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

[Joel Williamson]

Ernie Buford <ebuford at ...> writes:

> 
> 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 shows 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, despite the fact that my map extents had 
not changed.

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 overlay 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, and I get the same Z component 
regardless of which base layer I am using.

Again, I make no guarantees about the validity of this change, and would like 
to have someone on the OpenLayers development team review it.

Joel