Hello,

I am considering purchasing a DJI Phatom 4 Pro but am concerned about the terrain follow capabilities. I would like to be able to perform a grid survey, having the drone survey a grid (taking photos at selected intervals), while maintaining a constant height of ~2-3 meters above the ground. This requires being able to ascend or descend according to the terrain. However, I see that the terrain follow mode only allows it to ascend.

Is it possible to do a grid search from a constant elevation? Is this possible with third party software?


Thank you,
-Robert

I personally have never heard of a drone that can do as you describe. regardless of the flight app.   

Mark The Droner Posted at 2017-5-5 16:01
I personally have never heard of a drone that can do as you describe. regardless of the flight app.

I second that. There is no mechanism fitted to any of the DJI products that can accurately measure its altitude above the surounding terrain.

Geebax Posted at 2017-5-5 16:06
I second that. There is no mechanism fitted to any of the DJI products that can accurately measure its altitude above the surounding terrain.

The DJI Phantom 4 Pro has exactly that, a downward facing infrared sensor that can be used in Terrain Follow mode. Unfortunately it only will follow an upward slope and not a downward one. I'm curious if there is a way to implement it so it can follow upward and downward slope.

And Terrain Hugging is now an option on Arducopter 3.4, so it isn't out of the ordinary, especially if a drone has a built in altitude sensor.

citronade Posted at 2017-5-5 17:13
The DJI Phantom 4 Pro has exactly that, a downward facing infrared sensor that can be used in Terrain Follow mode. Unfortunately it only will follow an upward slope and not a downward one. I'm curious if there is a way to implement it so it can follow upward and downward slope.

And Terrain Hugging is now an option on Arducopter 3.4, so it isn't out of the ordinary, especially if a drone has a built in altitude sensor.

The sensor you refer to is the combined ultrasonic and visual sensor, as fitted to all Phantom aircraft from the P3A and P3P onwards. It has serious operational limitations, a quite short range (unusable above 3 Metres) and is not reliable enough for the task you want to use it for. If it was accurate, DJI would have made use of it already as a feature.

Have a look at the app DJI GS Pro. Rather than flying a grid pattern at a distance above the ground, it flies at a constant altitude taking photos to enable various surveys to be done.

Are you sure you need a constant elevation?
There are many apps that people use for serious photogrammetry surveys and none of them attempt to maintain a constant elevation.

Labroides Posted at 2017-5-5 20:06
Are you sure you need a constant elevation?
There are many apps that people use for serious photogrammetry surveys and none of them attempt to maintain a constant elevation.

This is a research project where we are using the drone to search for meteorite fragments in the field. So we feed the images taken from the drone into a machine learning algorithm to locate the meteorites. Maintaining roughly equivalent pixel density on the fragments is critical, so we would like to maintain constant distance from the ground.

Hi Citronade,  

There is a third party mapping app that downloads DSM tiles and uses these terrain tiles to drive the elevation of the UAV during a gridded photo mission to capture a constant GSD.  Only thing is that the terrain tiles used are using a 30m grid... This will create big problems for trying to map something from only 2-3m above the surface.   It is recommended to use this terrain awareness feature from 60m agl.  The app is called Map Pilot and it is great.  I have been using it for over a year.  The issues I experience with the terrain awareness stem from the date that the terrain tiles were collected (some time near 2001).  Any changes to the terrain since then are not reflected in the tiles which makes surveying active mine sites rather tricky.

Do you mind me asking what you are surveying that needs to be captured from such a low elevation?  I would be concerned about the UAV shadow mucking up the results from such a low altitude.

Hi Citronade,
I found this answer in a Youtube video: "With Terrain Mode you will need to set a height from 1 to 10 meters first, as you move uphill it will stay at that height setting, as you move downhill, pull down on the left stick and the Phantom will descend to the high you set.".

Bullflyer Posted at 2017-5-6 11:50
Hi Citronade,
I found this answer in a Youtube video: "With Terrain Mode you will need to set a height from 1 to 10 meters first, as you move uphill it will stay at that height setting, as you move downhill, pull down on the left stick and the Phantom will descend to the high you set.".

I guess it is done for safety reasons - to prevent the aircraft unexpectedly flying down and into someone's face without being told to do so by the pilot.

Bullflyer Posted at 2017-5-6 11:50
Hi Citronade,
I found this answer in a Youtube video: "With Terrain Mode you will need to set a height from 1 to 10 meters first, as you move uphill it will stay at that height setting, as you move downhill, pull down on the left stick and the Phantom will descend to the high you set.".

Thanks a ton for that information. I'm curious, is Terrain Mode available concurrent with using something like Map Pilot to conduct a grid photo survey?

Thanks!