Processing Aerial Imagery for use in GIS applications

Steve and Jason from CTCOG drove down to San Antonio last week to visit the firms who are collecting and processing our next set of aerial imagery.

Williams-Stackhouse Incorporated has been providing photogrammetric mapping and related services since 1960, and have produced imagery in our region in the past.  Their staff walked us around their offices, explained every step of the collection process, and patiently answered our questions.  The process from planning to final delivery is more complex than I would have guessed, but the attention to detail is what makes the final product so useful.

First, Williams-Stackhouse sits down with the aviation company to plan a series of flights that will cover the entire area, such that the images overlap.  This information is plotted into the pilot's GPS navigation system.

Once the route is planned, the pilot feeds the shot data, airspeed, and altitude into a custom-built controller that times the shutter release on the camera.  One of the pilots is showing Steve the display that tracks the flight path and allows him to control the camera.

Here's the camera, minus the film cartridge.  The mount is automatically gyro-stabilized in three dimensions, so any small corrections to yaw, pitch and roll don't cause distortion in the image.  For example, if the plane is flying in a crosswind, the pilot will adjust the rudder so that the nose is pointing upwind but the flight path remains straight.  The mount will swivel the camera opposite the direction that the plane is pointing, keeping the film square to the planned flight path.  However, the mount has a limit to how much it can correct, so the pilots only fly in the best possible weather.  

Weather is a consideration not just for a stable flight path, but also for ground visibility.  Clouds between the ground and the camera will obscure the features of the terrain.  High-altitude clouds can cast undesirable shadows beneath the plane. 

Once the flight is complete, the film negative is developed and sent to Williams-Stackhouse for processing. 

Steve is looking at the negative on a light table.  Even at this size, the detail is impressive.

The next step is scanning the film into a digital format.  This scanner automatically scans and advances the roll of film, scanning each frame down to a resolution of 2 microns.  

That means that each pixel in the scanned digital image corresponds to an distance of 0.0000787 (7.87 × 10^-5) inches on the film. 

The images are then adjusted for color, contrast, and brightness on the workstation to the left. 

You can see on the monitor how the image is tiled as the scanner processes each section of the image.

The next step is adjusting for variations in elevation.  The shutter is timed so that there is at least 60% overlap between images, which means we can use stereoscopic glasses to perceive the terrain in thee dimensions (very similar to the glasses you wear at a 3D movie).  Here's one of the technicians showing Steve how he can make adjustments to the images in 3D.

This is the 3D mouse used to process the images so that elevation differences can be calculated.

After several more steps that include controlling color variations, and determining where the edges of the separate images will be cut, another technician will mosaic the tiled images together. Here's a shot of the Temple High School football field being adjusted in Photoshop.

 If there are any smears or distortions that let you see where one image ends and another begins, the problem area is reprocessed and the mosaic is performed again.

Once the mosaic is complete, the image is stored on an external hard drive and shipped to the client.  Based on the number of images, and the resolution and file size, we expect to receive about 2 Terabytes of data when the project is complete.

The aerial acquisition for the entire 7 county region at 2 foot resolution, and the urban corridor between Copperas Cove and Temple at 1 foot resolution, is complete at this time.  They are currently processing the photos, and expect to have their final product delivered to us sometime in June.

Thanks again to Randy Holt, Dave Calkins, and all the staff at Williams-Stackhouse and Krawietz Aerial Photography for an enjoyable and informative visit!