Equation for adjusting sidelap/overlap to account for altitude change?


#1

The effective sidelap and frontlap decreases as the altitude above ground decreases (all other parameters being equal such as shutter frequency, etc). I’m flying a site that isn’t flat and at one point the ground elevation will be 100ft higher than the starting point of my mission. DD does not account for topography changes so if my AGL setting is 200ft, at that high point the drone will actually only be 100ft AGL. So if I want a 75/75 effective sidelap/frontlap I need to increase the setting in DD to be more than 75/75. I can’t set the AGL for the mission at 100ft as the drone will run into the ground.

How do I calculate the increase in frontlap and sidelap I need to get the same effective sidelap/frontlap at a certain lower altitude?


#2

Just do two flights. I always take off at the highest point for all my flights (if there is VLOS), but my aerials are not required to be insanely accurate or anything, just something newer than google earth.

There is a beta test for terrain following that will increase the altitude but not decrease.


#3

Hello Jeff,

Thanks for reaching out. Daniel’s suggestion is good in that you can plan two flights for each of the varied elevations to get good results and then combine these two maps. The most important thing here is to make sure you are getting good overlap from your images. I would take a look at this support article going more in detail about this.

We also have a couple of applications in our App Market that may help you with your flight. Our Overlap Optimizer is great when trying to make sure your data set has great overlap. You can also use our DroneDeploy Labs App Auto Flight Modes to allow the mapping of varied terrain (this app only allows the mapping of the highest point of the terrain and does not decrease the altitude of the drone).

Thanks!
Yusuf

@consultant @dwindham


#4

@consultant,

I fly my DJI Phantom 4 Pro missions at 300’ above the takeoff point. The 17 acres site drops 140’ at the Eastern boundary so the drone is at 440’ AGL but the trees below stick up 100’ so 340’ above the tree tops. On the Western side, the 100’ trees put the drone 200’ above their tree tops. I set the overlaps at 80%/80% which is based upon 300’ above the takeoff point. So this results in less than 80%/80% overlaps over the Western trees but more than 80%/80% overlaps over the Eastern trees. A reasonable compromise. I get good maps using these settings.

Can you do something like this on your site? Try flying at 300’ which will give you 200’ AGL at the high point of your site but up to 300’ elsewhere. If you set the overlap to 80%/80% which will be based upon the 300’ AGL of the takeoff point, then your overlaps will be similar to mine and you should get good maps. I do not think you need two missions for your site with relatively modest elevation differences like mine. But try it if you like and compare the results. If 300’ leaves some holes in your 2D map, try 350’.

I also recommend flying on overcast days in low wind with the drone max speed limited to 9 mph to avoid noticeable blurring in the photos. I do use automatic focus and exposure. I tried using the DroneDeploy defaults for flight speed but this ruined several missions (very blurry photos) as it did not fully compensate for the overcast conditions. Be sure to review every photo and remove any from the upload that are noticeably blurry. For me this results in the elimination of almost all the photos at the 18 turns in my mission on the 17 acres site. The turning is too fast and it badly blurs all the turn photos. So I end up with about 200 photos to upload which is 11.8 photos per acre.

Another nuance is that the RMSE of the mission may be better if you can fly with the long legs of your mission aligned with either constant latitude (my case with 1200’ long E-W legs) or longitude. This may be what is responsible for the 7X better Y vs X contribution to the overall RMSE in my case.

Regards,
Terry.