Making successful surveys

Surveys will be made up of a series of stitched photographs. Proper overlap, sidelap and other factors are required to avoid errors such as holes in the data (unstitched regions) or inaccuracies. Typical issues are: Motion Blur, Unfocused cameras, vignetting on images, insufficient image overlap, non-nadir photos (e.g. photos taken during turns), photos taken at low altitude, homogenous imagery, etc.

Stitching Defined – each photo taken from the drone contains ‘features’ such as crop rows, trees, buildings, trails left by equipment, or anything that is distinctly recognizable in the visual space. As the aircraft takes continuous photos during the mission, it captures multiple photos of each distinct feature, from multiple angles. These features are identified and matched by a mathematical process, and aligned on top of each other. This is not simple. You must adjust your aircraft and camera settings to take images that make the stitching process as simple as possible.

Some methods to assist the stitching process are:

1. Flying higher
   Flying higher gives the camera lens (with a given field of view) more land area to cover in a single image. This grants the drone more chances to cover common unique features in multiple images, which can help mapping in areas with homogeneous imagery (such as tree canopies or agricultural areas). It also enables higher frontlap to be achieved for a given camera. Flying higher is the single most powerful way to improve data quality.
2. Modifying flight path to fly down the longest lines
   The Flight Direction feature allows you to change the direction that your drone flies. Changing the flight path can assist when you are mapping a narrow shape so that the drone conserves battery life. Adjust the flight path to make the longest straight lines as possible.
3. Increase side overlap (sidelap)
   Flying with more sidelap between each leg of the flight is the easiest way to get more matched features in the imagery, but it does come at the expense of reducing the area that your drone can cover in one flight, and adding more photos to be stitched.
4. Increasing front overlap (frontlap)
   This will increase the number of photos taken during each leg by simply making your camera take photos more quickly. Your camera will have a hard limit on how fast it can operate, so after you hit that point you will not see any further improvements. At this time the limit is about 1 photo per second. With rotary wing aircraft the aircraft will slow its forward flight speed as required to obtain the requested frontlap. With fixed wing aircraft, it cannot slow below its stall speed. Most fixed wing survey planes cannot fly slower than about 30 knots.
5. Starting Waypoint
   With a rotary wing sUAS it is not critical to start the mission at a certain point on the map. For a fixed wing sUAS it is critical to set up the flight path knowing the wind direction. Most fixed wing sUAS conserve energy and fly longer if they fly perpendicular to the wind. Start downwind so that the sUAS is always turning into the wind. Turning downwind makes for very large turns which are very difficult for the autopilot to overcome. Turning into the wind makes for tighter turns and shorter overshoot. Overshoot is the distance beyond the border that the plane needs to turn around and get back onto the proper flight path again.
6. Check your camera settings and quality of individual photos
   Mission planning software typically attempts to make your camera capture imagery at its absolute best quality. However, ultimately image quality is governed by so many other parameters (some of them listed above), that it is useful for users to check the quality of the individual photos captured as the drone is performing the mission flight. By clicking the Automatic Camera Settings toggle button off, you can manually adjust your camera settings. Set the focus (if there is one) to infinity. Set the f-stop, shutter speed and ISO so that the exposure is just a little dark. Images can be lightened but an over exposed pixel is worthless. Since you are taking photos of the ground only, and not the sky, it is much easier to set the exposure without over exposing some areas.
7. Don’t fly when it is too windy.
   Usually about 10 knots is the maximum. While the aircraft can typically fly in much higher wind conditions the vegetation on the ground will be swaying back and forth and cause issues with stitching. If there is no vegetation or other things that can blow around, then it’s OK to fly at higher speeds. However, high winds usually indicate high gusts. Gusts are a problem because this causes the copter to constantly correct its flight path which moves the camera more which causes blur.