orthomosaic : wrong size buildings

hi,

je rencontre un problème assez conséquent pour mes activités de géomètre.

j’ai réalisé dernièrement une mission pour dimensionner des bâtiments industriels.
j’utilise pour caler correctement les reconstructions 2D et 3D des GCP3D.
précisions : j’utilise

  • DJI mavic 2PRO
  • pix4D capture V4.13.1
  • pix4D mapper V4.5.6

Pour reconstruire les orthomosaics, je fais une captation avec pix4D capture, capteur en position NADIR et des taux de recouvrement entre 70 et 80%
l’acquisition s’est parfaitement bien déroulée.
cependant à la reconstruction , je me suis rendu compte que les proportions des bâtiments sont totalement fausses (plus de 2 mètres de différences avec la réalité !!! cf capture jpeg)

précision : j’ai utilisé différents Template aboutissant tous au même problème de dimensionnement.
procédé de traitement : étape 1 puis référencement des 3DGCP puis ré-optimisation avant de faire les étapes 2 puis 3.

D’un autre coté, j’ai effectué la reconstruction 3D du site avec acquisition des photos manuellement directement avec DJI go4.
même procédé de traitement : étape 1 / référencement des 3DGCP puis optimisation / étape 2 (étape 3 non effectuée- je traite la 3D avec et non la 2D)
dans ce cas ci, les proportions sont parfaites.

précision : le dessin dwg superposé sur l’orthomosaic est juste et présente bien le problème rencontré avec le traitement 2D de l’orthophoto.

le problème est assez grave étant donné les engagements que j’ai vis à vis des ces études géomètres et je ne peux plus utiliser le logiciel tant que je n’aurais pas compris cette erreur.

j’espère avoir été clair dans mes explications.

donc, pourquoi cette erreur dans le traitement 2D?
merci pour votre aide

I encounter a fairly significant problem for my activities as a surveyor.

I recently carried out a mission to design industrial buildings.
I use to correctly calibrate the 2D and 3D reconstructions of the GCP3D.
Details: I use

  • DJI mavic 2PRO
  • pix4D capture V4.13.1
  • pix4D mapper V4.5.6

To reconstruct the orthomosaics, I make a capture with pix4D capture, sensor in NADIR position and recovery rates between 70 and 80%
the acquisition went perfectly smoothly.
however during the reconstruction, I realized that the proportions of the buildings are totally wrong (more than 2 meters difference with reality !!! cf jpeg capture)

precision: I used different Templates all leading to the same sizing problem.
processing process: step 1 then referencing of the 3DGCPs then re-optimization before performing steps 2 then 3.

On the other hand, I carried out the 3D reconstruction of the site with the acquisition of photos manually directly with DJI go4.
same processing process: step 1 / referencing of 3DGCP then optimization / step 2 (step 3 not carried out - I process 3D with and not 2D)
in this case, the proportions are perfect.

precision: the dwg drawing superimposed on the orthomosaic is correct and presents the problem encountered with the 2D processing of the orthophoto.

the problem is quite serious given the commitments I have with respect to these surveying studies and I can no longer use the software until I understand this error.

I hope I was clear in my explanations.

so why this error in 2D processing?
Thanks for your help

Hi Hugues,

Please could you share the project quality report?

I am looking forward to your answer.

Have a very nice day!
Best regards,

Heydi

hi,

thank you for your support;
here is the report:
JV2D _V31_report.pdf (2.9 MB)

hi everyone.
nobody has an idea of what happened?
I still try to generate good data but until now i still have the same issue…

thank you

Have you checked if yours GCP have the correct coordinates? It seems that you only put in 3 GCP, and a little mistake in the coordinates is enough

Hi jorge
and thank you for your help.
effectively, the mistake seems to come from the GCP.
I try without GCP et sizes are correct. But when i re-optimize the project with 3D GCPs, mistake comes…

But now, how explain that i have also used this 3D GCPs to generate the point cloud (in a new project and with 3 times more pictures) and it is pefect with the good sizes.

Hugues,

A few points and I hope that I am right. If I make a mistake in this answer I would appreciate people correcting me for a better understanding of Pix4DMapper!

  1. Jorge comment is absolutely correct. You only have 3 GCP’s, I would most definitely advise to use more.
  2. The GCP’s are not properly situated. These should be at the edges of the project area and also evenly spaced in between. On page 6 of your quality report you see that the relative camera positions error increases at the edges of your project area.
  3. The most striking point from your quality report is the absolute geolocation variance on page 7. There you see a Z error RMS as large as 5.7 meters! This table evaluates the quality of the image geolocation. A high error might indicate a poor GNSS receiver on the drone, or some other error such as noise or a poor connection between the camera and the GNSS.

My conclusion would be that the DJI MavicPro2 GNSS receiver is not accurate enough for precise aerial surveys unless you have very good GCP’s. When align the project without GCP’s you indicate that the building sizes are good (i.e. relative accuracy) , however in that case I would doubt the absolute accuracies.

One issue that I do not understand are the manual tie points (page 7 of the quality report). These also display a large error. Have you added these manual tie points?

How did you measure the GCP’s?

HI pieter,

really thank you for your complet analyze.
I am absolutly agree with what you, and jorge, said.
wrongs locations of GCP, not enough GCP… but it has been given to me like this…
(GCP have been mesured with a RTK trimble)
The drone used, is not made for arerial survey without GCP…

finally, in that study, without GCP, i have a quite good relative accuracy but un wrong absolute accuracy and for a good absolute accuracy, i don’t have enought GCP…
manual tie point have been added manually
you seemed to be really aware about arial survey. whant kind of drone do you used? (P4RTK or something else)

thank you again

Hi Hueges,

I don’t really use any drones myself anymore. I now teach photogrammetry and in the process of becoming a Pix4D Trainer. The drones I would suggest are the Phantom 4 RTK or the DJI M300 RTK with the P1 camera.

Hi. I use a Mavic Pro 2 with a PPK antena by Topodrone. But always use GCP as it’s mandatary to have correct data.