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GS RTK Minimum Altitude
315 7 6-10 08:23
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patiam
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We routinely need to do photogrammetric mapping at very low altitudes in order to obtain sub-cm GSD (for habitat classification and counting/measuring intertidal marine critters and plants). I'm talking as low as 2 m AGL at times, but routinely 5-15 m. For obvious flight safety reasons, many autonomous flight planning and control apps have a minimum altitude that you can set for a photogrammetry mapping flight. In the old Pix4D Capture and in Drone Deploy, it is 10 m, which we can live with for the most part; we use Litchi or hand-fly when we need to go lower.

I'm just planning my first intertidal surveys with a P4R, and of course I must use GS RTK for control. I was amazed to see that the minimum altitude for a 2D mapping mission is 25 m! This seems extreme, and is ironic because with RTK fixed, the P4R's altitude awareness is much more stable and accurate than non-RTK enabled aircraft that must rely on barometric pressure to measure altitude relative to takeoff.

I'd really like to use the P4R for low altitude mapping. I'm hoping there is a way to fly below 25 m AGL.

  • First question: does anybody know how to remove the minimum altitude limit?
  • If not, I do see where there is a "Relative Height (m)" setting (Default = 0), with an info blurb that says "Set the altitude of the shooting environment relative to the takeoff point. Take off in a safe area". It includes a dynamic indicator of "Actual Flight Altitude" that updates if the setting is changed. As one might intuit, positive values entered in the setting decrease the "Actual Flight Altitude". So presumably if I set "Height (m)" = 25 m (the min) and "Relative Height (m)" = 10 m, even though my takeoff location is within the area to be mapped (so has relative elevation = 0), I should end up with a survey altitude of 15 m AGL (at least that's what the "Actual Flight Altitude" value suggests). Has anybody tried this? My biggest concern is that while this approach may allow me to fly nat the desired altitude, the line and shot spacing will not be adjusted to ensure the proper overlap for 15 m AGL. I suppose I could tweak the overlap settings for 25 m AGL higher to account for this and provide the desired overlap @ my altitude.
  • I suppose I could accomplish something similar by doing a terrain aware flight and tweaking the DSM. Rather not go there but will if I have to. In theory this approach would also have the same overlap concerns as method #2. If someone has done this, I'd be interested to hear about it as well.

I'm well aware of the safety risks involved with very low altitude flight. Apart from bluffs/cliffs to the inshore of the study area, there is no topographic relief > 2-3 m. Some large boulders, but no trees, poles, powerlines, etc. and no dramatic slope across the area.


Any ideas or constructive advice much appreciated. Please, if you don't have knowledge of the P4R and GS RTK, don't bother posting suggestions that are not relevant.

Thanks!



GS RTK Rel Height

GS RTK Rel Height
6-10 08:23
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patiam
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Bueller?


*crickets*
6-13 13:21
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Guille_Meinero
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sorry , cant help u. Im not used to flying that altitude neither "relative height"
6-14 04:04
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patiam
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Guille_Meinero Posted at 6-14 04:04
sorry , cant help u. Im not used to flying that altitude neither "relative height"

Thank you @Guille_Meinero. Even an "I dunno" is better than crickets
6-14 12:44
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LV_Forestry
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Flight distance : 2879803 ft
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Hi,

Tested today with UGCS, 5m minimum, not below.
UGCS 5M.JPG
6-15 11:38
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patiam
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Haven't used UGCS... I don't have the SDK controller, so not sure if I can install it... But thanks for the idea!

I flew some GS RTK test flights yesterday using the relative altitude trick- flight plan altitude set to 25 m minimum, and the relative @ 15, 18, and 20 m to yield 10, 7, and 5 m AGL flights. The flights themselves went perfectly. The results after processing in Pix4D bore out my concerns; even though I cranked the overlap up some (to 90%/85%)  things began to fall apart @ the lower altitudes: @ 10m , 3/185 cameras couldn't be calibrated. @ 7m 63 failed, and at 5m 172.  The line and shot spacing for 25 m AGL just doesn't work @ 5m, even w/ 90%/85% overlap set for 25m.  I'll try even higher overlap next time, maybe even do the math to see what the limits are.
6-15 11:52
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Jason Chambless
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I just ran into this issue today due to the FAA only allowing me to fly at 50 feet.  As an easier solution for my project I will be filing for a waiver to fly at a greater height, but this is a pretty ridiculous feature.  I was told by DJI that is was due to the GS software not being able to process good final results from that height due to image smear etc.  
7-2 06:22
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patiam
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Jason Chambless Posted at 7-2 06:22
I just ran into this issue today due to the FAA only allowing me to fly at 50 feet.  As an easier solution for my project I will be filing for a waiver to fly at a greater height, but this is a pretty ridiculous feature.  I was told by DJI that is was due to the GS software not being able to process good final results from that height due to image smear etc.

Thanks @Jason Chambless. Looking at my imagery from 15 m AGL I'm beginning to agree that the 25 m minimum may be due to a system limitation.

My 5 mm GSD imagery is smeary in the along-track direction. Not sure if slowing down would help or not, and lighting was not great for these flights. Need to look into this more.

Do you have a contact @ DJI that gave you the info you shared here regarding the limit?
7-7 14:46
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