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Feb 01 2012

Ballon Aerial Photography, GoPro's and Photoscan


I stumbled on the website of a researcher Paul Illsley from Nova Scotia that works on remote sensing and has a hobby of photography.  He has a nice website description of a balloon aerial photography system and techniques for working with different cameras, including a GoPro Hero.  The picture at left shows a model he generated in Photoscan from a balloon survey of a building.  He suggests a technique for removing the GoPro lens distortion that I will definitely have to try out.

Photo source: http://www.paulillsley.com/airphoto/systems/B.jpg

Mar 15 2011

Learning Photoscan

I've really gotten my hands dirty in Photoscan this past week.  I've learned a number of things:

  • A periodic sampling regime ("Every third photo", etc) can produce a *SUBSTANTIALLY* worse pointcloud than every-photo for complex surfaces.  Simple surfaces aren't affected as much.  This could be applied selectively to cut down on runtime.
  • The "Estimating Scene Structure" time remaining display is only useful as a minimum bound, and may be 10-100x what is currently displayed.  The other estimators seem to be accurate.
  • Due to speed penalties at high imagecounts, choosing image subsets is going to play a very important role in synthing areas of interest, and we need to develop better methods for this.
  • Paused photoscan has a 'sleep mode' where it shifts down to a fraction of the memory (10GB -> 1.3GB) and no CPU, but it needs 10 or 15 minutes to enter it after pause is initiated, and uses full memory and 95% CPU during that time.
  • Tree trunks are readily identifiable in Photoscan given full-resolution pictures and a rapid frame rate, but care must be taken during turns to unify the synth using extra pictures
  • For small image sets, periodic subsetting (every other picture) may be attempted, and then supplemented with extra information in corners.
  • Photoscan is relatively noise-free for aerial photos, but anything that includes the sky will cause serious noise problems on silhouettes - points of sky and tree will appear in a distinctive projected pattern in what should be air.  Photosynth does not suffer from these problems.
  • Markers need to be sizable and textured to be seen in a synth.  Strings don't come close to working, though their directionality is very useful for walking consistant transects without good visibility.  The pin flags work very occasionally, but it would probable be better adding flat, 8.5x11 textured bullseyes as well.  GPSing those markers gets you a crude georeferencing, but in small forested transects this is not very useful due to the error bounds on the GPS.  Increasing the lateral dimension of the synth using a cross transect would make georeferencing much easier.
  • Symmetrically cropping the edges out of pictures (keeping the same centerpoint) can be an effective way to cut down noise and processing time in a camera held in a constant orientation.  Removing the top and bottom significantly decreased sky noise and processing time on a test dataset.
  • Our tree database is not very accurate.  It's missing stems < 12cm, but even the big ones are very hard to locate in the synth using relative positions.  A set of 'reference trees' would be very helpful here as a complement to GPS markers.
Mar 05 2011

Field Day with the 485 Class

We had a great field day with the GES 485 class on Saturday flying the Hexakopter at the Herbert Run site and developing field work and 'ground-synthing' techniques.

The weather was actually quite good for a data collection.  The sky was overcast and there was no wind, meaning that the Hexakopter was able to stay on track and the light was relatively diffuse so there are few shadows in the images.  I gave a large set of about 2000 photos to Photoscan for processing on Saturday afternoon and it is still running.  This is a great software, but I don't yet have enough of my own benchmarking data with large sets to really test out how it is going to work. I hope the point cloud looks good!

Regarding our Xbee testing, I used the MKUSB to upload waypoints, but then discovered that when I power down and then power back up to swap out the battery and plug in the wireless Xbee module I cannot read waypoints from the MK, or they are not stored on board.  But, I was able to upload waypoints wirelessly with the new Xbee configuration and the real-time telemetry communication during the flight was OK.  At least the current setup is no worse than what we had before.  More to come.