IE6 Cached Tile Transparency Problems

For those 18% of you using IE6 (or for those of you developing sites for the public to consume), you may come across a problem in which map tiles with transparency show up partly dark or partly black.

Our ArcGIS Server is handing out a .JPG base layer, and a PNG8 layer showing parcels to overlay on top. The top layer needs to have a transparent background so we can see the basemap, right? Great. Everything is peachy in FF3, FF2, and IE7. In IE6, *parts* of some of the transparent tiles are showing up black! We noticed that it seems to be worse in areas where the parcel boundaries are relatively close together.

In the ESRI JavaScript Extensions for Virtual Earth, we were also setting the parcel layer's opacity to .9 (layer.setOpacity = 0.9;).

I've also seen these transparency problems using MS MapCruncher to create a tilecache for consumption in Virtual Earth.

After searching blogs and forums, and after our team was about out of options, I stumbled across this page. The key line that helped me out was "With MSIE 6 you can only have transparency or opacity control, not both."

So ArcGIS Server is giving us PNG8s with transparent background, AND we were setting the opacity using JavaScript. No good.

I commented out the layer.setOpacity = 0.9; and, voila! Everything works in IE6+....

Hope this helps someone!

Welcome to GeoSpat!

This is my first attempt at blogging. I hope this won't be my only entry! This blog will discuss my work with Geospatial Technologies/Internet Mapping and everything that goes along for the ride.


I'm currently working on several Geospatial Web Applications:

• A Google Maps/.NET site to gather public feedback regarding public transportation options
• An ArcGIS Server/ESRI JavaScript API & Virtual Earth County Assessor's Parcel Viewer
• A PostGreSQL/GeoServer/OpenLayers Portfolio Tracking application

In the last year, I've had the opportunity to work on some cool projects, including the GeoEye GeoFUSE Search and Discovery Tool. This stack included SQL Server '05/ArcSDE/ArcGIS Server 9.3, the ESRI Google Maps Extensions and the Google Maps API.

One of my job responsibilities is to 'poke around' with new technologies and keep track of what is 'out there'. I've spent time poking around with:

• Deep Earth Silverlight Control
• SQL Server 2008 connection to GeoServer
• Using the native SQL Server 2008 Geometry Type with ArcSDE
• SMS Text (DOTGO) -> .JPG -> Virtual Earth 3D Billboard
• Creating .NET Web Handlers as REST endpoints for custom spatial data entry
  
• Using SQL Server 2008's XML capabilities to transform queried GML with XSLT