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Linear Mission Map Mission Large Section

This was my first attempt to map a two-lane road with a linear mission and my map is missing large sections. It is also not aligned well. I used only 2 passes but it seemed to easily cover the area I was interested in and provide enough overlap. I merged photos from two separate missions together to create the map. The weather conditions did change slightly between the two flights. The first flight was partly cloudy and flew at ~20 mph. The second flight was cloudier and flew at 13 mph. Any ideas why the map turned as it did?https://www.mapsmadeeasy.com/maps/public/ae613bc64aa443aab0065482280d174f 

https://www.mapsmadeeasy.com/maps/public/ae613bc64aa443aab0065482280d174f

 

Jacob

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Two linear passes is not enough. We recommend at least 3 passes in our Data Collection guidelines but even then you will need to up your along track overlap to make up for the lost coverage.

If you are using 75% across track overlap on two passes you will only bet getting 2X coverage on a stripe that is 3/4 the width of one image. Due to this you would need to up your along track overlap to 85%+ to get enough looks at the scene to reconstruct it. 

Also, the longer the linear mission the more passes you are going to need to get it to process properly. Think of it as a table. Does a table stand on two legs? Nope. 3? Kinda. 4? Getting there. 

The bottom line is you want about 15 looks at everything you want to include in your map. 

Zane

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Lots of good information posted about linear missions, but my test project in MME yesterday (2/6/19) errored out, so I haven't quite got it figured out. I used DJI Pilot app to create a 100-waypoint route; more or less 50 points going north and 50 points returning south; the two passes about 12 meters apart; roughly 1/4-mile in length oneway; flight elevation = 48 meters. I made a copy of that mission, and after moving the waypoints just slightly, I flew that mission in order to have a total of 4 passes, about 200 photos to process. 

My assumption is that - to use Zane's table analogy - my data (table) needs a couple more legs to stand on; I have a north and a south leg, but I need to spread out my passes to the east and west to get two more legs under it.

A client wants nighttime imagery of streetlights in a 5 street by 9 street project area, and I think photogrammetry of the street corridors makes most sense, and ignoring the dark building rooftops within each city block. But I am looking for advice!

Curt Meyer 0 votes
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Curt: Your job failed because you used Windows Photo Editor to reduce the resolution of the images and it killed the EXIF tag information. It also appears that the overlap was about 60% which isn't enough to work for a linear mission. 

 

Zane 0 votes
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Thanks for responding Zane. I reflew this section of road and collected over 1000 images. Used 3 separate takeoff locations, 4 passes with 85% overlap. The resulting map is severely distorted. Any ideas? This was an expensive job to process with disappointing results.  

Jacob 0 votes
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If you used 3 different takeoff locations and they are not all the same elevation but the ground reference images were still uploaded that is a good way to mess things up. 

The bigger issue here is that the aircraft was crabbed about 35 degrees during the passes. There was likely a strong cross wind up there to cause this. It is up to the pilot to check for this and adjust using the left stick to yaw the aircraft appropriately. This especially matters when doing linear flights where across track overlap is already limited. 

Due to this crabbing only a small straight section of the map actually worked out but got turned sideways making it look the way it does. 

Since your overlap and number of passes was good (ie. you followed the instructions) we will refund this job so you can try it again while monitoring the camera's alignment with the flight path. 

The only other thing I would keep an eye on would be trying to do mapping jobs that will take a long time to collect (especially linear ones) when the lighting will be consistent. If you do one pass while it is sunny and the next pass back it is cloudy the features will fail to match. 

The DJI aircraft used to do a better job at keeping the camera aligned with the path of flight but I think in the last year or so they have deprioritized that in the name of flight stability (which is probably a good thing). 

Zane 0 votes
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Thank you for the detailed answer. I'll look into correcting aircraft crabbing in windy conditions. I thought the gimbaled camera would correct for that. I don't have the budget on this project to refly the site but will attempt to process smaller sections being careful to include only one takeoff location photo. Thanks again for this great service and feedback. I look forward to processing many more jobs.

Jacob 0 votes