Lens burn when mapping over water

I am using a Mavic2 Pro to map dams. I have been using standard polar pro filters to enhance the image collection but I am regularly getting severe burn out on the images due to reflection from the water in sunny conditions even with ND32 lens in place.
Does anyone have any tips to reduce this problem as it makes what is otherwise a good map and model look bad and unprofessional.

Murray

When you say “lens burn” are you talking about blowout? Are you running nadir or obliques?

I am using enhance 3D mode so I suppose I am using a combination of nadir and oblique image capture. The problem I have is when the images are processed the map is fine but when you view the model there are areas that are just black shadows with no detail at all and it is almost always over water in bright sunlight.
I hope I am explaining the problem properly as it is a real pain when most of the mission is great and spoilt by one bad area.

Murray

If crosshatch in E3D is on then there are no nadir images. This is a good thing though because if the sun is high in the sky you will get complete blowout. Water is not photogrammetry compatible. The are no static features to create tie-points and reflections can actually have an effect similar to if you had the horizon and sky in shots. The machine does know what to do with the images and you get holes and large spikes or pits. Everyonce in a while I will get a decent surface (and DroneDeploy has worked hard on this), but that is rare when a body of water is large enough to include images of just water.

Michael

Thanks for these helpful comments, I can understand that if there is just a big plane area the software will have nothing to reference the images to.
Thinking about this I might try adding a few coloured floating objects such as plastic floats on a rope into the water where I am having problems, these might help to give better definition and will be easily retrievable on completion of the mission.

Murray

Great minds think alike, but I tried this with small 6-inch bouys and also tethered Home Depot bucket lids and the problem was that they moved just enough by the time that the next lane was flown so the machine discarded them as if they were a car passing by on a road. Unless it is a completely still pond it’s unlikely that it will work. I am not sure how it would be done in deeper water, but what worked for me was a 2-inch PVC pipe with the Home Depot lid fastened to the top. The water was only about 3-5ft deep, but we were able to hammer it into the bed and they were stationary well enough to work. It probably wouldn’t work for a large body of water because you would need to many of them.

Hmm, seems to be a bit of a problem that is not easy to get round. Most of the projects I fly are large dams and the water can be 50 -100 ft deep so no chance of getting something fixed to the bed.
I am mainly trying to get images of the concrete face of the dam both upstream and downstream. I usually end up flying both faces of the dam at 4k resolution and harvest images from the footage for my reports, quicker than taking individual still images.
I just like to do a 3D map of the structure as when it works the customer is usually blown away when you show them their structure and then rotate it and look all round it.
I think I might have a go with some good polarising filters and see if this make things any better.

Again thanks for your valued assistance on this issue

Murray

Our typically workflow involves bringing the point cloud into an editing software called Carlson Precision 3D Topo by which we remove the points from the water and replace them with a triangulated surface that can be textured if desired. This would be good for making a nice 3D model, but we are more on the analysis side and water is normally gets excluded unless we are tracking water retention.

Just curious where are you and how did you get authorization to fly the dam? I know it may be different here, but even the LCRA could not get a 107.45 waiver… Looking on the geozone sources it’s not even zoned.

Michael
My workflow is complicated as I have 2 separate interests in this. I run a specialist concrete repair company and my patch is Scotland and Ireland. We specialise in the maintenance and repair of hydro-electric dams and power stations. Our main customer has 78 major dams and countless minor dams that collect water from small rivers and divert them into collecting basins where they are carried by penstock or canal to hydro power stations.
With my other hat on I run a small Drone buisness that specialises in image capture of complex concrete structures to enable maintenance to be planned and executed based on good quality data.
Combining these two businesses allows me access to some amazing structures that nestle in the Scottish Highlands and both companies benefit from eachother. I still have to hold a full UK PfCO authorisation to fly professionally but it is great being able to use this on such great structures with the full support of the asset owners.

Regards Murray Soutar

Totally get it. We are a general contractor with an inhouse drone program that started off as a specialty civil concrete contractor. We have built many dams obviously not as big as the ones you are dealing with, but we build allot of bridges and other civil structures. If you are deep into analysis like we are you should definitely look into P3D Topo as a precursor to any of your CAD/BIM work.