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I am starting with maps but im a beginner. The answer is probably easy, but i could not find it anywhere. I cannot figure out why to use maxExtent. If I delete it from my application, its behavior doesn't change or I do not notice. So why should I use it? I understand purpose of restrictedExtent but not of maxExtent. Can you please somebody please explain it to me, thank you.

var extent = new OpenLayers.Bounds(-2003750.34, -2003750.34, 2003750.34, 2003750.34);

map = new OpenLayers.Map("map", {
        controls: [
                        new OpenLayers.Control.Navigation(),
                        new OpenLayers.Control.PanZoomBar(),
                        new OpenLayers.Control.ScaleLine(),
                        new OpenLayers.Control.Attribution(),
                        new OpenLayers.Control.LayerSwitcher()],
        maxExtent: extent,
        //restrictedExtent: extent,
        maxResolution: 156543.0399,
        numZoomLevels: 19,
        units: 'm',
        projection: new OpenLayers.Projection("EPSG:900913"),
        displayProjection: new OpenLayers.Projection("EPSG:4326")

var osm = new OpenLayers.Layer.OSM.Mapnik("Mapnik", { 'attribution': 'Provided by blop' });
var gmap = new OpenLayers.Layer.Google("Google Streets", { visibility: false });

map.addLayers([osm, gmap]);
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I'd like to just point out, that there are incorrect bounds in your example - those are 10x smaller than should, check CaptDragon's answer for correct bounds for Google projection – user1702401 Nov 1 '13 at 10:08
up vote 2 down vote accepted

maxExtent is used for other projections.

For instance by default it's the whole world in decimal degrees (-180, -90, 180, 90)

But if you are using a different projection such as EPSG:3857 You will need to modify the maxExtent. One that is valid for that projection. Which in the case of EPSG:3857 could be (-20037508.34, -20037508.34, 20037508.34, 20037508.34)

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