[OpenLayers-Users] why no overlay on Virtual Earth basemaps?

[Ernie Buford]

I only received one response to my question, and it was from someone who 
came up with their own patch.  I have not yet tried it myself.  Would 
love to hear from the dev team or expert programmers about this fix.  My 
original post is below, followed by the response.

On 5/17/2012 12:20 PM, Ernie Buford wrote:
> Having fun getting started with OpenLayers, but I've hit a curious 
> snag....
>
> I have a set of map tiles created using Arc2Earth, and they overlay 
> nicely on Google, Yahoo, and OSM basemap data.... but no luck with 
> Bing/Virtual Earth.  Is this not currently possible, or (more likely) 
> am I missing some critical detail?
>
> I have modified several OL examples (e.g., 
> OL_spherical-mercator-example.html and some from the 2.10 Beginner's 
> Guide) by adding an XYZ layer like this:
>
>     // create slope overlay layer
>     var slope = new OpenLayers.Layer.XYZ(
>         "VT Slope",
> "http://myserver/slope/${z}/${x}/${y}.png",
>         {
>             'opacity': 0.6, visibility: false,
>             'isBaseLayer': false,'wrapDateLine': true
>         }
>     );
>
> At times I've added the 'sphericalMercator: true' option, but it does 
> not seem to matter.
>
> I've added this exact chunk of code to several working examples.  The 
> slope layer shows up over any of the aforementioned basemaps except 
> Bing (which I've tried accessing via OpenLayers.Layer.VirtualEarth as 
> well as directly with the newer OpenLayers.Layer.Bing).
>
> Why won't this work with the Bing/VE basemap?
>
> Should I be using a different method for using this type of tile cache 
> as an overlay in OL?
>
> Ernie

> Ernie,
>
> I had the exact problem that you described - my XYZ overlay layer 
> displays splendidly with a Google or OSM base layer, but nothing when 
> using Bing or VirtualEarth.  I have a suspicion as to the cause of the 
> problem, and have tried a patch that seems to correct it, but I am not 
> an expert in the OpenLayers code, and cannot guarantee that my theory 
> & fix are truly valid.
>
> But here's what I have found.
>
> I ran my test case in Internet Explorer, and used its "F12 developer 
> tools" to look at the tile requests being sent from the browser to the 
> XYZ tile server.  The important difference that I saw when using Bing 
> instead of the other base layers was that the Z/zoom component of the 
> request to the server was off by one with Bing relative to the other 
> base layers.  So I might see a tile request with a "Z" component of 9 
> for Google or OSM, and after switching to Bing, that "Z" component 
> became an 8.
>
> Bing/VirtualEarth layers differ from many of the other base layers in 
> how they designate the zoom level.  The other layers start with zoom 
> level 0, while Bing starts with zoom level 1.  I poked around in the 
> OpenLayers library code, and found that map layers can have a 
> "zoomOffset" data member which I believe accounts for this difference 
> - for Google & OSM, the zoomOffset is set to 0, while for Bing it is 
> set to 1.
>
> After some experimentation, I found a patch that corrected the problem 
> for me (disclaimer: by no means does this mean it is an appropriate 
> fix!).  In the OpenLayers.Layer.XYZ class, there is a function called 
> 'getXYZ', and it includes this statement which calculates the "Z" value:
>
>   var z = this.serverResolutions != null ? 
> OpenLayers.Util.indexOf(this.serverResolutions, res) : 
> this.map.getZoom() + this.zoomOffset;
>
> Note that "this.zoomOffset" represents the zoomOffset value of the XYZ 
> layer which is not displaying for us.  For my layer, it is set to 
> zero, which matches the Google/OSM base layers, but doesn't match the 
> Bing value.  I replaced that line with this code:
>
>         var zOff;
>
>         if (this.map.baseLayer.zoomOffset) {
>
>             zOff = this.map.baseLayer.zoomOffset;
>
>         } else {
>
>             zOff = this.zoomOffset;
>
>         }
>
>         var z = this.serverResolutions != null ? 
> OpenLayers.Util.indexOf(this.serverResolutions, res) : 
> this.map.getZoom() + zOff;
>
> The replacement code checks the zoomOffset value of the base map layer 
> (if it exists) and uses it if it's present; otherwise, it uses the 
> zoomOffset of the overlay layer.  With this code in place, I find that 
> my overlay layer now displays properly for all my base map layers.
>
> Again, I make no guarantees about the validity of this change, and 
> would like to have someone on the OpenLayers development team review it.

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