Various Flight Altitudes Affecting Stitching Process

I can’t find on their website but I’ve been alierted that flying multiple missions, that will be stitched together, to create a 2D ortho for 3800 acres…at various altitudes, will cause issues when trying to stitch all missions together for final / single 2D ortho.

The area consists of several small mountains that range from 200-350 ft higher than valley floor. There are various structures on both these mountains that are critical to capture, as well as same importance of valleys. The thought was to fly the mountains alll at a higher altitude (for safety and clearance) and then fly the valleys at a lower altitude…both areas would be flown relatively the same approximate distance to the ground. The UAS being used doesn’t have terrain follow.

ANY specific insight or details would GREATLY be appreciated.

Hi @davida,

Thank you for your question.

For the project you described, I agree with you and would suggest flying multiple flights attempting to keep the ground sampling distance (GSD) the same across the entire project area. With such changes in elevation, this would mean flying above the mountains at a higher altitude than you would over the valley. The goal is to maintain a similar distance between the camera/drone and the ground across the entire project area so that the GSD remains the same. Since your drone does not have terrain follow, this will have to be done manually.

To go a little further, as you can see in the diagram below, if you keep the same altitude across the entire project area, the GSD will substantially differ between the valley floor and the mountains. If you do not change the flight altitude to accommodate these changes, then several risks will be introduced. The first is that the drone might crash into one of the mountains. The second is that if you set your altitude based on A, then there might not be enough overlap for images captured by B. Since the distance between the ground and the camera will be reduced, the overlap might not be sufficient. Alternatively, if you set your GSD and altitude based on point B, then when the drone arrives toward A, there will not be enough detail in your images to properly reconstruct the project area.

I hope that confirms your initial suggestion. You have entirely the right idea to change the altitude based on the landscape to maintain a similar GSD.¨


​Dr. Ryan Hughes (He/Him) | Technical trainer and content creator​

Though I appreciate your response, I am in tune with all that data. My original question was specific to any known issues w/Pix4D in altitude variances…and what are the maximum tolerances to avoid known issues.

If anyone has this specific information, I would greatly appreciate your response…

Hi @davida, there can be issues related to the ground sampling distance (GSD) of your images. In what follows, I am assuming that you apply an image overlap that is ideal for your image content.

If you fly the whole area in one continuous mission while keeping a constant distance between camera and ground, the GSD in your images should not vary too much, and there should be no matching issues between your images to create the model. However, this is difficult to achieve in a mountainous region.

If you break up your projects area into various flights at various altitudes, we usually recommend following the recommendations in How to process images taken at different Flight Heights / GSD?: the difference in height between two consecutive flights should not exceed 2*the higher flight height.
In other words, our photogrammetry software will have more chances to match a feature captured in two datasets with different GSDs if those GSDs are not too different by a factor of 2.

I hope this helps, and good luck! Do not hesitate to let us know how it goes, we are always happy to showcase interesting projects.