High absolute camera position and orientation uncertainties Z component

Hi
With the use of Pix4Dmapper so far, we have not encountered any problems that I will briefly try to describe, so please help.

An area with low altitude differences (flat area) was captured with dji phantom 4 advanced in 2 sets of photographs with a flight height of 75 and 160 meters. In the initial processing of both projects, a high value of mean Z component of the absolute camera position and orientation uncertainties was present in the report: for the mission with 75 meters 0.208 m and for the mission with 160 meters 0.183 m (in this mission new parameters for camera had to be used because of high difference between initial and optimized internal camera parameters and restart the first processing step was needed). After processing 2nd and 3rd steps (after loading the GCPs and reoptimizing) the Z values for the 75 m mission was 0.429 m, while for the 160 m mission was 0.772 m.

Part of the subject area was recorded using classical surveying methods, and there is a large discrepancy in the height of the captured details and the same details from both models, the details of the models from photos from 70 meters are on average 15 cm lower from the same details taken by the classical surveying methods, while the details of the models from photographs from 160 meters are on average 30 cm higher than the same details taken by classical methods. The height difference of 2 models is approx. 0.5 m

In attachment I am sending you quality reports for initial processing and model for both missions.

19_SELNICA KAMING 75 inicial report.pdf (1.5 MB) 19_SELNICA KAMING 75 report.pdf (1.6 MB) 19_SELNICA KAMING 160m initial report.pdf (2.2 MB) 19_SELNICA KAMING 160report.pdf (2.3 MB)

Hi Damir,

I assume you did not acquire an RTK/PPK image dataset. If your dataset has been acquired with a consumer drone that relies solely on a standard GPS receiver, then the absolute accuracy will lie within a few meters. Note that image geotags are usually measured with an accuracy between 5 m (X, Y) and 10 m (Z).

In projects with image geolocation and without GCPs, the absolute camera position uncertainty will be similar to the expected GPS accuracy. As all images are positioned with similar accuracy, the sigma reported in the table should be small compared to the mean (which is also your case). The absolute camera position uncertainties are also expected to be bigger than the relative ones in the table Relative position and orientation uncertainties.

In your case, I would recommend using GCPs. GCPs are definitely more accurate and reliable than image geotags recorded with standard GPS. They make the model more robust and accurately georeferenced. If you are using GCPs, it is likely that some of the cameras will be moved significantly with respect to the initial geotags. This would be expected due to the low accuracy of the image geolocation.

Hopefully this helps.

Best,
Teodora

Please in theory what does absolute camera uncertainty mean?

Hi Eddy,

Welcome to the community, and thank you for your question.

Absolute camera uncertainty refers to the degree to which PIX4Dmapper is confident in the computed position and orientation of each image.

PIX4Dmapper considers many different sources of information to produce a result that exhibits the least amount of error possible. Examples of different sources of information PIX4Dmapper takes into consideration include manual tie points, automatic tie points, initial image positions (geotags), and ground control points.

If all of those sources of information agree with each other, PIX4Dmapper will generally have a greater amount of confidence in the computed position and orientation of each image. On the other hand, if the inputs conflict with each other or point PIX4Dmapper in different directions, PIX4Dmapper will have less confidence in the position and orientation of each image, which corresponds with a greater amount of uncertainty.

In projects only with image geolocation, the absolute camera position uncertainty should be similar to the expected GPS accuracy. As all images are positioned with similar accuracy, the sigma reported in the table should be small compared to the mean. In such projects, the absolute camera position uncertainties may be bigger than the relative ones in the table “Relative position and orientation uncertainties”.

In projects with GCPs, a large sigma can signify that some areas of the project (typically those far away from any GCPs) are less accurately reconstructed and may benefit from additional GCPs.

For visual reference, the dark green ellipses illustrate the absolute position uncertainty. You will notice how the amount of uncertainty increases or decreases depending on the distance from positions of higher confidence as well as its proximity to the edge (less information).

References:
Quality report specifications
Quality Report Help

I hope this info helps, and look forward to seeing you in the community more.

-Jon

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