A Word About Tile Caching in ArcGIS Server 10

I have recently been involved in creating a web mapping application that displays environmental monitoring locations over a very large area (approximately 7 million hectares).  Of course one of the primary objectives in developing any application is optimization and performance and, with web maps, that usually means tile caching.  So the development team came together (two of us in this case) and quickly decided what our zoom scale requirements were.  We then tried to implement this which is where it all fell apart.

Because we were using the Bing Map Service as our base layer we were committed to using the zoom scales dictated within that service.  Originally we decided that the minimum scale could be set to 1:9,244,648, which is one of the Bing mid-range zoom scales, however, we felt that the maps should be tiled the the largest scale possible; 1:1,128 in this case.  Here in lies the problem.  If you do a quick calculation to determine how many tiles are created using this range it is approximately 89 million, YES I said 89 million.  Of which 98% come from the scales above 1:9,027.  When we tried to tile with those scales the process would run for a number of days without finishing.

So how did we overcome this you might ask.  By taking a more realistic view of our requirements and removing the unneeded scales from the tile cache.  Once we removed the 3 scale values above 1:9,027 the process took less than an hour.  We also made sure to have Tile On Demand checked for each service so that the tiles could be created on an as needed basis, prior to running the tile caching.

Lesson Learned: Before creating your Map Service infrastructure develop a tile caching strategy which should identify the largest scale you NEED, the number of tile cache layers and which layers MUST be cached.  DO NOT include scales that you don't need and cache only the Map Layers that will be accessed FREQUENTLY.

ArcGIS Server vs. Mapserver/PostGIS

I know that ArcGIS server is supposed to be a true example of SaaS and the Mapserver/PostGIS example I have given is likely considerd something else but I would like to share an experience on a project that spans both worlds. About 5 years ago the company that I was working for at the time got a contract to build an online database/spatial data managment system for a Canadian Government Agency. Originally they required a proof of concept so we decided to build this thing using open source technologies. We very quickly found that UMN Mapserver combined with PostGIS/PostgreSQL and a couple of other nearly free software titles would do everything we needed. By the time we were finished we had a highly interactive tool that gave the user access to such things as dynamic cross sections, dynamic drill logs including down hole temperatures, the ability to upload photos and add them as points on the map and a dynamic 3D visualization tool. The grand total for software expenses was under $2000 CDN.

Anyway, to make a long story even longer, about a year and a half ago the client decided that they wanted to host the application themselves. As they are completely an ESRI shop we were contracted again to migrate the application to ArcIMS and ArcGIS Server. This task did not come without several headaches that's for sure but we managed to get much of the application migrated. The one thing that was impossible was the ability to migrate the 3D visualization tool. Apparently ArcGIS Server won't allow you to consume their ArcScene objects without a thick client such as ArcGIS Explorer or full blown ArcScene. The client did not want their clients to download any software for this purpose so we ended up going with the much less versitile solution of creating a number of static views as jpeg files. This also required some additional programming to create a viewer for accessing the images. Needless to say this required an adjustment to the scope of the project and some creative negotiation with our client.

So why am I telling you all this; keeping in mind that the software we were using with the original application cost about $300 to generate the 3D views. Why is it that ESRI couldn't provide this functionality in thier much more expensive software. Please don't get the idea that I am anti ESRI because I think ArcGIS Server is a phenominal product but I don't understand why, with the advent of such technologies as Google Earth and Microsofts Virtual Earth, ESRI didn't throw in the ability to view snapshots of the 3D scenes without a thick client.

As the project manager for this application I made an official improvement request to ESRI but I haven't heard whether this improvement has been made as of yet. I understand that ESRI as the largest supplier of GIS software can't include everything into thier applications but I just thought it was a no brainer given my comments about virtual earth technology and the web.

Please let me know what you think about this especially if you are involved in creating rich web applications for your clients or employers.

-Gerry-