OpenLayers Minutiae

At least in my mind this is "minutiae." How in the world one is supposed to know what makes an OpenLayers map application display properly for the various scenarios that arise is still beyond me. To get the answer I needed today I had to send an email to a colleague who is much more experienced than I am. Thanks, Chad, for helping me with this.

Scenario was as follows: A certain server exposed a number of WMS base layers that represented data coverages over the world. They could be displayed with a custom-built "tileserver" OpenLayers class that inherited from OpenLayers.Layer.Grid. I wanted to use the standard Metacarta world vector layer as a base layer and then have 5 of the tileserver layers as user-selectable overlays.

The Scott Davis, "GIS for Web Developers" book was a great help in walking me through the process of how you add the LayerSwitcher control and pass in the correct parameters for each layer to make them visible over the base layer. Part of what slows me down in this experimentation is that the data I am trying to access is behind a PKI-enabled firewall. You can only get to it if you have a CAC card reader on your PC and a CAC card with the proper credentials. Then it seems that Internet Explorer tends to get picky about timing of the sessions or something because for a while it will do the proper thing of reading and authenticating the CAC but then all of a sudden it quits working. It pretends that it doesn't know it needs to ask you to authenticate and just won't display the data or tells you the page is "not found."

In any case, getting around those hurdles and finally getting a consistent display of the data (or a workaround when it wouldn't display) was a great triumph. But then I saw that the tileserver layers weren't lining up properly with the WMS base overlay. These things were supposed to be in the same projection. What was I doing wrong?

Here's the answer I got back from Chad:

I was using this statement (copied from who knows where) to construct the map object:

var map = new OpenLayers.Map('map', {'maxResolution':'auto'});

Chad suggested using this constructor:

var map = new OpenLayers.Map('map', {
maxResolution: 146543.0339,
projection: "EPSG:4326",
maxExtent: new OpenLayers.Bounds(-180, -90, 180, 90),
scales: [442943842.5, 221471921.25, 110735960.625, 55367980.3125, 27683990.15625, 13841995.078125, 6920997.5390625, 3460498.76953125, 1730249.384765625, 865124.6923828125, 432562.34619140625, 216281.17309570312, 108140.58654785156, 54070.29327392578]
});

Yikes! Now you tell me where do all those numbers come from? I don't think I could ever have discovered this on my own. But I tried it and, sure enough, the layers now line up perfectly with the overlays.