Tilted results

We need the orthomosaic of a small (~300x400 m) urban area (park, max. 10-15 m high buildings, streets, square).

We have made a vertical image acquisition (camera pointing to the nadir direction) of the area, grid flying at 60 m altitude with a DJI Phantom 4 - the result was about 350 pics. Overlapping of the images was as high as recommended by Pix4D. We used high precision GCPs (collected with RTK GNSS) for  georeferencing.

The generated orthomosaic was odd, as some of the building facades (facing to the same direction) was viewable on that, although that should not, as the facades were perpendicular to the ground.

We have checked the densified point cloud - realized that these facades were also generated as not perpendicular to the ground. Checked the result and the report of the initial processing - realized that neither pics facing to the nadir direction according to the Pix4D, but all of these are tilted a bit (for some degree, all with the same amount), not parallel to the XY plane.

Do you have any idea why all the results are tilted and how to correct it?

Hi Laszlo,

We discussed this question in more detail in the ticket you have sent us.

Keep in mind that the Phantom 4 has a linear shutter camera that can be affected by rolling shutter effect which is most probably the cause of the issue you are facing. You can find more information about the effect here: https://support.pix4d.com/hc/en-us/articles/208460436

In a rolling shutter readout, the sensor is exposed and read line-by-line, instead of the entire sensor being exposed at once. This can lead to distortions when capturing fast-moving objects or when capturing the image while moving at a high rate of speed.

The rolling shutter effect becomes significant when flying the drone fast or when flying at a low altitude. In these cases, it is recommended to use the rolling shutter algorithm that Pix4D has developed to acquire more accurate results.

As a side note, it is recommended to use the Linear Rolling Shutter option if the Vertical Pixel Displacement is bigger than 2: https://support.pix4d.com/hc/en-us/articles/115000409383

If we take into consideration your camera model and the flight altitude of 60m then the Vertical Pixel Displacement for your specific project could be, depending on the speed of the drone, around the value 7.

We processed the images you have sent us and we were able to get better results, with the building perpendicular to the ground, after applying the rolling shutter algorithm.

Best regards,