GSD & flight line height variation in Highely undulated areas.

Anoop_Singh · August 14, 2018, 3:40pm

Hi,
I would like to know about the flying of hilly areas. I have flown a hilly area of 1 sq km. When I have aligned all the photos then I have found that aligned flight lines have hight variation between the missions and finally got the GSD variation in Orthophoto.
Pease suggest me, how to solve this issue and got no height variation in highely undulated areas.
Thanks&Regards,
Anoop.

momtanu.chakraborty · August 14, 2018, 6:41pm

Hi Anoop,

An easier way to deal with hilly terrain is to launch from the highpoint on your site, or adjust your consistent flight altitude so the difference between the UAV and the terrain will be constant. This post might help: https://support.pix4d.com/hc/en-us/articles/204656209-Image-acquisition-plan-for-terrain-with-height-variations

Anoop_Singh · August 16, 2018, 1:02pm

Hi Momtanu,
Thanks a lot for your response. Suppose flying altitude has differ at the time of flying because highest point UAV launching location selection is not possible for 10-20 sq km area.
In this case, can we control this difference by using ground control points ?

Thanks&Regards,
Anoop.

momtanu.chakraborty · August 17, 2018, 6:31pm

Hi Anoop,

GCPs won’t be useful in this. GCPs increase the accuracy of the project, but in this case, the ground sampling distance is getting messed up. So if it’s a single flight and the height difference (which is related to GSD, you can see the GSD calculator) between the highest or lowest point is twice or higher, the reconstruction might fail.

The best would be if the flight can follow the terrain. At the moment, Pix4Dcapture does not create flight plans that follow the terrain elevation variations. In order to do this, either another application should be used, or it could be done manually. For the latter, a professional drone pilot should make the mission.

Instead, we would recommend using the second option described in the article (https://support.pix4d.com/hc/en-us/articles/204656209), which would be to create several grid flight plans, each at a certain altitude, with a bit of overlap between the different grids. This would enable you to use Pix4Dcapture for the images acquisition and should still enable you to have accurate results.

Anoop_Singh · August 18, 2018, 8:06am

Hi Momtanu, Thanks a lot for your response.

I have another concern related to overlap and side lap. For example, i want to keep 70% overlap and 50% side lap. How is it possible in Pix4D capture because i think there is no option to define the side percentage.

Please explore this…

Thanks&Regards,
Anoop.

momtanu.chakraborty · August 21, 2018, 7:54pm

Hi Anoop,

When you adjust your overlap setting in Pix4Dcapture on Android you adjust your forward and sidelap at the same time. If you set your overlap to 70%, both your side overlap and your front overlap will be 70%. Please read this (https://support.pix4d.com/hc/en-us/articles/203873435–Android-Pix4Dcapture-Manual) to learn more about using Pix4Dcapture on your android device. Unfortunately, side overlap and front overlap cannot be set to different values on Android devices, this feature is available on iOS.