Tuesday, April 20, 2004

View from above

Tonight, I stumbled across one of those online destinations that I get hooked in to deeper than a Louisiana chemical plant worker playing video poker.

It's the National Map Viewer brought to you by the fine folks at the U.S. Geological Survey.

Essentially, it's a big, click-to-zoom database of all sorts of geographic images, maps and other thingys (pardon my technical language). The cool thing is that for a lot of metro areas (including Atlanta), included in the image set are really detailed aerial photos run through some process called "orthoimagery" that ... well ... changes the image ... into ... some kind of ... ...

OK, I have no idea what "orthoimagery" is. But, as the very well-written federal government explanation clearly states:
An orthoimage is remotely sensed image data in which displacement of features in the image caused by terrain relief and sensor orientation have been mathematically removed. Orthoimagery combines the image characteristics of a photograph with the geometric qualities of a map. For this dataset, the natural color orthoimages were produced at 0.3-meter pixel resolution (approximately 1-foot). The design accuracy is estimated not to exceed 3-meter diagonal RMSE (2.12m RMSE in X or Y). Each orthoimage provides imagery for a 1500- by 1500-meter block on the ground. The projected coordinate system is UTM with a NAD83 datum. There is no image overlap been adjacent files. The naming convention is based on the U.S. National Grid (USNG), taking the coordinates of the SW corner of the orthoimage.


So anyway ... here in Atlanta you can drill down to some pretty damned detailed images, and download them for free (my tax dollars at work).

To wit:

That's my old house in the center there. And that's the wife's old red car sitting in the driveway/yard out front.

Kind of freaky, ain't it?

The metadata that downloaded with the image even tells me when the photo was taken - April 7, 2002. Hey, that was the day after our wedding! At the time, the wife and I were still in NashVegas doing a post-ceremony brunch with her family.

And, that same day, a plane was flying over Atlanta taking hi-res images. Or, in fed-speak:
The aerial platform used during the photo acquisition for this project was a Rockwell Turbo Commander turbine-powered aircraft capable of cruise speeds of around 215 knots. This capability is very important for good production on a very large photo acquisition project such as this one. A Jena LMK 2000 lens high-precision photo-grammetric camera was used as the photographic instrument. This camera has a nominal 6-inch focal length with Forward Motion Compensation (FMC,) gyro-stabilized mount, airborne GPS (ABGPS,) and Inertial Measurement Unit (IMU). Dual-frequency GPS observation data was collected on-board the aircraft at a one second epoch. Additionally, inertial data was collected during all periods of flight and is collected at a rate of 0.005 seconds. The midpoint of each photo exposure was precisely captured as an 'event' by the GPS receiver. All ABGPS and Inertial data was then post-processed to provide accurate positional and rotation data of the camera for each exposure. Effectively, the three dimensional position (x, y, and z) of each exposure was determined from the ABGPS data while the three-dimensional rotation (omega, phi, and kappa) of each exposure was determined from the inertial data. The IMU data (which includes adjusted position and orientation of the camera at time of exposure) were orthorectified using the relevant USGS Digital Elevation Models. These were processed using Z/I's OrthoPro package. The orthorecitifed images were then mosaicked (if necessary, to reduce the effects of micro-relief on the final product). Product tiles were then extracted from the orthorecitifed images or mosaic and converted to GeoTIFF format. Product RMS accuracy was determined by measuring the metric displacement of common features in adjacent tiles or measuring the ground control that was collected. Metadata files were then created and populated to reflect the relevant tile and project data. Product tiles and metadata were then written to DVD for delivery to USGS.

Who knew?


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