Kamis, 08 Mei 2008

Los Angeles Web Mapping Service

In Monday's post I talked about processing image and LIDAR data for rapid response mapping. Around the time time as the exercise in China Lake, Leica Geosystems also flew downtown Los Angeles. A colleague and I had the opportunity to process the image data from raw imagery through to a final orthophoto (orthomosaic) product, and then we had it served up as a WMS using IWS technology.

Here is the WMS link:

If you have a decent web connection it is fairly straightforward to fire it up in ERDAS IMAGINE using the same workflow I walked through here. If you're prompted for a resolution or GSD, you can use 0.127 meters (this may happen if you have 9.2 SP1 loaded). Here's the entire project area loaded up in a viewer:

Since the imagery was flown relatively recently (late summer '07, I believe it was around August), it is quite a bit more recent then imagery in Google Earth or Virtual Earth. Check out the area just north of the Staples Center:

Google Earth (credits to Sanborn):No construction in the parking lot to the north of the Staples Center.

Virtual Earth (credits to USGS):Construction has started, mainly digging.

RCD105 WMS:Construction not yet complete, but buildings have started.

It is interesting to see the sequenced development over time. Which brings me to an enhancement request I've seen several times out there: Google Earth and Virtual Earth should really think about including the "date of image capture" as a displayable layer (or some other means). Right now it seems like guesswork to find out when an area was actually collected. This is "kind of" in version 4.3 of Google Earth - the dates of some images are displayed in the status bar, but not all (I think only satellite image collection dates are displayed). Since Sanborn is largely an aerial provider, there are no dates on their downtown LA imagery.

One of the interesting things I've noticed about the LA datasets in both Google Earth and Virtual Earth is that both seems to take the "high brightness, high contrast" approach to radiometric processing. This is good for making a pretty, visually appealing image, but tends to blow out some image detail in "light" areas (e.g. white and light colored areas). Here's and example from Google Earth (also downtown LA):
You can see that the RCD105 imagery looks quite a bit "darker", as I didn't bump up the brightness/contrast too much. It was also hazy on the day of data collection, which can also cause issues. However, keeping the brightness and contrast down allows for image details to be picked up - although I'm thinking of making a "pretty picture" version as well. Shows that radiometric adjustment is very much an art as well as a science!!

A few notes on the processing: the final orthomosaic for the project area in the RCD105 WMS was about 4.5 gigs. We used ECW compression (10:1) in IMAGINE to compress it down to a 220 megabyte ECW file, which was the final image used for the WMS layer. While is a lossy compression format, there is little visual difference between the IMG and ECW files. If you really zoom in close, it is possible to see some compression artifacts - but I suppose this is the trade off between speed and quality. For general purposes is seems like a decent method for prepping up imagery.

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