Public Lab Research note


Balloon mapping notes from a first-timer

by jbest | November 06, 2013 23:36 | 66 views | 9 comments | #9742 | 66 views | 9 comments | #9742 06 Nov 23:36

Read more: publiclab.org/n/9742


First flights - problems and solutions

I wanted to share some basic problems and solutions we found in our first and second balloon flights for aerial surveys. These notes might help other first-timers and may also elicit input from those who are more experienced and have better solutions.

My attempt and results

Helium tank and regulator - since a regulator should not be used when filling the balloon, I ordered the helium delivery without one (it cost a little extra), but I recommend you get a regulator that includes a pressure gauge so you can keep track of how much helium you use. We will be getting one next time.

String reel - the Public Lab kit includes a basic reel, but we decided to get a wooden reel with handles ($30 from Into the Wind - http://www.intothewind.com/shop/Line_and_Accessories/Bulk_Line_and_Reels_for_Traditional_Kites/Windbreaker_Kite_Line_Reel ). This was worth every penny.

String markings - we decided to mark the string with a permanent marker every 10 meters (1 red mark each increment, and one - 9 black marks to represent multiples of 10). This helped us estimate altitude.

Zip ties - the reusable zip ties provided in the Public Lab kit worked fine on the first flight, but came loose on the second flight and our balloon slowly lost helium and altitude. We refilled it and tried with the reusable ties, but they were not getting a satisfactory seal and would sometimes open up one click or more after we tried to cinch them down. We decided to use standard single-use ties which worked great. We used wire cutters to cut them off.

IMG_2903.jpg
This is the kind of tie that was provided in the balloon kit. Some of them did not hold reliably.

Cones - we put orange traffic cones in the survey area at 20 meter intervals so we could 1) determine scale of the photos and 2) determine altitude that the image was taken (given a known FOV or AOV).

Picavet and balloons - since I had originally planned on doing a kite survey, I built a picavet system rather than the liter bottle pendulum system described in the balloon mapping kit. With a balloon and little wind, this system didn't work as well as I hoped. Because the tether was almost vertical, there was no horizontal distance between the two attachment points so it allowed the camera to spin. Many others probably had the sense to not try this setup, but the problem wasn't obvious to me until we launched. The result was that about a third of our images were too blurry to use because they were captured during a camera spin.

Questions and next steps

We wanted to measure the buoyancy of the balloon with a spring scale, but ours didn't arrive in time. We have them now and plan on finding the minimum buoyancy needed for a given payload so we can minimize the amount of helium needed. Others have probably done this so I'd appreciate any input.

We'll be conducting another survey of the site in a few days to compare the results of horse grazing on an urban prairie restoration of the site: http://goo.gl/maps/muPzD

I'll be posting a full writeup soon but for now, I'm adding our first stitched image of the survey. This was done using VisualFSM as shown on the tutorials at FlightRiot.com: http://flightriot.com/uav-mapping-post-processing-tutorial-1-visualsfm-cmvs/

BRIT-aerial-2013-10-23-small.jpg


9 Comments

Hi there, jbest!

Did you get usable images? Sounds like you had a good flight. Just a couple of notes:

1) I use electrical tape on the neck of the balloon to get a good seal and not let expensive He escape. I thread the flying line under the tape. I then use zip ties as a back up.

2) Nice use of traffic cones. I have used orange speed squares that are exactly 12 inches each way. http://www.sears.com/johnson-12-in-glo-orange-plastic-rafter-angle/p-00981569000P?sid=IDx01192011x000001&kpid=00981569000&kispla=00981569000P

3) Great thought about minimizing the amount of helium. I usually just clip the camera on and see if it floats. I try to use as little as possible too, especially since our prices are about $200 for a large tank here in NC.

Good luck! Got any maps in the works?

Adam

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Adam, thanks for the tips! I'll try the electrical tape next time. Aside from the minor problems, we had a great couple of flights, both with some very nice images. On the second flight, we did a survey both at 100m and 200m of our urban prairie restoration, the day before a dozen horses arrived that we've employed as lawnmowers. They've eaten down a lot of the vegetation and hopefully are distributing seeds and pressing them into the soil for a nice regrowth in the Spring. When they leave in a few days, we'll do another survey and soon after that I'll be doing a full writeup and will share here.

Here is the survey site: http://goo.gl/maps/muPzD

And I'm adding to the note above a low-res image of the stitched survey images.

Thanks, Jason

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Hi Jason,

That sounds like the perfect application and we look forward to anything you have to share! Good luck!

Adam

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Oh wow - that tutorial is awesome! I've had some problems with the 64-bit version on SFM and this might help. I was getting stuck at the SIFT or multicore bundle adjustment, I can't remember. I'll check it out!

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Glad you mentioned that you had problems with VisualFSM because I did too. I also don't remember the specific problem but eventually I had success by using only half of our images instead of trying all of them. May have been a memory issue or something, but on my next run I'll take some better notes for myself and to share so others can benefit.

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Yes, I actually joined the SFM discussion forum. It looks like I had trouble with the pba and dll installations. I was trying for a 64 bit version with GPU acceleration, but ended up giving up since this is peripheral to my promary job responsibilities. Here is the string of messages from the google group: https://groups.google.com/forum/#!searchin/vsfm/griffith/vsfm/9SIUUVMnf0k/n4ELPzmz-VkJ

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we recently switched from side tab to top tab ties. could you post a photo of the ones you used? The old ones were not as reliable.

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mathew, I added a picture of the kind of zip tie that didn't work well for us.

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thanks. Yup, those have a problem with the latch weakening, making them less-than-reuseable, and easy to accidentally push while tightening. We switched to these. e-mail me your address and I'll throw some in the mail for you. (mathew at pub lab)

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