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[DJI Phantom 4 pro] Surveying an area with large level differences in one survey

Hi, Im looking into investing in a drone to survey an opencast coal mine.  

My question is, when I survey the backfill, the level difference between the top of the backfill slope and the bottom of the slope is between 100m and 110m.  If I fly at say 50m above the top level will this be a problem with G.S.D. Will the accuracy at the bottom of the hole be badly impacted?

Hi Peter,

In general single flight at a constant altitude regardless of the terrain changes in altitude is not recommended.
In the article Image Acquisition Plan for Terrain with Height Variations you can find the following:

  • It will affect the GSD: The GSD will not be the same over the whole project a cause of the variations in elevation of the terrain. 
  • It will affect the overlap: The overlap will be lower in the higher point B, if the image rate is the maintained.
  • It may affect the reconstruction or even fail, if the height difference between the higher and lower point is twice or higher (if the highest GSD exceed twice the lowest GSD).

One of ways to survey an opencast coal mine is to follow the changes in the terrain as shown here:


Followup Question. 


So.  In planning a “stepped” flight above. 

If I am taking off from near the “B” location (higher msl portion of a site) how can I set the height in Pix4d Capture to a correct photo capture rate?  The capture software is trying to increase my overlap (thus flight time) and battery usage. 

For the specific real-world application.  I have a building campus (28-ac) on a hillside area with 392-ft vertical distance from the lowest point of ground to highest point of a building.  I have trees, open space, building rooftops, parking lots, roadways, open space, etc.  I have planned out 3 flights with steps and overlaps.  I am also planning a 4th (maybe also 5th) flight around the building to merge into the dataset. 

I am planning to take off from a building top, near campus center, mid-campus elevation (between lowest & highest msl) to ensure no communication losses.  I have to go over the last building (the tallest building is at the “top” of the hill) near b (your above figure, thus the highest MSL) and capture a low area and open space behind the building.

I assume I would want to fly all flights at the same average ground distance which I calculated to be 210-ft.  

  • Flight 1 (704-ft msl) - ranges from 114-ft above structure (building) to 272-agl (open space at lowest campus msl),
  • Flight 2 (737-ft msl) - ranges from 87-ft above structure (small patch of tall trees) to 319-ft agl (roadway), 
  • Flight 3 (779-ft msl) - ranges from 74-ft above structure (highest building) to 303-ft agl (loading dock and open space behind the building)

In summary, when taking off from a structure my elevation setting (set at 159-ft, 192 and 234 for each flight respectively) is needed to achieve an average height of 210 in each flight.  Since my purpose of the flight is surface mapping ortho and contours (using this for a buried utility map base) I am not concerned about the loss of fidelity when crossing some of the taller structures.  I am more concerned with an accurate ground structure footprint.  I was planning to take off from a building of 545-ft msl (70-ft agl one side, 40-ft agl another side…remember this is a hillside campus).  If I say my height is 210-ft, then the drone (Matrice 210 RTK) will fly the mission on each zone at 210-ft+building height relative to my takeoff point.  I want it to go out to the flight 1 zone and fly 210 feet asl not 210-ft + building + change in elevation.  This would mean there are locations on the “lower end” of flight 1 where I will likely exceed 400-ft asl.

I thought about flying out to a location and starting the mission, however, this would be:

  1. in active parking, yard or drive lanes of the campus and
  2. I may lose signal or line of sight

I believe it would be safer to take off from a building roof or parking garage location.  Since the campus has many buildings, tree patch areas, etc. I assume it would be better to fly from the “higher end” of the campus or from the top of a structure to ensure I maintain excellent signals.  

Please your suggestions are welcome.   


Perhaps this will make more sense related to what I tried to describe.  Flights planned (uniform elevation areas) and flight takeoff location (blue star). 

Follow Up…after I hit send I had a crazy idea. 


I had my overlaps set at 90%.  Do I just reduce percentage since it will acutally be better than 90% once the bird flies out to the area for the flight and the ASL of the drone increases?  In essence adjust the percent to a uniform cm/pixel number as if I was taking off from the lower elevation zone?


Thank you for a detailed description of the project! It is a very good example that shows how much planning is needed in order to get the job properly done.

As you already mentioned, if you take off from a building then you somehow need to account for the elevation difference between the take-off point and actual ground level. If not, the mission will be planned as if the ground level is the at the altitude of the take-off point. 

There is no straightforward way to do this in Pix4Dcapture but you could try to artificially reduce the overlap in the mission settings.

This way the overlap will better reflect the actual overlap that is needed to cover the ground level. Since it could be hard to determine the overlap you could:

  1. Plan a mission based on the altitude above ground level and count the number of flight lines.
  2. Decrease the height of the mission to the altitude above take-off point (building).
  3. Change the overlap in the settings to match the number of lines from 1.
  4. Fly the mission with the reduced height and overlap.

Hope this helped and you will succeed.

Let me know how it goes, it will be interesting to see the results,