Tuesday, October 28, 2003

An ugly blue tarp, as seen from space.

I'm a big fan of aerial and satellite photography, and was thrilled to discover the original TerraServer several years ago. Though its datasets are black and white and a bit dated, it's still amazing to see familiar areas as photographed from above.

There's now another site called TerraServer, with more recent datasets, including some in higher resolution and color. I'm not sure if it's a successor to the old one or a competitor, but this one offers access to very high resolution data (1 foot in some areas) for a fee. Still the same basic interface, though: web form that lets you search for an address and then click to pan or zoom.

It occured to me that the web is hardly the optimal platform for applications like this. (I happen to believe that it's not the optimal plaform for any application, but that's another discussion.) Specifically, I suspected that a local application could let you zoom and pan smoothly over datasets like this, by fetching image tiles on demand from a server. It's such an obvious idea, I figured somebody must have done it.

Turns out I was right.

I asked a friend of mine if he knew of such an application, and it he grinned. "I worked on one once." TerraVision is a cool open source app that does exactly what I expected: it lets you browse multiresolution image/elevation datasets on the earth interactively. You can load more than one dataset, to provide coarse data over a large region, and more detailed data in specific areas. It all works in a small memory overhead by managing a multiresolution cache of tiles.
TerraVision can fetch tiles on demand from a server or operate from datasets on the local disk. Unfortunately, there don't seem to be any active TerraVision servers out there, so you're left to download the handful of datasets they provide (or convert your own from USGS data.) In the bay area, for example, I was only able to browse 25 meter terrain data, which isn't enough to resolve individual houses. Still, very cool, especially for an open source app.
Kiril also pointed me to a more polished commercial app that does the same thing. Last night I tried a free 7 day trial of Keyhole LT, and was blown away. Because it's a commercial app, they provide a server with really high resolution data over most major cities, and coarser (but decent) data elsewhere. If you live anywhere near a major city, you can probably zoom in to see your house.
Keyhole also lets you do address queries (like MapQuest), which makes it easy to find familiar places. I spent several hours last night exploring all the places I've been, continually amazed at the detail in their dataset.
For example, the house we just bought is easily discernable in Keyhole. In fact, behind the house there's a bright blue rectangle, which I realized is the blue tarp on top of the awning in the back yard. As seen from space!
If you have a Windows machine with any half-decent graphics hardware I highly recommend you check out Keyhole LT. (It works on my laptop with Intel i815e, so it'll probably work on your computer too.) Find your house. You'll thank me for it.


  1. Nice! Do you know how often they update their datasets? We recently took down some trees - ones that would completely obscure our property as seen from space. And yes, after installing the trial, my house is completely covered. :( I quickly checked their site and I find it rather disturbing that I could examine the Las Vegas strip at 3 INCH resolution. Well, there goes my afternoon.

  2. HELLO
    THIS IS COOL, please reply ;>)