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Author Topic: Systematic Z Error in Proportion to Altitude (Phantom 4 RTK)  (Read 125 times)

Dragline

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Systematic Z Error in Proportion to Altitude (Phantom 4 RTK)
« on: December 08, 2018, 08:49:16 PM »
I recently acquired a Phantom 4 RTK, and have been working with it for the past week. However I seem to be encountering systematic errors in the Z axis. The X & Y values seem to be spot on though.

For instance, I just created a map of an area with significant vertical relief (100m within 30ha). Within this area I have 3 points that I have measured with our survey equipment, I know these points to be good within ~2cm vertically. When I compare these points to a DEM produced by photoscan I find results like this:

Surveyed point, Elevation (ft)   DEM, Elevation (ft)      Difference (ft)
1557.6                                          1557.6                        0
1428.1                                          1430.4                        -2.3
1382.6                                          1386.1                        -3.5

Some ideas that I have to fix this problem:

*Enable Dewarp in flight planning software before flight. I believe that in the drone the camera calibration will be applied
*Import the dewarp camera parameters into Agisoft, somewhat covered at the end of this thread (http://www.agisoft.com/forum/index.php?topic=9910)
*Very heavy use of the gradual selection tool (http://www.agisoft.com/forum/index.php?topic=8016.0)

If there is any interest in taking a look at this data set, I can send it out along with control points.

Paulo

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Re: Systematic Z Error in Proportion to Altitude (Phantom 4 RTK)
« Reply #1 on: December 08, 2018, 09:40:33 PM »
Dragline,

it would be very interesting to have some data set to test it... Using RTK to just fix the camera perspective centers can often bring a problem in elevation as focal is directly correlated to flight altitude... So the higher you are from ground, the Z error will increase relative to some error in focal…..
Best Regards,

Paul Pelletier

Dragline

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Re: Systematic Z Error in Proportion to Altitude (Phantom 4 RTK)
« Reply #2 on: December 08, 2018, 11:31:09 PM »
Paulo,

Thanks for the consideration. I was planning to collect a test data set today with lots of control points. However it has begun to snow quite a bit. However, when I do collect this data, I will be sure to send it on over.

Paulo

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Re: Systematic Z Error in Proportion to Altitude (Phantom 4 RTK)
« Reply #3 on: December 08, 2018, 11:50:18 PM »
Great,

to decorrelate the calibration parameters as well as to eliminate bowl effect, I would suggest capturing additional obliques photos to the traditional grid pattern coverage, maybe a few going up to the block and a few after terminating the grid collection....
Best Regards,

Paul Pelletier

Dragline

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Re: Systematic Z Error in Proportion to Altitude (Phantom 4 RTK)
« Reply #4 on: December 13, 2018, 09:23:47 PM »
Hello Paulo,

I have had a busy few days! However I got a chance to collect another dataset yesterday. It includes obliques. This seems to have fixed most of the problem, however there is still some error there. The error seems to be just a vertical shift though. I'm not quite sure where this is coming from yet. I will PM you a link to the data for you to take a look at.

The control points are in EPSG: 26734. I just use the Agisoft coordinate calculator to convert the photo locations to this system.