Last weekend I experimented with taking a digital aerial photo mosaic. The area covered by the mosaic image is a little over 22 acres.

I used my modified Multiplex Twinstar II model airplane with a Canon Powershot digital camera. The camera was set up with the CHDK firmware and a simple basic script to take a photo every 1.5 seconds. The camera looked downwards through a lens sized hole in the airplane's wing. The plane has a FMA direct Co-Pilot flight stabilizer to keep the plane level and reduce wing rocking.

At launch I set the Canon camera to photo mode, and enabled mountain mode ( infinity focus ), 100 ISO, sun white balance, turned off continuous photo mode and turned on ALT mode and started the photo taking script.

I wrote an easy to use intervalometer script called countdown intervalometer. Countdown intervalometer allows you set the delay between photos and the number of photos to take. If you set the number of shots to zero it will take photos until your memory card is full.

Here are two simple CHDK intervalometer scripts I use depending on how many photos I want to take.

The script timed.bas has a variable Interval in Seconds that lets you set the interval between photos.

@title Timed Photos

@param a Interval in Sec

@default a 0

while 1

shoot

sleep a*1000

wend

end

The other script rapid.bas simulates the shoot_full button press and works well for taking photos rapidly. For the rapid.bas script I recommend not using continuous mode but regular mode for a consistent timer interval.

@title Rapid Photo v2

while 1

press "shoot_full"

release "shoot_full"

wend

end

Note: If you want to take a whole lot of photos with lots of overlap you could set the camera to continuous mode and then hold the shutter button down with an elastic band and something small to apply pressure on the button. Just make sure to focus the camera on a distant object when you take the first photo or all of the images will be out of focus.

The plane was flown up to an altitude of approx 300 feet high then guided on a gentle hippodrome shaped flight path. In all about 200 pictures were taken during the flight from initial launch to final landing.

Here is one of the individual photographs the plane took. Basic color correction has been applied. (Click on the image for a full resolution version )

After I transferred the images from the digital camera to my laptop computer I selected  36 overlapping pictures that were taken at the desired height. I used PTGUI to automatically align and stitch the 36 images using a stereographic projection on the output.

Then I cropped the mosaic image in Photoshop and saved it as a high quality JPEG image. Here is the cropped photo mosaic image scaled to a reasonable resolution for the web. ( Click on the image for a full resolution version )

The next step is to load the image into Google Earth as an image overlay. The image is moved into place by adjusting the control handles on the image to slide it around and rotate it. It helps to adjust the transparency to check that the image is properly overlaid and aligned over distinctive ground features.

Below is the finished result in Google Earth. The nice thing is that you now can save the image overlay to your My Places section and it will display by default when you load Google Earth in the future.

If you shoot your own aerial mosaics I would love to hear about it. What techniques do you use? Do you have any tips or suggestions to share?