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I am new to this group and have a question for anyone knowledgable about how drones use elevation data in their flights. Is it loaded into the software onboard the UAV? What data is currently available and used. And would very specific elevation data of the ground surface area along with obstacles present be of any value in that process? Here is what I am talking about .It is a collaboration between Lufthansa and Intermap Technologies in creating a global database for Drone/ UAV usage called NextView. The data specifications are listed at the bottom...

 https://www.intermap.com/aviation-survace-data

Thanks in advance as I don't know (but am trying to learn quickly) all the nuances involved with a wildly growing industry.

 

Bill Raymor

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The issue with elevation data associated with UAV imagery/flying height is an interesting issue.  At the time of takeoff, the UAV measures the barometric pressure at the take off location and using the standard of 29.92 mmHg sea level pressure calculates the elevation of the take off point. UAV then uses changes in barometric pressure as it ascends to calculate the height above ground level (AGL).

During a 20 minute or so flight, local barometric pressure is unlikely to change dramatically so this system works for calculating flying height. Unfortunately it has no real world association with the actual elevation of the take off point. 

If you fly multiple missions using the same take off point over the course of multiple days/weeks or months, the calculated takeoff elevation, for the same location, will be different for each flight due to varying barometric pressure. The flying height will always be correct because the change in barometric pressure as the UAV ascends will be calculated based on the take off barometric pressure for that flight. 

Imagery collected from repeated multiple flights using autonomous flight software and always taking off from the same location when processed in Structure from Motion (SfM) software will result in point clouds of the scene displayed at various elevations when viewed in 3D. For the most part, horizontal alignment of the point clouds is quite good but vertically (Z) the values are not accurate at all. 

The only way to control and correct Z values in the resulting SfM models is by using Ground Control Points (GCPs) that have been accurately located with differential GPS positions for latitude, longitude and elevation. 

Although I do not have access to an RTK equipped UAS my understanding is that even with RTK UAS system GCPs are necessary to get high quality Z data. GeoCue company reports 5cm vertical accuracy with their modified DJI Phantom 4 RTK system which is pretty impressive. 

I get good results  with my system, DJI Phantom 4 Pro, 5-10 GCPs with positions measured with Trimble H-Star GPS and differentially corrected in Trimble software referencing local CORS data. 

Hope this helps.

Dave

 

 

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All countries keep obstacle (obstruction) data in their official aeronautical databases.  These are used in all VFR and IFR charting as well as in terminal instrument approach databases.

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1 hour ago, Cyclops55 said:

All countries keep obstacle (obstruction) data in their official aeronautical databases.  These are used in all VFR and IFR charting as well as in terminal instrument approach databases.

Which kind of begs the question why DJI doesn’t use Jepessen!?  To my knowledge no DJI uses elevation data.  

3 hours ago, dtewksbu said:

the UAV measures the barometric pressure at the take off location and using the standard of 29.92 mmHg sea level pressure calculates the elevation of the take off point.

This is true, however you can use other instruments to calculate MSL that also provide very accurate and consistent AGL.  

3 hours ago, dtewksbu said:

The flying height will always be correct because the change in barometric pressure as the UAV ascends will be calculated based on the take off barometric pressure for that flight. 

Although mostly correct, we fly a lot of missions on ships where the DA can change during a flight and it doesn’t take much to effect the accuracy of the altitude.  Oddly, on the larger ships we mark out a legend which is the first and last thing we shoot as a reference image so we can manually calibrate.  A poor mans kollsman window...

3 hours ago, dtewksbu said:

The only way to control and correct Z values in the resulting SfM models is by using Ground Control Points (GCPs) that have been accurately located with differential GPS positions for latitude, longitude and elevation. 

I guess this depends on the accuracy your trying to achieve.  We use GCPs when we can and rarely use RTK, but our models are within 1.2cm in all axis even without them.  All of this is to overcome the innaccuracy with GPS and there are better ways to account for that variable.  

 

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