When you create a map to visualize data, there are some important things you need to take into account: projection to use, available zoom levels, the default tile size to be used by the layer requests, and so on.
Most of these important things are enclosed in the so-called map properties and, if you work in the
allOverlays mode, you need to take them specially into account.
This recipe shows you how to set some of the most common map properties.
Before you continue, it is important to note that instances of the
OpenLayers.Map class can be created in three ways:
var map = new OpenLayers.Map("map_id");