Lowest possible mapping altitude

Hello all,

I have received an unusual mapping request and am wondering is anyone here might have experience with a similar project.

The job is to map aggregate stockpiles for inventory estimates at an asphalt plant. The site is dead flat (other than stockpiles) but is located within controlled airspace with a max LAANC altitude of 50 ft. Yikes! I have not reconnoitered the site yet but am told that the stockpiles may approach that height. If so, then obviously, game over. However, if they are lower, say 30 feet, I wonder if any have had success attempting to map at this altitude/clearance and, if so, if they can provide any suggestions. GPC’s will be established prior to flight. Site is only about 4 acres so high overlap and slow speed is feasible.

If I have sufficient clearance for the flight I was thinking maybe a cross grid with overlap of 75%. If this makes sense at all, would gimbal be best at nadir or maybe an angle, say 65 degrees?

Any input appreciated.

Steve

With that small of an elevation difference between the tops of the piles and the flight altitude you will have to run an oblique angle mission and I think the crosshatch is the right idea. Even with that you are going to need to turn your overlaps up to max or you will not get enough matches on the tops of the piles. Or you might consider running two missions with at least 60/60 overlaps, but the second mission direction would be 45 degrees to the first. Also, raising the gimbal pitch a little will help with overlaps. Without seeing the site it’s hard to be certain.

In my opinion if you are just going after stockpile quantities then GCP’s are a waste of time. Stockpiles are small local areas of A minus B so the error in that small of an area is negligible.

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Thanks Michael. Great input as always. If I get the project I will report back results. When you say two missions, do you mean that each mission would be a crosshatch mission (so four passes altogether)? Both missions with gimbal at 65 degrees? With that many passes, I should certainly have enough data.

Thanks again

Steve

My pleasure. Yes, two crosshatches. One North-South-East-West and the other 45 degrees to that. You might only need 70/70 overlaps, but it will ensure that you get all “sides” of the piles. Like I said before the tops of the piles will be the key. I have flights that if I go below 200ft above the highest objects you can actually see the affects in the accuracy report coverage map. Another thing this will help with if there are any crushers or screeners.

Thanks again Michael. Hope I will get the chance to fly it. Again, I will report back on lessoned learned if I do.

Steve

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Hello all,

I just wanted to take a minute to report back on results of a very low altitude mission (see above) for mapping stockpiles. Although I didn’t win the contract, I did fly a test mission (without GCP’s) and am happy to report that I got excellent results (see link below).

The mission was flown at 50 ft AGL with the highest stockpile at about 30’ in height. In addition and because this was a only a test mission, it was flown with an excavator parked on top of one of the stockpiles for a flight clearance of 5’ or so (yes, I checked clearance before flying automated mission - see attached photo from the mapping set :slight_smile: ). All in all, it was one of the tightest and most demanding I have flown with LAANC max of 50 AGL, an interstate highway adjacent to the site, and trees very close to mission limits. Needless to say I stayed very close to the drone with eyes up all of the time.

Mission parameters were: P4P; 50’ AGL, 85/80 overlap; 4 mph, manual focus (at 25 feet) and manual exposure (ISO100, f5.6, 1/500) in full sunlight.

If anyone wants additional information, please let me know.

Steve

https://www.dronedeploy.com/app2/data/5d61124706340d143680492c;jwt_token=eyJhbGciOiJIUzUxMiIsInR5cCI6IkpXVCJ9.eyJvdmVybGF5X2ZvbGRlcl9pZCI6IjVkNTQ2MjU5YmZhYjk3MTE1MGUzZTQ3MiIsInNjb3BlIjpbImYxMThiZjYyZDBfNDk1NDA3N0U4N09QRU5QSVBFTElORSJdLCJ0eXBlIjoiUmVhZE9ubHlQbGFuIiwiaWQiOiI1ZDYxMTI0NzA2MzQwZDE0MzY4MDQ5MmMiLCJleHAiOjI1MzQwMjMwMDc5OX0.sjacWft7uCjJJrKJbUjMZ5j6ZZJbdHfebL05Zdn9eM2ayaMQAR195efShoDUlAzLkJsOl7gH0pNCZ7Ak5wto0Q

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Very nice! This is proof-positive of how altitude and overlaps interact. Everything is beautiful until you get just a 15’ variance on a tight flight like this. You can see what happened to the excavator. Could you describe you final mission plan? Kudos and thanks for coming back to share!

Thanks Michael

Regarding the mission plan, I considered your recommendation of flying a cross-hatch pattern but, because I realized that I was going to have to fly a test mission regardless, I reasoned that I wouldn’t learn anything if I flew a very conservative pattern to be sure things would work. My goal was to win this project as a repeat customer (inventory every 3 months on a number of sites - this one being by far the most challenging) so I wanted to fly the quickest mission with a reasonable chance of success. I intentionally flew in full sunlight because I figured it would, again, be the worst case test for future missions. At 1/500 shutter and 4 mph, I reasoned that I had some wiggle room for a slower shutter on future missions in cloudy conditions. Plus, cloudy conditions are generally better for mapping.

The hardest part was setting the geofence. I set the initial draft of the geofence on the desktop DD app before going to the field. Once in the field I got LAANC clearance using AirMap, then went to the Go4 app for DJI clearance to fly. The site was in an area that was not fully restricted (not within the airport “bowtie”) but that allowed flight with an acknowledgment and release agreement.

Here I learned something that many might find valuable. DJI’s release process is very obscure: a pop-up screen says something like, “take-off not allowed, you must obtain release from DJI” or words to that effect. I spent the next 30 minutes sifting through endless DJI Airspace pages looking for the clearance release with no luck. Finally, in frustration I went back to the “no take-off” pop-up and ignored it, pushing both sticks down and in to start the motors. Hosia!! The release form popped right up! I checked the box and was good to go. BIG LESSON HERE: don’t believe the pop-up that says the bird won’t fly! Try it anyway and yee shall be rewarded!

I got the bird in the air and opened the DD app, then carefully flew along the edge of the geofence while visually checking the clearance to the adjoining interstate and trees. When I got as close as comfortable I adjusted the geofence to match the indicated position of the drone on the display. Obstacle avoidance was left on for obvious reasons. Finally, once clearance was confirmed, I landed, installed a fresh battery, and flew the mission without incident.

The 50’ AGL requirement was a given. I chose 85/80 overlap and 4 mph as a best guess at what might work vs. the amount of time the mission would take (actual flight time was about 1 hour - 3 batteries - 4 acres). I figured If the mission worked with these parameters, I could live with not chasing further time savings.

Alas, I didn’t win the project. My understanding is that the previous pilot that had been flying the mission had been flying with no questions ask of him (ie: probably unlicensed and uninsured). I think he got cold feet when DJI implemented their new “bowtie” clearance scheme and he had begun renting a ground based laser scanner to complete this site. I assume my price probably was used to get him to undercut his laser scanner price to complete the work. So be it, fortunately, I am not hungry enough to need to chase unprofitable work.

Again, let me know if questions. I learned a lot on this one along with gaining a few more gray hairs.

Steve

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Great write-up Steven! Good experience that I’m sure others will benefit from. As a word to everyone, never sell yourself short! That’s how people get bad reputations in construction and in the end it will bite them in the ass. Quality work for a fair price is where it’s at. There’s plenty of work for everyone out there and if you do a good job you’ll get repeat business from referrals for sure!

Is it good if your mapping larger area, maybe some building on local to do a couple maps at diffrent altitude or degrees???

If you are trying to get a more realistic 3D model of the structures then multiple altitudes is a good practice. If you are wanting a standard 3D model focusing on the terrain then it won’t make much of a difference and actually oblique images can hurt.

Say I wanted to do a detailed 3D model of a 1,000,000 SF warehouse. Are you saying I should do multiple flights at different altitudes, like 150, then again at 200, 250 & 300?

Yes, I would suggest a 150ft and a 250ft or 275ft. Just running a 150ft, which you think would give you what you need, may have trouble stitching the map together as a whole. Building roofs in particular can be pretty homogeneous and hard to stitch so flying higher will get more objects in view to tie together. Running the higher flight will also help reduce warping. You’ll want to keep the height of the building in mind because your closest subject may only be 110-120ft from the drone.

A 1m sqft building may also be a good example that DroneDeploy really doesn’t have a good flight plan for. I imagine it is pretty long so trying the Enhanced 3D Perimeter selection will struggle because at the ends it is still looking the central point of the flight plan and the perspective of the walls will be at a very sharp angle. Ideally you’ll want to be perpendicular to the faces of the walls for the best detail and stitching so strafing each face of the building would be better.