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Author Topic: Mavic Pro Survey - Exaggerated heights  (Read 604 times)

Millsy1

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Mavic Pro Survey - Exaggerated heights
« on: July 23, 2018, 09:22:34 PM »
I have been trying to use the Mavic Pro for quantity survey for the last little while. (6 months).

For the last few years, we have used the Phantom 3 with great success. To this end I thought the Mavic Pro would be just as good, but a lot easier to carry around.

But all of the survey's I have done with the Mavic Pro have greatly exaggerated heights of piles, buildings and vehicles. Initially I thought this was an issue with the survey on the ground, however, I have now had a few sites flown at the exact same time by a different drone, and I am seeing nothing unusual in that survey.

https://puu.sh/B1Mo3/8be8e1e008.jpg
https://puu.sh/B1MBz/8c065367c4.png

As you can see in these screen shots, the vehicles in the point cloud look extremely tall. In fact my own vehicle is shown to be 4.3m tall. when in reality, it is 2.4m 
 
I get the same result for each pile as well. The peaks are nearly double the height of the real thing. 
 
This specific survey I used 6 GCP's, the entire area was only 170m x 130m. The GCP's were surveyed using an RTK setup with an accuracy less than 3cm.

https://puu.sh/B1MI2/eb785938aa.jpg
https://puu.sh/B1MKw/b1bdc9aabd.png 
 
I used a 90% forward and 80% sideways overlap for the flights. There is also lots of area outside the survey zone that was flown.
https://puu.sh/B1MPz/2d7461baa5.png


Millsy1

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Re: Mavic Pro Survey - Exaggerated heights
« Reply #1 on: July 24, 2018, 02:44:21 AM »
For additional information, here is a previous survey done, this one with 15 GCP's over a larger area (400m x 400m).

This is a cross section of two large barns, the Mavic survey is the dashed line, the other survey is an RTK enabled drone survey done about a week later.

Clearly I can't get on top of the building to put GCP's down, so there has to be some other solution.

Both survey's were done using a 2.5cm GSD


Erik Holmlund

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Re: Mavic Pro Survey - Exaggerated heights
« Reply #2 on: July 24, 2018, 07:31:26 PM »
Hello Millsy1,
I've had similar issues with analogue nadir air photos, due to the focal length being poorly constrained because of the little changes in perspective. Have you tried adding oblique images to the survey? These greatly reduce errors that are otherwise inherent to the camera calibration estimation. See James et al. 2017 (DOI: 10.1002/esp.4125) for example.

Also, are you shooting JPG or raw? I've read here that the lens corrections in the JPG processing will mess up your camera model in PhotoScan, so that should be avoided.

Hope it might help.