I have 2 layers of points in the point cloud. The upper layer have detail of the terrain such as roads and mine pits, but much less points than the layer below. The lower layer has much more detail, however it seems to terminate (last image), with the upper layer continuing to cover a majority of the survey area.
How do I deal with cleaning this up? Add mtp and reprocess, or delete the upper layer where there is overlap and re-run step 3?
Hi APEX Geoscience,
Thanks for posting screenshots and for describing the issue. Could you also post the quality report of your project (e.g. a link to a Google Drive or Dropbox document)? This would give some more hints on what can be improved.
I would not recommend to delete the points, because the model was not correctly calibrated, so it might not be accurate even if the points are deleted. Instead, I would try one of the following (once I see the quality report I might be able to tell more):
Add Manual Tie Points (MTPs) and mark them in images that were used in the reconstruction of both layers. I would add about 5 and then click Process > Rematch and Optimize. Here is how MTPs can be added: https://support.pix4d.com/hc/en-us/articles/202560349
This type of issue tends to happen if there are more than one flight over the same area with different illuminations, which results in the software having some issues recognizing the same points. The project seems also rather large, so you could try splitting the project into smaller parts (that should still overlap between each other), process step 1. separately for each of them, and then merge the project. More about that here: https://support.pix4d.com/hc/en-us/articles/202558529
Note, you can make a copy of your project with Project > Save Project As…, so that you can try changes without losing your current project.
Same thing happened in this thread, https://community.pix4d.com/t/3387-My-project-was-divided-into-two-blocks-with-a-vertical-gap-of-about-5-meters#gsc.tab=0 and since it was just 1 block, the MTP option didn’t work.
I bet this project has multiple cameras like the other one and thus we are seeing a trend where letting your camera run in “auto” across significantly different conditions will seriously hurt processing. Once we see the quality report here then we will see how similar these two projects are in their data collection variables.
I had added 5 manual tie points. I optimized and am now re-processing steps 2-3. I cannot upload the quality report, as I sometimes have an issue where the .pdf does not generate ( issue for another time). Step 1 did generate 3 blocks, georeferencing has a mean RMS erros of 1.615m. This is including 5 3D GCP. I cannot remember is this was an issue with the original processing. I will wait until it completes to look at the final result.
APEX, even a screenshot of the first page of the Quality Report helps. Since you have 3 blocks then those certainly need MTPs or done as sub-projects and merged.
When using MTPs, I suggest trying to match with at least 7-8 images with a preference of 10-12 to achieve the best results.
If you need a more in-depth look at your data then shoot me an email at Adam.Jordan@nhiae.com so I can process on my “super” computer. I am pretty sure my rig is better than what Pix4D has in-house :)
@APEX Geoscience: how did it go? I agree with Adam that blocks should be removed, usually adding MTPs between them and to Process>Reoptimize helps. If they are not removed, you will have different parts of the project that are optimized separately, which results in some areas not perfectly fitting together.
How should I go about removing blocks? Removed the physical photos that make up those areas? remove the key matches or points from the point cloud?
I added 12 MTP @ ~ 15-30 photos matched for each. The results is exactly the same as before. I noticed int he orthomosaic that where these 2 layers overlap there is a contrast difference. I expect there was either a large shadow, or these 2 layers were flown on different days with a vast lighting difference.
But even if there was a difference in lighting, wouldnt the georeferencing of the raw image still have an elevation of roughly the same value? If one set of photos was flow at 100’ and the next day the same area was photoed again @ 300’. Wouldnt Pix4D match them together based on the terrain features and give the ground surface the correct elevation across both sets of photos?
Thanks for the help everyone
APEX, unfortunately with the project I was given the photos for in the link above, I couldn’t fix it with MTPs. Every project is different but this is VERY similar to the one I helped Roy with so you can either process as sub-projects and merge or fly new pictures with consistent exposure and focus.
The geo-referencing is just an initial start, which I have had it a few hundred feet wrong and Pix4D fixes it by visual matching. I am not the software coder here but I am 99% sure that GPS won’t over-ride the flaws in visual matching (GPS can be off by 5 meters and visual matching is 1X-3X GSD)…I even tried setting the MTP to GCP and no luck there either.
Agreed with Adam.
APEX, normally blocks can be removed by adding MTPs and using Process > Reoptimize. When there are two layers like this, Process > Rematch and Optimize should help merge the two layers. However, if these two did not work, I would suggest the merging procedure as in the first comment. It’s a shame that the quality report does not get generated, would have been helpful.
Hi,
I think I have the same issue, which is also described here: Point Cloud Error - Overlapped points.
In several areas of my projects I got a situation when resulting orthophoto was skewed and there were unfixable errors with GCPs. Orthomosaic becomes blurred in such areas. I think I found the reason for it. It seems that the generated point cloud becomes sliced and discontinued (see pictures) at some points. It is strange because there is only one block reported.
I tried to add manual tie points (several) in those areas of discontinuities but it had no effect on the cloud. I also tried to introduce one of such tie points as a GCP. This led me to the big error for that GCP, showing no effect again.
I tried different point cloud resolutions but it also does not help.
Which option should I use and on what stage to correct this behaviour?
Unfortunately I am not able to add more pictures.
Regards,
Hello and happy new year!
Please attach the quality report of your project after adding the Manual Tie Points, so that we can further investigate.
Happy new year, Christina!
At the moment the problem is gone and the report is overwritten since I’ve made some attempts from the initial step.
Thank you.
Dear Christina,
I have the same issue:
When the drone can not be flew on the same elevation, so the images also have different elevation, but we tried to put them altogether in one project, the result almost always is an double layered pointcloud as you can see on the oicture below.
Despite of several MTPs, the Reoptimizing did not solve the problem, and if we use more photos, some of them displayed as an incorrect mark, but in real it is a correct one (as you can see below).
We tried to divide the project into parts, then run initial processing, created MTPs finally merged them into one model, but it did not help.
Can you suggest something to solve this problem? I hope we do not have to survey again…
Many thanks,
Laszlo
Hi @abraham.laszlo,
Thank you for your request, and welcome to the Pix4D Community 
.
This can happen when there is an important change in illumination between the two flights.
When creating Manual Tie Points, make sure you mark the same point in images that were used in the reconstruction of both layers.
Also, would it be possible for you to share the quality report with us?
Regards,
Daniele
Hi Daniele,
It is a newer flight, but the same issue. We had to pause the flight during the survey, due to airspace restriction, therefore there was a little time gap between them.
Firstly, I tried to do the initial processing for the whole project, but the “double layered” effect had formed. Then I tried to split it into two. At one of them, I only had two GCPs on the area, so first I georeferenced the other one and then I created two more GCPs from MTPs. (I created two MTP on the second one, then create the same two point to MTP on the first one, finally I transformed them to GCPs with the second one’s coordinates). Then I did the reoptimization on the second one too, which went well.
Finally, I merged these two well georeferenced project into one, but the merged project was “double layered” again.
Here is a printscreen of the merged project’s GCP:
I also attached the quality reports of these projects.
First part name: M1-Páty_szeptember_1
Second part name: M1-Páty_szeptember_2
Merged project name: “M1-Páty_szeptember_v2”
I hope you can suggest a solution for this issue.
Many thanks,
Laszlo
M1-Páty_szeptember_1_report.pdf (1.9 MB)
M1-Páty_szeptember_2_report.pdf (1.6 MB)
M1-Páty_szeptember_v2_report.pdf (1.8 MB)
Hi Lazlo,
if you compare the GCPs that you created on both projects, do you see the same elevation?
If you want to merge two projects, please use Manual Tie Points as explained on this page:
Hi Daniele,
Yes, the GCPs have the same coordinates (elevation as well).
I thought the GCPs work as MTPs during the process of merging two projects. So shall I transform back the GCPs, which I created, into MTPs after reoptimizing and before merging the two projects? Should I create some more MTPs too, can it help maybe?
Many thanks,
Laszlo
Hi Lazlo,
Yes, the GCPs work as MTPs during the process. GCPs, same as MTPs need to share the same name among projects. Also, MTPs and GCPs needs to be marked in several images (minimum: 3 images, recommended: 5 images, better if more).
Comparing the two quality reports (screenshots below), it is possible to see how only two GCPs have the same name. In the first project, the GCPs are marked in only 3 images, in the second project, the GCP 6 is verified in only 1 image, which is not enough.
Please mark the GCPs in more images and create more MTPs in the shared area between the projects.
Note:
Verified: The number of images on which the Manual Tie Point has been marked and are taken into account for the reconstruction.
Marked: The images on which the Manual Tie Point has been marked.
Hi Daniele,
I think I did not generate a quality riport after I rename the GCPs, so they had the same name when I tried to merge them, but in the reports they were different.
So I marked the GCPs on more images, and I created more MTPs as you suggest.
And I noticed that, I missed the reoptimeize step, after I created the MTPs as described in the description you sent. Maybe this was one of the key moments I missed, but it was necessary.
I also attached the refreshed quality reports.
M1-Páty_szeptember_1_report.pdf (1.9 MB)
M1-Páty_szeptember_2_report.pdf (1.6 MB)
M1-Páty_szeptember_v3_report.pdf (1.8 MB)
Many thanks again,
Laszlo
Hi Laszlo,
Thank you for the additional information.
Yes, you need to Reoptimize each subproject after adding GCPs and MTPs and save them. Menu Process > Reoptimize
Please let me know if you are getting better results.
