Back in the era of analog aerial surveys, spanning most of the 20th century, aerial cameras and aerial images were basically standardized. Image size and focal length determine a cameras field of view which, together with flying heights, defines ground coverage. With such standardized equipment, the quality of orthophotos could be easily specified by defining the overlap between images (forward overlap) and strips (side overlap), together with the camera’s focal length.
However, the substantial growth of digital aerial surveying over the last decade has produced many different digital airborne cameras that offer a wide variety of frame sizes, focal lengths, pixel sizes and FOVs. The new developments have made overlap obsolete as the sole parameter for specifying flight planning for orthophoto production.
VisionMap’s Chief Scientist Dr. Yuri Raizman has taken the liberty of putting forth a new approach, which takes into consideration the new realities of the digital era.
The main parameters that determine the quality of orthophoto are occlusion and image resolution. A camera’s central projection causes building facades to be visible in the image. The buildings seam to lean over, so that facades are visible on one side of the building, but objects on the other side are occluded.
This effect increases from the center of the image to the borders, and as such, the border parts of the images, where leaning and occlusion are most severe, are unusable. Instead of being able to use the entire FOV, we can only use smaller views defined by the permissible angles across and along strip. These angles, which determine the usable image area for orthophoto production, are therefore the main geometric parameters defining the metric quality and readability of orthophotos, rather than overlap. Finally, these angles directly define a building leaning for orthophoto.
In fact, for the same forward and side overlaps, building leaning varies greatly between different digital cameras. For example, For 60% forward overlap building leaning ranges from 5% to 32%, for 40% side overlap the values range from 15% to 84%, while for 25% side overlap leaning ranges from 19% to 105%. The smaller the building leaning, the better the orthophoto readability.
In order to compare the main parameters of flight plans for various cameras, a flight plan for an open area was created. The following design criteria were used: forward and side overlap should be at least 60% and 25%, respectively; ground sample distance should be 25cm; the permissible angle should be less than 60°; forward overlap of 60% should be achieved at an affordable frame rate (FPS) at an aircraft ground speed of 250 knots. You can read the complete findings here.
VisionMap’s A3 Digital Mapping System can achieve the specifications with a distance between flight lines of nearly 10 kilometers.
Understanding the geometric principles of flight planning explained in this post allows an accurate analysis of the performance of digital cameras – and saves valuable time and resources.
Until next time,
Sarah Hakim, Marketing Team
Special Thanks to VisionMap’s Dr. Yuri Raizman for this post.