TILING 101: 2010 Tile-Based GIS Chapter
After reading an article called “How Google Earth Works” on the great site HowStuffWorks.com, it became apparent that the article was more of a “how cool it is” and “here’s how to use it” than a “how Google Earth [really] works.”
So I thought there might be some interest, and despite some valid intellectual property concerns, here we are, explaining how at least part of Google Earth works.
Keep in mind, those IP issues are real. Keyhole (now known as Google Earth) was attacked once already with claims that they copied someone else’s inferior (IMO) technology. The suit was completely dismissed by a judge, but only after many years of pain. Still, it highlights one problem of even talking about this stuff. Anything one says could be fodder for some troll to claim heinvented what you did because it “sounds similar.” The judge in the Skyline v. Google case understood that “sounding similar” is not enough to prove infringement. Not all judges do.
Anyway, the solution to discussing “How Google Earth [Really] Works” is to stick to information that has already been disclosed in various forms, especially in Google’s own patents, of which there are relatively few. Fewer software patents is better for the world. But in this case, more patents would mean we could talk more openly about the technology, which, btw, was one of the original goals of patents — a trade of limited monopoly rights in exchange for a real public benefit: disclosure. But I digress…
For the more technically inclined, you may want to read these patents directly. Be warned: lawyers and technologists sometimes emulsify to form a sort of linguistic mayonnaise, a soul-deadening substance known as Patent English, or Painglish for short . If you’re brave, or masochistic, here you go:
There are also a few more loosely related Google patents. I don’t know why these are shouting, but perhaps because they’re very important to the field. I’ll hopefully get to these in more detail in future articles:
3. DIGITAL MAPPING SYSTEM
4. GENERATING AND SERVING TILES IN A DIGITAL MAPPING SYSTEM
5. DETERMINING ADVERTISEMENTS USING USER INTEREST INFORMATION AND MAP-BASED LOCATION INFORMATION
6. ENTITY DISPLAY PRIORITY IN A DISTRIBUTED GEOGRAPHIC INFORMATION SYSTEM (this one will be huge)
And there is this more informative technical paper from SGI (PDF) on hardware “clipmapping,” which we’ll refer to later on. Michael Jones, btw, is one of the driving forces behind Google Earth, and as CTO, is still advancing the technology.
I’m going to stick closely to what’s been disclosed or is otherwise common technical knowledge. But I will hopefully explain it in a way that most humans can understand and maybe even appreciate. At least that’s my goal. You can let me know.
Big Caveat: the Google Earth code base has probably been rewritten several times since I was involved with Keyhole and perhaps even after these patents were submitted. Suffice it to say, the latest implementations may have changed significantly. And even my explanations are going to be so broad (and potentially out-dated) that no one should use this article as the basis for anything except intellectual curiosity and understanding.
Also note: we’re going to proceed in reverse, strange as it may seem, from the instant the 3D Earth is drawn on your screen, and later trace back to the time the data is served. I believe this will help explain why things are done as they are and why some other approaches don’t work nearly as well.
Late last week ESRI announced that its ArcGIS Online services will be migrating to the Mercator-based tiling scheme used by Google Maps and Bing Maps by the end of the year. Previously these services have used the WGS 1984 geographic coordinate system with a 512 x 512 pixel size. Google Maps and Bing use a modified Mercator projection with a 256 x 256 tiles size. The differences between ArcGIS Online and the popular Google Maps and Bing tiling schemes caused problems for many organizations. According to ESRI,
Some organizations struggle with choosing either the ArcGIS Online tiling scheme to match their ESRI software stack, or the Google / Bing tiling scheme to match a better-known standard. With a unified tiling scheme for the three services, the decision gets a lot easier.
The post goes on to discuss the advantages and challenges of switching to the new tiling scheme and also discusses a workflow for caching your maps in the new scheme.