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DJI A2 Based Photogrammetry Results
4587 15 2015-6-8
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HMArnold.msn
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United States
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I have been getting a lot of email questions and forum posts lately about using the PC Ground Station for photogrammetric work, so I thought I would start a thread that summarizes what I have discovered so far.

The good news is that I have been able to get the entire S1000+/A2/Lightbridge/IOSD Mk II/Datalink 900mhz/Zenmuse gimbal/Dual Futaba 14SG system to fly reliable autonomous missions with rational grid distances using the Photogrammetry tool that produce flight information exported using the IOSD DataViewer application.

Combining this flight info with the images from a Sony A6000 camera can be used to produce a mosaic image of the entire grid area using the Agisoft software package.

Although I don't have any way to test it, I assume that these same results can be obtained with any other combination of hardware as long as it flies, takes pictures in response to a standard servo signal, and uses an A2 controller coupled with an IOSD Mk II unit.

The absolute minimal system for flying blind I can think of that could work would not need the Lightbridge, the camera gimbal, or the Futabas because you can see most of the required info on the PC Ground Station screen as you're flying, basic throttle/pitch/roll, etc commands are available on the PC keyboard, and the "RETURN HOME" button causes the platform to return back to the home point with remarkable accuracy and grace without any operator intervention.

All that being said, the bad news is that there are quite a few little operational issues that you have to learn to work around, but nothing that prevents you from bringing home the goods.

I have now flown 50 or so autonomous missions, all with happy endings equipment-wise.

More than half of those missions have been my attempts to characterize the mission planning in terms of altitude, distance between the grid rows, distance between the image points along each leg, and the speed of the platform during the mission such that the resulting images and flight info can be used in Agisoft to create a seamless orthogrammetric image of the target area.

These are the two main things I've learned:

1) The limiting factor to make a mosaic image commercially is the battery. I fly agricultural fields in southern Texas, and some of them are upwards of 250 acres. For an autonomous mission at 75 meters, I can cover about 35 acres with a 15000 mah battery before I'm looking for a soft spot to crash in. The images you get at 75 meters with a 50mm lens show individual leaves on the plants, weeds with enough definition to identify them, areas of standing water, etc. If I fly at 200 meters I can cover almost 70 acres with one battery because of the wider grid spacing, but the resolution at that altitude is limited to standing water and area color changes, which are both important to agricultural imaging.

2) The Agisoft software package is great stuff. I have been thinking of trying the Pix4D product, but haven't gotten around to it because I haven't seen anything written down that says it's better than Agisoft, and so far the Agisoft package has been reliable and repeatable.

The ability of the software to reconstitute a mosaic image is controlled by a lot of things like image quality, availability of GPS coordinates, etc - but the most important for me is the amount that the images overlap, even if you have flight parameters.

There are two different overlap values, and they have completely different risks and benefits.

The most important overlap value is the horizontal overlap, which comes down to the distance between each row of your autonomous mission grid. The greater the distance between each row, the fewer rows you have to fly, so the more acreage you can cover on a single battery. The best of all possible worlds would be such that the distance between each row is such that the image taken covers an area on the ground that just touches the area covered by adjacent picture on the previous leg, and just to the point where the adjacent image on the next leg will start. This would be 0% overlap, and if you sit down and position the images yourself by hand, you could re-create the mosaic image.

I have been able to make mosaic images using Agisoft with overlaps of as little as 35%, which when you really look at a single spot in two adjacent images, isn't a lot - but when you consider that 35% overlap means 35% wasted imagery, it hurts.

If Pix4D or any other software could work reliably with less than 35% horizontal overlap I would be very interested in hearing about it.

The other overlap value is the vertical image overlap, which is the distance between images as the platform flies along each leg. One would think that with a camera like the Sony A6000 that can take images at 10 frames a second until the SD card fills up or the battery runs dry, this wouldn't be a problem, but with an A2 controller, it is.

The only way I know of to trigger the camera shutter on an A2/PC Ground Station setup is by using the "GP SERVO ACTION" definition, which as I understand, presses a button that shoots an infrared signal to the camera that clicks the shutter.

The problem with an A2/PC Ground Station system is that the minimum time you can set for a "GP SERVO ACTION" point is one full second.

If your autonomous mission gets defined in such a way that the image points are closer together than one second, the A2 records the GPS location, but the "GP SERVO ACTION" point has not recovered from its one second cycle, so the camera doesn't take that image.

If you are right at the edge of usability with the vertical overlap, losing a few images messes up the whole show, offsets the GPS coordinates to the wrong pictures, Agisoft gets confused, and the entire mission is basically wasted.

The only solution to the one second restriction that I know of is to slow the platform down along each leg so that one second will still give you the vertical overlap you need to make the mosaic.

Since the lower the altitude, the faster the images need to be taken to overlap, for a 30 meter mission I have to fly at 2 meters a second to have any hope of stitching things back together.

Taking pictures at walking speed is never going to be economically viable.

I don't fault Agisoft for this, I fault the DJI minimum shutter time for having a camera that can take images 10 times faster than that limping along while the A2 finishes it's one second cycle.

At higher altutudes, say 75 meters and above, it doesn't hurt so bad because the fastest I can get the A2 to go is about 8 meters per second, and at that speed there is more than one second between image points.

There are 3 different places you can enter a platform speed on an autonomous mission, but no matter what you speed you enter, after each turns its flies at about 3 meters per second then slowly builds to a maximum of 8 meters per second.

If anyone knows of a way to get the platform to fly faster, please let me know because at 100 meters altitude you could still set great image resolution, stay ahead of the 1 second "GP SERVO ACTION" restriction, and finish a mission in less time if you could fly faster than 8 meters a second.

Also with the Agisoft, every time I have a question or a problem that I send to them in the form of an email, I get an intelligent, thoughtful response within a few hours that either tells me what I'm doing wrong of asks me to send them what it takes to see what I'm doing wrong - then after they see what I sent, I get an intelligent, thoughtful response.

Agisoft support and DJI support are as different as night an day.

When I figure something out and post what I've learned, I have been posting my email address and offering to do what I can to help. It's been a great opportunity to interact with people all over the planet, and we all have the same hopes and dreams

HMArnold@msn.com


2015-6-8
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ggoodlett
Second Officer
Flight distance : 23857962 ft
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United States
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Can you switch to banked or adaptive turns so the copter does not slow down at the corners?

I have aspirations to do photogrammetry with the DJI equipment but I am way behind you. I appreciate your post.
2015-6-14
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HMArnold
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United States
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There are 3 types of turns available that I know of for autonomous missions:

1) Stop and Turn
2) Banked Turn
3) Adaptive Banked Turn

All 3 types in a photogrammetric context are silly.

1) Stop and Turn : The good news is that the platform actually flies to every waypoint. The bad news is that as near as I can tell no matter what value you enter for the hold interval, the platform hovers at every waypoint for at least 5 seconds. Five seconds of absolutely wasted battery is huge when you have 20 or more waypoints in a 15 minute flight. Sometimes, for reasons I have not been able to duplicate and quit wasting time with, I think the A2 holds at some waypoints for as much as 10 seconds before continuing, even if the hold value is zero or one.

2) Banked Turn : I have absolutely no idea what the plan was for this type of turn. For each waypoint, the platform heads that way for a while, then when it gets to some value I don't understand, like 50 meters, it figures it has covered that waypoint well enough and clicks over to the next waypoint in the mission. What that means for a photogrammetric mission is that it covers a portion of the middle of the target rectangle, but so many corners get cut that it really just flies around in the middle for a while.
              
In typical DJI fashion I cannot insert an image because I'm using Microsoft Internet Explorer, but here is a link to an example of a Banked Turn mission:

https://www.flickr.com/photos/125540011@N06/18196215334

The straight red lines show the target area defined by the mission waypoints, and the green line shows the actual flight path. The green triangles show the image points.

In photogrammetric terms this is so silly that it's useless.

3) Adaptive Bank Turn : This is the least silly of the 3 turn types, hence the one I use. The good news is that it covers most of the area in the target rectangle, but the bad news is that it cuts off the end of every grid leg by about 20 meters. The Bad/Good news is that since this cutoff is repeatable, you can lay out your target area with this knowledge, and with time, draw the target area with overrun sections at both ends so that when the leg gets cut off, it's right at or just past the end of the actual area you want covered. With practice, I'm getting pretty good at estimating how much is going to get cut off, and compensating properly.

The other problem with Adaptive Bank Turns is that the very first waypoint positioned by the Photogammetry Tool always gets cut off in terms of both lat/lon and altitude. What this means is that the first half of the first leg almost never gets proper image coverage because the platform rounded it off about 30 meters before it arrived and had not yet reached the proper altitude. There is a difficult work around where you manually add a throw-away waypoint some distance away from the first actual waypoint so that after lift off, the platform cuts THAT waypoint off and lines up much better with the first actual waypoint.

Here is a link to a mission using Adaptive Banked Turns:

https://www.flickr.com/photos/125540011@N06/18632656559

Again the red lines are the target area, and the diagonal red line is because for this mission the home point is in the upper right and the first waypoint was on the far left bottom corner.

As you can see, about half of the first leg was missed by the platform.

If I had to guess, I would say that the guys that designed Adaptive Banked Turns set it up so that the waypoints were a distance MAXIMUM, as in an imaginary box that the platform will fly safely in without ever touching the sides. To do this, the A2 has to cut off it's trip to the waypoint in time to complete the 180 degree turn BEFORE it arrives at the side of the invisible box.

That concept might have some validity if you're flying inside the confines of 4 tall skyscapers, but it makes planning a photogrammetric mission that actually covers the target area a quest of guesses and estimations.

In actual operation it means that if you fly a mission with an A2 platform set up with the Photogrammetry Tool that has 50 waypoints defining your target area, when you finish you will not have a single image that was taken over any waypoint.

It would be nice if DJI added a 4th type of turn that actually covered the actual waypoints, but as far as I can tell, they aren't into making improvements on existing products, just making new products the will in their turn be left needing improvements.
In my opinion, they are letting a huge market of comercial drone use slip away....

As soon as somebody else comes out with a product that will lift a regular camera and doesn't take 6 months of intense trial and error to get working, I'm jumping ship.




2015-6-14
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yoyoma
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United States
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Hi HMArnold-

I appreciate all the good information you've been providing. Was curious if you looked at the 3DR X8-M? I am considering it. I don't believe it can carry a DSLR, just a 12.1 MP Canon Point and Shoot, but its seems to be a total turn-key aerial mapping package. One drawback of the X8-M is the short flight time of 14 minutes (depending on altitude can cover 25 acres), but then again in comes with a couple of batteries.

Have you flown a mission on your DJI where you have to switch the battery out and resume flying? Is that a fairly quick transition or are there additional steps to make sure the craft resumes where it left off?

Thanks - YYM
2015-6-15
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gmarc007
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HMArnold Posted at 2015-6-15 10:28
There are 3 types of turns available that I know of for autonomous missions:

1) Stop and Turn

HM,

You can set hold time on each way point. I use adaptive over the other two because it cuts down on mission time.

If you use "stop and turn"  you would have to go into each way point and adjust hold time.
I set it to 0.

Also on every mission I Delete waypoint Zero " 1 then becomes zero" which saves time as well.
2015-6-15
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HMArnold
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United States
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YoYoMa,

There may be a way to pause the mission, return home, change the battery, then pick back up again but I have never tried that.

When you land and disconnect the battery, when the A2 wakes up again I don't think it has any memory of the unused waypoints from the previous mission. If you were to press "CONNECT" to hook up the Ground Station software, the pause button isn't active because there is no mission in progress.

When I'm flying an area that needs more than one battery I just note the place it left off and try to recover that leg with the first leg of the next mission.

If you or anyone else has a way to accomplish such a battery change, I would appreciate hearing about it.

2015-6-15
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gmarc007
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United States
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HMArnold Posted at 2015-6-16 04:55
YoYoMa,

There may be a way to pause the mission, return home, change the battery, then pick back up ...

Just  reload the mission again and use the starting waypoint feature.  So if you ended at 14,set to start at 15.
"You can set which waypoint you want to start at."
Let me know if you have any questions.
2015-6-15
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HMArnold
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That's a very good idea

I'll give it a try

Thanks
2015-6-15
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steve
lvl.2
Flight distance : 1276983 ft
United States
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Hi,

To speed up the rate of shooting how about:

- use the Sony "time-lapse" app. It is $10 from the Sony app store. The shortest time on it is 1 sec, but you can also set it to "continuous slow" which works out to 1/3 sec.

- Get an external invalometer from Amazon and set it to trigger the camera at whatever interval you'd like.
Either way with a big SD card the battery on the copter will run out first.

I'm eager to get this kind of thing up and running with my s1000+/a2/ios/ usual suspects copter.

I don't know if integration with the A2 is important or not, but in the software I use to stitch large numbers of images together it isn't.
2015-6-21
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HMArnold
lvl.4
United States
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I tried the Sony Timelapse app early on.

If you don't care about GPS and other flight data for each image point, that could work.

Since I don't normally fly low enough to need that kind of rapid coverage, to me the 1 second restriction is a silly hassle, but its not stopping me from flying data sets that stitch back together.

I have tried using Agisoft to create orthogrammetric mosaics without GPS/Alt/Heading info, but it seems to get confused about things like the height of trees without it.
2015-6-22
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steve
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Flight distance : 1276983 ft
United States
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HMArnold Posted at 2015-6-22 19:07
I tried the Sony Timelapse app early on.

If you don't care about GPS and other flight data for eac ...

Ah - I reread the post - I misunderstood the rate limiting step.

How to get the copter moving faster I don't have any insights into.

2015-6-22
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megt
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Norway
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I'm trying to set GP Servo Action points from autonomous missions but can not get the gimbal/A2 to shoot the camera, I have Lightbridge + A2 + IOSD Mark II + zenmuse Z15 A7 if someone can help me to configure this  GP servo action i will appreciate a lot!
For the Sony camera i have bought timer remote shutter and it work perfect
http://www.dx.com/p/1-2-lcd-digi ... lack-2-x-aaa-338796

2015-6-22
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HMArnold
lvl.4
United States
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First, does your shutter release work in manual flight through the Zenmuse?

Next, when you want to take GPSA images, you have to run a 3 wire jumper from the F2 port on the side of the A2 main controller to the SHUT port on the side of the Zenmuse.

Next, have you disabled the A2 gimbal control in the A2 Assistant?

See number 7 here...

http://forum.dji.com/thread-7679-1-1.html

For my Sony A6000, in PC Ground Station I use default = -1000 and then the action is +1000 for one second
2015-6-23
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megt
lvl.1

Norway
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Thank a for the asnwer and the link! HMArnolD

I do have that configuration and it doesn't work  

Im still trying  to find out what can it be....

2015-7-6
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Geronimo13
lvl.3
Flight distance : 505878 ft
France
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Hi HMArnold,

Your description issue with the speed, type of turn  and trigger setting is perfect. I also spent 3-4 month to understand how the setting in  the Ground Station software is working.
As a conclusion, There is a lot of bug in the DJI software (Ipad or PC Ground station).
What is your best configuration to manage the speed of your uav during its autonomous fly ?Mine is Default speed in adaptative, with automatic and repetitive 1 second shutter.

my config is s900/a2/ligthbridge/datalink with GH4


2015-11-8
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skyvideoct
Second Officer
Flight distance : 1135939 ft
United States
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I have 3 DJI birds, but use my 3DR X8 w/ Canon xs260 and the open source groundstation to do Class A2 survey work and render in Pix4D. It works great every time and I am also a qualified pilot for Skycatch/Workmode.
2015-11-12
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