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Single Grid vs. Double Grid Flight for DTM
Posted by DandaMan on January 23, 2017 at 1:57 pmUsing a Phantom 3, we’ve successfully created orthos from images, utilizing VRS to capture GCP’s every 500′ along road routes.
Next project entails a 60 ac. farm with rolling fields and wooded creeks in the valleys. Max elevation difference of 150′. Ultimate goal is to extract contours to marry up with the conventionally captured data in the wooded areas. Two quesitons in my mind:
1. Ground Control. I’m planning to keep at 500′ max, placing at perimeter and also at the low and high points (i.e. at the top of ridges and down toward draining lines). Does this make sense?
2. Single Grid vs. Double Grid. I was planning to do a single grid with 80% front and 70% side overlap, flying at 150′ to 300′ AGL, camera at nadir (perp. to ground). Is there any advantage to flying a second grid at 90 to the first?Any comments would be appreciated…
chris-mills replied 7 years, 7 months ago 7 Members · 47 Replies -
47 Replies
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300′ AGL with a P3 is not going to work well at all. The GSD will be too larger, then the error will increase 3x. You will need a terrain follow app.
Not only do you need those GCP, you also need at least check shots. Get 5 checks in each quadrant (20 total) of the survey. Do NOT use them as GCP’s. Compare then survey check points with the DEM data.
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I would do 80% both forward and back and side to side. As far as 300″ – that depends on your camera. We’ve flown higher than that with our UX5 and gotten good results.
A lot of people have been telling me to do the double grid, with the flight lines perpendicular to one another. Supposedly this produces much cleaner vertical data; I haven’t had a chance to try it yet.
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leegreen, post: 410482, member: 2332 wrote: 300′ AGL with a P3 is not going to work well at all. The GSD will be too larger, then the error will increase 3x. You will need a terrain follow app.
Not only do you need those GCP, you also need at least check shots. Get 5 checks in each quadrant (20 total) of the survey. Do NOT use them as GCP’s. Compare then survey check points with the DEM data.
Thanks for the reply. Double gird necessary, in your opinion? GSD at 300′ AGL = 4.0 cm. Yes, that is too high. I’ll look into terrain follow apps, any suggestions? The thought would be to topo the full site and use drone data as a comparison. If results are promising, deploy drone to capture DEM data for remaining 80 ac. of site.
The P4-Pro is intriguing to me because of the better camera and lower price-point than the Inspire (assuming DJI family).
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DandaMan, post: 410484, member: 11506 wrote: Thanks for the reply. Double gird necessary, in your opinion? GSD at 300′ AGL = 4.0 cm. Yes, that is too high. I’ll look into terrain follow apps, any suggestions? The thought would be to topo the full site and use drone data as a comparison. If results are promising, deploy drone to capture DEM data for remaining 80 ac. of site.
The P4-Pro is intriguing to me because of the better camera and lower price-point than the Inspire (assuming DJI family).
The P4-Pro with Map Pilot + Terrain Feature awareness by Maps Made Easy, run double grid. Process with Pix4d. Map Pilot and Pix4d Mapper software are not Free.
80 acres (rule of thumb always get 20% more with drone mapping) will take at least 3 flights for a single grid. You will need 6 flights (and 6 batteries) for a Double Grid. Depending on your computer specs, it may take 4-24 hours to process the data. You are reaching the limit of drone mapping, where conventional aerial mapping may be more efficient.
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leegreen, post: 410488, member: 2332 wrote: The P4-Pro with Map Pilot + Terrain Feature awareness by Maps Made Easy, run double grid. Process with Pix4d. Map Pilot and Pix4d Mapper software are not Free.
80 acres (rule of thumb always get 20% more with drone mapping) will take at least 3 flights for a single grid. You will need 6 flights (and 6 batteries) for a Double Grid. Depending on your computer specs, it may take 4-24 hours to process the data. You are reaching the limit of drone mapping, where conventional aerial mapping may be more efficient.
Thanks Lee, that is very helpful information.
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I should have should you are reaching the limits of entry level drone mapping.
With a high end drone (likely a fixed wing), and a high end PC you can map 300 acres.
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I understand. Yes, we don’t do a lot of these large sites (anymore) so this one is more of a one-off. I’m hesitant to go fixed wing, high cost, until RTK is standard and possibly some lidar applications as well. Just remeasured area at around 50 acres. Will look for a sunny day to try the flight, hopefully on Weds. We have used Pix4D in combination with DroneDeploy (just to do the flight planning).
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leegreen, post: 410492, member: 2332 wrote: I should have should you are reaching the limits of entry level drone mapping.
With a high end drone (likely a fixed wing), and a high end PC you can map 300 acres.
Using a Quest Q-200 fixed wing I’ve covered 300 ha. in three hours on site – 3 flights at around 350′ There’s a marginal improvement by using double grids but they mainly serve to fill in any areas where the coverage on the main grid wasn’t what it ought to have been. That normally happens at the end of the grid lines when the wind gets up, so you can potentially eliminate any problems by adding a loop around the periphery of the site – much quicker than flying a second grid.
The other problem with a second grid is that if you flew the first one cross wind, so that camera stations remain equally spaced then the other grid is upwind/downwind and the camera spacings will be very variable (unless you are triggering the camera by position, rather than by time).
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Camera quality, sensor size, and megapixels is the DATA IN for this partiuclar workflow. I typically shoot 12mp JPEGs from the Phantom 3 Advanced. How much should one play with camera settings? I typically set camera to auto for ISO and shutter speed selection and for the image I just shoot standard. I’m curious if anyone has played with the photo recording methods.
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DandaMan, post: 410758, member: 11506 wrote: Camera quality, sensor size, and megapixels is the DATA IN for this partiuclar workflow. I typically shoot 12mp JPEGs from the Phantom 3 Advanced. How much should one play with camera settings? I typically set camera to auto for ISO and shutter speed selection and for the image I just shoot standard. I’m curious if anyone has played with the photo recording methods.
I have not used the P3A. My P4Pro has a lot of camera options. I turn on the “show over exposure” option and fly up to my preferred AGL. This shows in camera view where light is too bright. I can then change the exposure to reduce it.
If you don’t have this option in P3 then use auto settings and highest resolution. Your P3 camera has a non-mechanical shutter, therefore subject to rolling shutter error. You will need to control the air speed to prevent this. Map Pilot does a great job of detecting wind speed, then it will correct the air speed of your drone to reduce image blur. This is why a P4Pro is much better, it has a better sensor, and mechanical shutter. With your P3A you can get good results with low altitude and slow flights. Also suggest you try using the rolling shutter correction in Pix4D.
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I have that setting in the P3A also (the overexposure). I will try the Map Pilot App, prev. used DroneDeploy, which was just okay. Good to know on the rolling shutter too, thanks.
The P4Pro sounds great for the price. If only it accepted the P3 batteries. The Inspire has a even better camera but cost is much more and it seems to be geared more to movie type applications.
Waiting on sun to fly..
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Lee, I’m in Ohio (Cincinnati). Haven’t seen much sun in a few weeks. Peeked out this morning but it’s clouded back in.
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leegreen, post: 410761, member: 2332 wrote: I have not used the P3A. My P4Pro has a lot of camera options. I turn on the “show over exposure” option and fly up to my preferred AGL. This shows in camera view where light is too bright. I can then change the exposure to reduce it.
Lee, I assume that you don’t use your camera in the auto setting and manually adjust your apeture so the shutter speed falls where it may? I know that with hdr and panorama photography you want a constant apeture for all photos being stitched so I assume creating orthomosiacs is the same. I’ve been wondering about this for awhile. Sorry for the hijack Dan.
Gregg
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DandaMan, post: 410771, member: 11506 wrote: Lee, I’m in Ohio (Cincinnati). Haven’t seen much sun in a few weeks. Peeked out this morning but it’s clouded back in.
I have a project in that Cincy, and my twin brother lives just across the river in Walton, KY.
By the way, have you read Drone Deploys disclosure statement? It is not something my clients would frown upon.
“By using the Service, you grant DroneDeploy a non-exclusive, irrevocable, fully paid and royalty-free, transferable, sub licensable, worldwide license to use, copy, reproduce, process, adapt, modify, publish, transmit, display, and distribute your User Content.”“
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GMPLS, post: 410773, member: 8404 wrote: Lee, I assume that you don’t use your camera in the auto setting and manually adjust your apeture so the shutter speed falls where it may? I know that with hdr and panorama photography you want a constant apeture for all photos being stitched so I assume creating orthomosiacs is the same. I’ve been wondering about this for awhile. Sorry for the hijack Dan.
Gregg
Gregg,
No. The software is a bit forgiving. It will adjust the jpgs when creating the mosaic image.Dan,
I would not be afraid to fly on a partly cloudy day, this time of year. With the low sun angle, a bright sunny day will yield long shadows even at solar noon. -
GMPLS, post: 410773, member: 8404 wrote: Lee, I assume that you don’t use your camera in the auto setting and manually adjust your apeture so the shutter speed falls where it may? I know that with hdr and panorama photography you want a constant apeture for all photos being stitched so I assume creating orthomosiacs is the same. I’ve been wondering about this for awhile. Sorry for the hijack Dan.
Gregg
Gregg, the aperture on the P3 is set at f2.8, I assume the same on the P4?
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leegreen, post: 410774, member: 2332 wrote: I have a project in that Cincy, and my twin brother lives just across the river in Walton, KY.
By the way, have you read Drone Deploys disclosure statement? It is not something my clients would frown upon.
“By using the Service, you grant DroneDeploy a non-exclusive, irrevocable, fully paid and royalty-free, transferable, sub licensable, worldwide license to use, copy, reproduce, process, adapt, modify, publish, transmit, display, and distribute your User Content.”“
And I assume by “User Content” they mean all of your photos? I’ve only used DroneDeploy to fly, never to process. Use Pix4D. This is good to know.
And we have a project in Walton right now! Small world…
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DandaMan, post: 410777, member: 11506 wrote: Gregg, the aperture on the P3 is set at f2.8, I assume the same on the P4?
I don’t think it’s set a f2.8 unless you manually set it that way. That’s just the maximum for that lens. I believe the all of the P3’s and the P4 (excluding the P4P) have the same camera as far as still photos are concerned.
Gregg
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Lee,
I thought bright days were best as you can lower ISO and ensure good image quality? Do the shadows cause issues in Pix4D?
Thanks, appreciate your posts. Be curious what you got going on in Cincy.
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