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3D Survey of a Dam - Pointers Welcome

Discussion in 'General Discussion' started by DanWeston, Mar 13, 2018.

  1. DanWeston

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    Hi folks,

    I'm due to have another go at a photogrammetry survey with my H520. This one is of a dam in Wales which obviously slopes quite steeply. I'm looking for some general guidance/pointers, mainly around altitude. When planning it in Data Pilot, the altitude is said to be 'relative'. I assume this is relative to the take off point? My concern is that as I start working down the damn, if I've set my altitude relative to the take off location at the top of the dam, as it works its way down, it will be too high. In other words, if I'm at 50m at the top of the dam, and the dam is 100m high, I'll be at 150m altitude at the bottom of the dam. Can I get it to decrease altitude as it works its way down? I'm afraid that this will affect how the images are stitched together. Or will it just work itself out?

    Any other pointers gratefully received.
     
  2. Steve Carr

    Steve Carr Moderator
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    @Bob520 do you have any suggestions?
     
  3. NorWiscPilot

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    Dan,

    I am still heavy in the “learning” phase so please excuse me if my questions and comments expose my ignorance.

    My first inclination is to ask what type of imaging is being requested. If a basic aerial mosaic, would the elevation (altitude) staying constant be the expected flight plan?

    If 3D modeling is the expected use of the images, then I would hazard a guess we would want to invoke altitude changes dictated by the subject.

    I am a firm believer in multiple missions. Keep it simple. The more complex a single mission, the more likely problems might be incurred.

    Looking forward to reading this discussion.

    Good luck!

    Jeff
     
  4. PatR

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    Have you tried using negative values; -25’?
     
  5. Bob520

    Bob520 Moderator
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    Relative altitude changes from Above Ground level at the take off point to Mean sea level, I suggest not using this for a preprogrammed mission unless you know ecactly how high or low you are flying in MSL. I have not seen a negative altitude mission accomplished however, if you were to have the mission start with a negative sign in theory the aircraft would descend and work it's way ascending the dam face. As far as the images go, it depends on what stitching software you are using. Most will automatically use the metadata and pixel mapping to create the correct stitch.
     
  6. jscotcha

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    I not sure what type of image you are after. If as an example you had a 10 foot diameter circle at the top and bottom of the dam, in the view from above the dam the circle on the bottom would appear smaller. If you didn't fly the mission at a fixed elevation the images taken at the bottom would be larger and the stitched photo would appear distorted if it was even able to be stitched together.
     
  7. DanWeston

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    Thanks guys... all stuff to think about.

    I think I'm going to split the mission in two but keep the altitude the same for both missions at a level which will require a smaller spacing between the grid lines at the top of the dam where it will be closer to ground level.

    I'll let you know how I get on!