do these two option do?

Answer 1

From the OpenLayers Beginner's guide:

This is a property that is used mainly by controls which show coordinate information. By setting this property on the map object, any control that has a displayProjection property will be set to this value. Controls, such as the MousePosition control, can display coordinates in the displayProjection. So, your map could be in a different projection than what you wish to display the coordinates in. However, to use a displayProjection other than EPSG:4326 or EPSG:900913, Proj4js must be included on your page. This property comes in very handy if, for instance, your map is in a spherical Mercator projection (i.e., EPSG:900913), but you might wish to display coordinates in another projection, like EPSG:4326 (to display lon/lat coordinates).

The bold parts should give you an idea. I hope it helps.