I have been fortunate enough to be part of the ESRI South-Africa Summit 2013 held at the Champagne Sports Resort. I had the opportunity to fly the ESRI UAV there and see it as the highlight of all my efforts in this blog. I could not only do what I love, but also had the opportunity to finally test the results against proven equipment.
I could do a few flights photographing both the front lawn and a part of the golf course. I was then assisted by Ian from Leica to capture a set of control points on the lawn using their excellent GPS hardware. The accuracy of the Leica GPS was in the region of 4 to 8 mm when making use of differential GPS! We also captured a whole host of cross-check points to test the final results against. Without Ian’s help, I would not have been able to get such a good quality set of points to test against.
I then built an ortho photo and a digital elevation model using a trial of Agisoft’s excellent Photoscan software. I imported the GPS measured control points and cross-check points into ESRI’s ArcMap and measured the differences between my Digital Elevation Model and the measured points. I think the trial has made it’s point and I hope to acquire a licence of this software in the very near future.
I was stunned by the results! Height accuracy of the model was between 25mm and 95mm in the control point area. Accuracy was between 40mm and 400mm in the area outside the control points. The worst accuracy was at the edges of the photographed area – as expected.
These areas would typically not be used as there is a lot of distortion there due to that area being made up mostly of oblique photos. Other large errors was due to the stereo process measuring the height of vegetation and not of the ground under the vegetation. This is a limitation of this type of height model generation that is not present using traditional methods and advanced methods such as LAS mapping.
Even though the process has limitations, it should be considered against the cheap cost of the platform compared to traditional hardware. It should also be noted that the results were obtained using low quality runs on the model to keep the processing time low, enabling us to get results in less than a day. Also of note, is the fact that the plane was flown manually and without the assistance of the auto pilot as I am still awaiting the replacement auto pilot after the servo failure and crash I had last year.
The week end yielded a total of 9GB of video and photos taken from the UAV. All in all a very fun and productive week.
Here is a compilation of some of the video taken from the plane:
Throughout the process of building the UAV I had been experimenting with SPAD (Simple Plastic Aircraft Design) planes. Since they are simple and quick to build and cheap material wise, one can build a plane to try out a concept and discard it if it does not work.
I started off with a fairly traditional layout sans the V-tail which worked quite well and flew quite well. The biggest flaws were the too short tail moment which could bite you if you got the GC only a little bit wrong and the too small size to carry any usable size stills camera.
The second design was better. It had a pusher prop, leaving the nose free for camera equipment. Also a V-Tail design, that paid off at least once when I had a servo come loose, while still maintaining enough control for a safe landing. Since the two panels of a V-Tail doubles as both elevator and rudder, losing one, means you only lose 50% of your elevator and rudder control. It’s thus naturally redundant. Due to the pusher design, it had to have a fairly deep section body so that the prop can clear the tail boom. Much the same as the current plane I use for camera work. It had tricycle undercarriage, which made it a breeze to take off on a good surface.
The disadvantages of this plane is that it’s still too small for a stills camera, the wheels are too small for rough field take off and landing. Also, like the current UAV plane, the deep section body doesn’t like crosswinds, and give funny pendulum effects while flying, making it overall not a very nice plane to fly.
My latest SPAD is again a much more traditional plane. Even though I don’t carry the camera in it, it will carry the camera easily. Of all the planes I have, this is the sweetest one to fly. It handles well and is always a pleasure to fly. It has one small flaw that led to the name I gave it : The Lawnmower. It’s a fairly long plane which means that it stands on it’s wheels in a near to level attitude. The plane is a tail dragger which means I cannot increase the angle of attack on the main wing on take off by rotating when I have sufficient speed. This all means that I need quite a high take off speed to get into the air, even though it can fly quite slow once in the air. So, when doing take-offs on grass, it tends to look like a rogue lawnmower as it tries to gain enough speed whilst chopping and throwing up grass with the fairly low to the ground propellor.
I also found that the large tail fin is not nearly stiff enough for high speed flying as I got a scary amount of flutter on the one flyby I did. As luck would have it, I had the keychain camera pointing right at the tail when it happened! (See the video below).
All in all, it’s still an enjoyable plane. It glides surprisingly well and seems to have fairly low drag compared to the UAV I use for camera work. This is quite surprising if you take into account how badly it’s finished aerodynamics wise!
I am already formulating a layout based on an American spy drone as the next plane I will build. I will keep this blog updated once I start with that.