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Drone Crash

This is embarrassing… I was mapping a mountain side and forgot to set my altitude higher to adjust for the slope and crashed into the canopy. Not proud of that. Obviously, the mission aborted and I then retrieved my drone. Was able to successfully process the first 20 though.

Hi Benjamin,

Sorry to hear about your drone colliding with the tree canopy. I hope it’s still flying fine.

To avoid a similar situation in the future, I recommend taking off and landing from the highest point within the extent of your mission. You’ll notice that Pix4Dcapture does not allow you to set a negative mission altitude that is below your drone’s home point. When taking off from the highest elevation in your mission, you will no longer be concerned about the safety of your drone and the overlap of your images. Instead, when taking off at the highest elevation in your mission’s are you must consider the ground sampling distance of your images.

You can see an example of how to plan flights to achieve a consistent ground sampling distance at Note that you do not need to fly multiple overlapping missions at different altitudes if your ground sampling distance at the base of the slope is detailed enough to get the job done at a single, safe, altitude above the entire project area.

Thank you Andrew, though I didn’t post this part of my other question here at 5:14 this morning, this is helpful.

I see in the article you cited, Figure 1 shows what pix4D recommends as the best way to capture imagery above a slope:

Can you describe to me how you would go about this recommended method of keeping a constant height above the ground?

Others: note we are talking in terms of height above ground, not absolute altitude above ground level which most UAVs do not in fact measure. Important to note for execution of mission at planned GSD quality and for compliance with FAA altitude restrictions in the USA.

Back to Andrew: In your words, Pix4Dcapture does not allow you to set a negative mission height. Does it allow me to set a drop in altitude to follow the terrain at a constant height?

I imagine to execute your recommended Image Acquisition Plan for Terrain with Height Variations,"Single flight following the terrain elevation variation (recommended)", I would need to use the Free Flight Mission option? This is definitely a workable solution, just want to know if there are autonomous flight mission options as well.

…I imagine to make it autonomous, I could just run the mission manually in the Free Flight Mission option once, and then re-load the same mission plan each time I need to revisit the site?

Thanks for all your help!


I took the opportunity to split your original post to ensure nothing fell through the cracks.

In an ideal world, you would be able to capture images across your project area at a constant altitude above ground level. However, it is not always necessary to have a consistent ground sampling distance throughout your entire project, and in other cases, it isn’t worth the additional effort. Accounting for changes in terrain becomes necessary when the ground sampling distance at the lowest elevation in your project area would be too coarse for your needs if you were to fly at a constant, single, altitude across the entire project.

Pix4Dcapture does not allow you to set an iterative drop in altitude within a single flight to approximate the changes in elevation of the underlying terrain. If you would like to create a mission with Pix4Dcapture that follows your project’s terrain, you must plan individual flights at different altitudes that approximate the elevation of the underlying terrain. In this case, I recommend that each adjacent mission have at least one redundant flight line to ensure that there is sufficient image overlap so that all of your images can be automatically tied together.

 If you would like to pursue a Single flight following the terrain elevation variation (recommended)_ _I recommend that you do not use Pix4Dcapture’s, Free Flight. My experience has demonstrated that nine times out of ten, flight computers are much more patient and consistent than pilots who are flying manually. Instead, in the meantime, I recommend that you consider another flight planning application that supports your drone and terrain following. I’ve taken the opportunity to share your comments for consideration as part of the ongoing development of Pix4Dcapture.

Free Flight for Pix4Dcapture does not record waypoints based on a manually flown flight path. All Free Flight missions must bemanually flown each time. All the more reason to not use Free Flight for this application.

Please note that this all hinges on the assumption that you are unable to safely capture sufficient image overlap and a fine enough ground sampling distance throughout the extent of your project area with a single flight.

If you are still unsure whether it is necessary to account for the changes in terrain elevation;  ask yourself if the ground sampling distance at the base of the slope will be detailed enough to accurately reconstruct what you need to get the job done when capturing your images at a single, constant altitude. Any ground sampling distance calculator will help you answer that question, including Pix4D’s Basic GSD Calculator. There is no golden ground sampling distance, so if you do not know what ground sampling distance you would like to capture, consider the relationship between ground sampling distance and the accuracy of your final reconstruction as a starting point.

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