Inconsistent contour data

PIX4Dmapper
1

I have been using Pix4dmapper to generate contour lines and have had little to no success with consistency between flights that are sometimes flown in the same day. We take at minimum 5 ground control points and use them in the flights and the contour lines are sometimes 5-6 feet off in the area around these points. I have rematched and re-optimized several times but i’m still getting one point in particular that is 6 feet off of a confirmed GCP.

We do use the rapid/low res function to run all of ours flight because we haven’t seen a dramatic difference between running the standard processing. With this flight in particular I have areas that are dead on when comparing GCPs to the surrounding contour lines and some that are way off.

We are trying to determine quantities from an overburden stripping project and it is nearly impossible to do when these lines are so inconsistent. Any insight on this is appreciated.

2

Would you please upload the quality reports and several screenshots?

3

Here are the Quality reports for the two separate flights of the same area.

The first (Holcim 2021_quality_report.pdf (3.6 MB) ) is the flight before the overburden stripping was done and the second (Holcim Quarry East 4-9-21_quality_report.pdf (1.6 MB) 4-9-2021) was done last week to track how much has been excavated.

We used some of the same GCP’s and also added a few more in the second flight as we were planning on mapping a larger area this time.

Here are the two DTM contour maps that were generated from each project.

First flight:

4-6-2021 Flight Contour
4-6-2021 Flight Contour1138×907 79.8 KB

Second flight:

4-9-2021 Flight Contour
4-9-2021 Flight Contour756×921 38.9 KB

These were two flights of the exact same area last week with identical GCPs including several on a paved road that clearly generated different contours.

Close up of road with both flight contours shown.

4-6 vs 4-9
4-6 vs 4-9855×469 40.8 KB

4

Thank you for uploading the quality reports and screenshots. It is helpful to understand the issue. According to the report, the GSD and the coordinate systems are different which could be the reason why there is no consistency. Would you please use the same settings and see if it works?

Kind regards,

5

The coordinate system is the same. However, the GCP’s for the 4-9-2021 flight came in too far above the generated map so I had to adjust the output so I could successfully locate and mark the GCPs in pictures. I don’t see how changing that part of the coordinate system would affect the contours that are generated seeing as they were generated using the same GCPs.

6

Thank you for the explanation. There could be a difference even if the images are taken on the same day as the weather condition or sun directions are different. I understand that there are areas that are not well reconstructed on your project. To improve the result, I recommend adding MTPs in the problematic areas that could help to have a better result.

How to add / import and mark manual tie points (MTPs) in the rayCloud

Warm regards,

7

I have attempted to do that and while the contour data did seem to turn out better, I am curious if there is an advantage/disadvantage to using the included DTM contour generation or using the DTM file to create contours on a different program such as QGIS?

8

What I can think of is the efficiency as if you can create both DTM and contour lines in one software. I will let you know if anything else comes up.

Warm regards,