There are several basic principles for acquiring aerial data for 3D modeling:
The higher the image accuracy, the better the 3D effect. The lower the drone, the higher the resolution of the photo, the larger the CCD sensor format, and the better the three-dimensional result obtained. Meanwhile, the 3D effect of the image speaks louder than the video.
The drone must move and should not rotate the camera at a fixed point to take a separate set of images like a panorama. We must let the drone move to take multiple sets of images.
The overlapping degree between photos should be intensive - 70% is basic, 80%~85% is recommended, more than 90% is wasteful on image resources.
There are two basic shooting methods which can be flexibly combined to apply in most scenes as follows.
Surround Flight - If you are using a DJI drone, DJI Go's fixed-point surround flight is recommended.
Poly-line Flight - If you are using a DJI drone, Altizure is recommended.
We are going to explain both shooting modes in more details below.
Surround Flight is a circular flight around the object to be modeled with the camera targeting on the subject being modeled.This shooting method is especially suitable for shooting a single building or landmark with good 3D reconstruction effect and few images required. It is quite economical and practical.
Here's a surround flight example of a single building:
If the building is relatively tall, multi-layer circular photography can be adopted to ensure that both the top and bottom areas can be well covered by high-precision images. E.g.
Poly-line Flight is a path flight that lets the drone go through the zigzag route and scans the entire area to be photographed. This method is more suitable for shooting large-area scenes.
The following is a poly-line flight example.
These two basic flight models have their own individual advantages and disadvantages, and should be flexibly applied in practice to achieve better results. For example, in one case, the creator used a poly-line flight to cover a large area of the scene with surround flight focusing on the main building.
Flight Altitude and Ground Accuracy
The main measure of ground accuracy is the ground sampling density GSD, which is measured in centimeters per pixel. For the same camera, the lower the altitude, the higher the ground accuracy, and the more detailed the three-dimensional reconstruction results.The following table shows the number of photos and flight altitudes required to capture a square kilometer at 5, 10, or 20 GSD with 70% overlapping.
The number of acquisition photos above is for reference only. The actual number is closely related to the accuracy of the flight path, the weather, camera shooting speed, the shape of the acquisition range, and the operation ability of the collector.
The drone needs to fly over the target area in accordance with the illustrated folding line. The flight path needs to be expanded slightly beyond the actual acquisition area. The extended distance is equal to the flight altitude. For example, if the flying altitude is 100 meters, it needs to expand 100 meters outwardly.
- Target Area
- Flight Path
Heading and side image overlapping requires at least 70%. To ensure the best 3D effect, take aerial photos instead of aerial video.
In addition to keeping the camera perpendicular to the ground, you also need to tilt the camera at a 30-40 degree to the vertical for shooting the image in east, south, west and north directions.