Photogrammetry is a science that enables detailed object analysis from images. Drones are probably one of the most convenient ways to gather all the necessary data (images) for big scale objects. For this reason, drone industry is tightly connected with photogrammetry. There are situations when companies once decided to start with photogrammetry, buys the latest drone in the market. Not a rare case when purchasing the newest drone one expect a stunning result, but the expectations might result in quite an opposite situation. That’s because there’s a big chance that it won’t be compatible with the existing flight planners yet.
UNMANNED AERIAL VEHICLE (UAV) – IS IT REALLY UNMANNED?
Drones that are usually known as unmanned aerial vehicles often managed through the flying apps. For this day, there are many flying planner apps that offer to select the desired track and set the drone in automated flying mode. Such apps are the best to ensure the track to be fully executed by a drone, set the necessary amount of pictures to be taken, ensure the necessary picture overlap. But what happens when a drone is not yet compatible with the flying planner? Our partner – a professional UAV pilot Lukas is sharing his insights on what to do in such situations.
TIPS AND TRICKS
In photogrammetry, images taken by drone have to comply with some requirements. It needs to fit quality standards, overlap, consistency, etc. Seeing as recently released DJI Mavic 2 Pro had no compatible flight planner shortly after its release, the only option was to fly manually as consistently as possible. To achieve reasonable consistency, one has to use a few features and settings:
- GPS flying mode for stability if flying high – 30 meters or more. Tripod mode (if available) if flying lower than 30 meters.
- Enable the rule of thirds grid on the app. When taking nadir (downward) pictures it is important to use the grid to see how much did the drone move between the shots.
- Keep an eye on the map view and the flight track that the drone is making. If the conditions are windy you may have to compensate for the wind. If the drone is not making straight lines but there’s no wind – calibration is required.
Using these tools one can achieve a good overlap and consistent scan quality. Also having manual camera settings is a bonus as it’s taking the pictures in raw format. With manual settings, one doesn’t have to rely on auto exposure and with raw file formats, can have the ability to tweak white balance and other settings after taking the pictures. This gives much flexibility and quality even if something goes wrong during the flight.
Manual focus, on the other hand, is not optional but obligatory. Always use manual focus (not any automatic mode) as misfocused or inconsistently focused pictures can ruin the whole process.
To begin flying manually, first set the required altitude that is dependent on the scene. Then face the camera down. Choose a flying direction. It can be just like a compass– north for example, or it can be directed according to the scene, perpendicular to the object or nearby road.
When the direction is set (and the drone is facing that direction) fly to such a position so that the scanned object or area is in the top right corner of the screen (as seen in picture 1).
When the object is in the required place – fly forwards (which is the chosen direction) and take a picture every time the drone moves for 1/6 of the initial view. For that, just track where the center marker is. Then take note of what is under the grid line above the center marker (picture 1).
One can fly as much as the center marker would be over the object that the grid line used to be and take the picture.
Repeat this process of flying 1/6th of the view from the center to the grid line and taking pictures each step. When taking pictures you can either choose to stop and press the shutter button or keep going at a consistent speed – most important thing is to take the pictures every 1/6th part of the view.
If you take the picture not exactly where you wanted, don’t worry and just keep going. Small deviations from this technique will not ruin the scan, but it’s always better to take more pictures than less. When the scanned object is at the bottom right corner of the view, it’s time to move sideways (picture 2).
Move sideways 1/6th of the picture using the vertical grid line and the same center point. Then fly backward using the same 1/6th rule with the help of the center point and grid lines (picture 3).
When the object is right at the top of the view, again move sideways to the right. It should become clear by now that you are flying in a grid pattern (picture 4). The same grid pattern the flight planners usually suggest.
To verify that look at the map in the app where the flight track is displayed. By taking pictures every 1/6th of the image you ensure enough overlap to recreate the object and make a true orthophoto image.
This technique should work in most cases of nadir scans. The technique should not be difficult at all for experienced fliers and approachable for beginners. Keep in mind that the larger the object the more pictures it is necessary to be taken. Flying manually straight for 20 minutes is not such an easy task to be always comfortable, beware of the surroundings and know local rules and regulations regarding drones.
Flight track should be always used by checking the alteration of the object in time. If this cannot be guaranteed, the part of photogrammetric capacity can’t be fully exploited. It is until the flight tracking apps are restarted.
It’s obvious that sometimes even the technological novelty and all the latest updates can interfere with the use of the technology itself.
More info: https://pixprocessing.com/pixprocessing/