The most basic thing to keep in mind when scanning a scene is to cover the parts you want to be included in the finished model. To make sure no details are missed during a scan, we recommend using continuous shooting mode on your camera. If that is not possible you might be able to use a time-lapse to capture photos as often as possible. If you want to scan a larger scene it’s best to divide it up in several scans. When scanning in this way it’s important that scans overlap so that it’s possible to align them to each other.
Below are some quick tips to remind you of the most important things to bear in mind while capturing a scene. For more advanced instructions, consult chapters following this one.
- Take photos on a cloudy day rather than in direct sun light.
- Orient the camera to maximize the space of the building in the image.
- Take photos in sequences, using a lawn mower pattern to get several overlapping photos of each part of the building.
- Avoid stationary rotation of the camera while photographing. Instead use smooth motion to capture corners.
- Remove photos from the start and landing that contain little information about the scene.
- Retake photos that are over-/underexposed or unfocused. The more time you spend shooting, the better the results will be.
One of the most important things to bear in mind is how crucial overlap is for good reconstructions. Overlap in this case means how much of the the content of another. An overlap of 50% means that half of the visible content in a photo is visible in the next photo. For an example see the image below.
CONSECUTIVE PHOTO OVERLAP
In order to get a good reconstruction, su cient overlap between photos is required. To achieve an accurate reconstruction of an area, it has to be visible in at least three cameras. To ensure this, an overlap of more than 75% between consecutive photos is recommended. To get a good overlap when ying around corners, y at a slower speed when passing the corners. We also recommend using a “lawn mower pattern”, like in the image below, when covering an area so that overlap will occur on all sides of photos.
To enable good reconstruction when flying in sequences, it’s important that sequences have a good overlap. It’s recommended that at least 10% of photos have a 75% overlap in between sequences. To achieve this we recommend that, if sequences are divided into building sides, corners are passed in each sequence so that overlap occurs when shooting the next building side.
To increase the quality of the reconstruction, it’s essential that all objects of interest are photographed from multiple angles, that the photo sequence has an internal overlap, and that in-place rotations of the camera are kept to a minimum.
Facade sweep sequences are meant to capture all vertical surfaces as well as complex details. These can be performed using the same “lawnmower pattern” as used in the ortho flight described below. We recommend turning the camera a bit towards the ground when shooting these sequences. As the camera passes the base of the roof you can angle it a bit further down to capture less sky and more roof in each image. At the camera’s highest altitude, turn it almost straight down (see the figure) to get a better angle toward the roof.
If UAV photography is used for data collection, it’s recommended to photograph both ortho and facade sweeps. The ortho flight is basically traditional aerial photography where the camera is angled directly toward the ground. These flights are meant to cover the entire area of interest, to make sure the ground and all the roofs are reconstructed correctly, and to simplify alignment of the facade sweeps.
To get full coverage of the area, fly in a “lawnmower pattern” covering the entire area, and then straight back to the start position, to connect the flight. In the lawnmower pattern, 75% sequential overlap is recommended, while 60% overlap between each row is sufficient (as illustrated in the image above).
Because of the rotational problem mentioned above, take special care not to rotate around a building when passing a corner. The figure below illustrates how to correctly pass corners when shooting a sequence.
Avoid in-place rotations of the camera at all costs! As consecutive photos taken from the same position affect the reconstruction negatively, it’s important to avoid rotating the camera when shooting. Instead, photos of closed-off areas should be taken by sweeping them multiple times.