Data Collection Recommendations for Photos
This section outlines Skyline’s recommendations for optimal 3D PhotoMesh production from collected data:
§ Camera and lens combinations (hardware preferences)
§ Camera settings and image format
§ Flight planning and execution
Camera and Lens Combinations (Hardware Preferences)
The following factors in sensor and lens selection are critical for 3D model geometry and quality:
§ A high quality lens, with minimal distortion.
§ Fixed focal length - to avoid alternating focal lengths in photos which are meant to be processed in the same dataset.
§ For a standard grid collection pattern, it is important to have a lens with a field of view (FOV) at least 90 degrees wide in the long axis (60 degrees in the short axis, ~108 degrees in diagonal).
§ A FOV < 120 degrees in the long axis, since these lenses tend to be fish-eye and too distorted for high-quality aerotriangulation.
§ A lens which can maintain sharpness in the edges of the image - for proper aerotriangulation and texturing of facades in the 3D model.
§ A sensor which can collect images rapidly - to maintain high overlap collection in low altitude flights while achieving proper overlap.
Camera Settings and Image Format
The following camera settings are recommended:
§ Disable any "auto-rotate" options, to ensure that all of the images maintain a uniform orientation during the flight.
§ Avoid slow shutter speed (generally < 1/1000) since it will cause significant motion blur, resulting in photos that cannot be used in the PhotoMesh production process. Instead of allowing a shutter speed drop to 1/640 or 1/500, it is preferable to open up the aperture value as wide as possible, even at risk of slight softness at the corners of the image. It is also possible to increase the ISO value in high quality cameras, and still maintain non-grainy images with high dynamic range in bright and dark areas.
§ Disable any settings which might affect the relative location of the lens to the sensor, such as anti-shock or vibration features, since this will create a random principal point for the camera model. Vibration or motion in the UAV stabilization platform should be addressed with the camera and lens combination as a static object.
§ Avoid auto-focus or automatic zoom. Focus the lens (usually to infinity) and lock it in that position.
§ Use full resolution format, so that it is not down-sampled from the sensor pixels.
§ Collect the photos in full uncompressed format. If JPEG format is necessary, use minimum compression to avoid artifacts or missing information in the images.
Flight Planning and Execution
The following flight patterns and execution are recommended:
§ For single building or small area collection, use a circular flight pattern, with the target area in the center of the image from all perspectives.
§ For large scale collection, use a grid pattern which allows for efficient area collection with the goal of creating 3D products.
§ For nadir cameras with a field of view of approximately 60 degrees, tilt the camera 10-15 degrees forward to achieve the necessary 40-45 degree oblique perspective in all directions.
§ Maintain a frontal (vertical) overlap of 75 - 90% (recommended - 80%).
§ Maintain a side (lateral) overlap of 70 - 90% (recommended - 80%).
Mobile Photography
The following perspective and overlap are recommended for optimum reconstruction results:
§ Perspective – Capture required objects and facades at a perpendicular angle and at a 30 degree oblique variance (60 and 120 degrees) to maximize triangulation quality, while minimizing perspective distortion. In close-range data collection, any perspective which captures the object or façade at more than a 45 degree oblique will most likely be at a considerably lower resolution to provide effective input for the reconstruction.
§ Sky – Limit the sky component in the photo to no more than 30%. This can be achieved by setting the tripod to a higher altitude when possible, as well as by slightly angling the camera down (instead of pointing the camera directly at the horizon, 90 degrees from the ground, it can be angled 70-80 degrees from the ground).
§ Overlap – Maintain at least an 80% overlap, so that each object will appear in at least five photos. This provides PhotoMesh with the data to fill in holes in planes and facades that are partially obstructed by objects (such as trees and street furniture) which are closer to the camera.