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Author Topic: RTK and PPK with P4RTK for mapping  (Read 2985 times)

geomaticist

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RTK and PPK with P4RTK for mapping
« on: September 16, 2020, 12:26:14 AM »
greetings. I am a land surveyor and use a P4RTK to do typical mapping. It is to map and survey 2D generally and not modeling 3D building etc. I use Agisoft to process the images into the ortho-mosaic. Been doing this for about 3 to 4 years. This year we finally got the ability to use RTK and PPK. And yes it works great.

The point of this post is that for some reason I will see a RTK and/or PPK dataset of images for a mission result in a miss of photo control on the ground. I always have a few photo control points. First time this happens I was puzzled and ended up just shifting final ortho to a photo control point. So eventually I made a local drone research area on the edge of town. I am in Fairbanks Alaska. I have very accurate and precise survey control I set there and triple checked to make sure no blunders. And I have around 20 photo identifiable points on the ground surveyed in. I originally intended to collect drone data to study various ways to process it in Metashape to try to better understand it all. but first flight mission, that captures the whole area (only 195 images), at about 380ft or 115meters above ground, when i processed it in Metashape the ortho-mosaic misses the photo control. i will just use metric units. I processed the RTK corrected images. And I also PPK corrected the images as well and processed those separately. now it is worth noting the coordinate for reference positions for the cameras was basically exactly the same (because of how many digits it shows.) During RTK about 10 images were during a RTK drop. That is reflecting when I load in the images, a few show 'accuracy' of about 1.5 meter. Where the rest are in and around 1-2 cm. However the PPK all came in as 0.5cm or better (on the accuracy column). point here is that the reference positions are the same for same image, except a few where the PPK worked on an image where the RTK had dropped. Other than that, the PPK supposedly is might tighter precision. I realize it is not in real life.

Processing in a typical manner the RTK - the ortho missed the photo control by an average of 15.2cm. but after shifting the image, zeroing out the average move, the standard deviation is 2cm. Since the ground sample size of the ortho is 3cm, this is a very good fit (after shifting the ortho.) processing the PPK images resulted in an average move of 23.4cm. But after moving the ortho it has a standard deviation of 1.5cm. Again very good, but it required being moved.

Right after i collected two more flights of a smaller area (to make processing faster.) And had 45 images. Anyhow the point is the RTK images land right on top of the photo control. And flight after I did with RTK off, and then PPK them and the result lands right on top of the photo control. And the resulting precisions were both 2-3cm.

So the real point of this post is that most of the time when I do RTK or PPK the results land accurately on the photo control with the expected precisions. however, sometime, and I cannot at all explain why, some drone data will not land on photo control and I will have to shift it.

Anyone have any idea why this happens? you can assume I know how to measure and do control. And that I know what base coordinate is being used to send RTK corrections the drone, and so the RTK corrected images are what I expect. And that I know how to PPK correct the images as well. It just is not making any sense to me when sometimes the processed data will land on the photo control.

I am NOT using any control markers in metashape - yes I know how to. I should not need to I would think. The images loaded already have reference positions corrected and have 'accuracies' as well.

Am I doing something wrong? if the rtk or PPK corrections are muddled some how and the reference positions are off - how in heck are they all still very precise?

Anyone experience the same thing or know anything about this?

Thanks. (keep in mind most of the time there is NO shift, the final ortho is centered over photo control. so I am really not sure how it happens sometimes.)

JMR

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Re: RTK and PPK with P4RTK for mapping
« Reply #1 on: September 16, 2020, 10:26:13 AM »
Are you using pre-calibrated camera? are the photos strictly nadir only? I've found that Metashape (as did Photoscan) is not as robust as it could be at aligning cameras with direct reference (PPK/RTK) in non-calibrated case. I have reported this a few times but nothing seems to have changed.
calibrate your phantom with a very well controlled (plenty of GCP) test field and save your params for future use as fixed or semi-fixed and see if it improves your result. It looks like ground elevation and focal lenght are hard to be estimated at the same time in projects with nadir photos only. thus some vertical offset is to be expected whtn no ground control point is invited to the party.
BTW, sadly, I've found that a competitor, does it a bit better.
Regards.

Corensia

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Re: RTK and PPK with P4RTK for mapping
« Reply #2 on: September 18, 2020, 05:05:13 AM »
Not sure it'll make much of a difference, but I was curious if you were using NTRIP or the D-RTK Mobile Station.

Darko

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Re: RTK and PPK with P4RTK for mapping
« Reply #3 on: September 18, 2020, 10:34:50 AM »
Hi,

JMR is right about the camera calibration. Make your own calibration field with as many as you can GCP-s. Make sure what is the precision for your camera coordinates (should be about 2 cm). Collect your image coordinates for GCP-s. Set the precision for your GCP-s to the same value of 2-5 cm. Make the alignment and save your Camera calibration in file for your future use.

Next time when you have your real field data and photos before the alignment go to the Tools, Camera calibration, load values and FIX THEM ALL. That should solve your problems.

I don't have any experience with P4RTK but I am using my own design and PPK with Sony A6000 and 25mm fix focus, fix aperture lens and this works fine.

Good luck,
Darko