Aerial Photography – Oblique vs. Vertical and Other Considerations

When many people think of aerial photographs that are taken for commercial purposes, they tend to think of vertical photos, also referred to as straight-down or nadir images: photos that are taken directly over the subject. This is seldom the case, however, unless one considers photos that are being taken by satellites. Even satellites having advanced technologies are capable of taking the more common type of aerial photographs or videos, and that type is oblique imaging.

 

When Vertical Is Best, and When It Isn’t

When you take a photo of a plot of land, the vertical view is often best, since there will be no distortion of distances, and distances can in some cases be critical. When, on the other hand, the subject is a structure, oblique camerawork is by far the best approach, and when taking videographs, it is normally the only approach. Oblique photographs show detail, such as heights, that vertical images cannot. Ideally, oblique photographs are taken at an angle of 45 degrees. If the precise angle is known, processing software can adjust for distortion when structural dimensions need to be calculated.

Our real estate and land developer clients prefer oblique imagery because of the greater detail that is given. City planners, who in times past worked primarily from maps, need both vertical and oblique imagery, and we of course are more than capable of supplying both. Oblique images are by far the best way to keep a record of how major construction efforts are progressing, and this is an area in which video images really shine. Whether conventional digital cameras or video cameras are used, to achieve optimum results often requires the use of several cameras and requires a significant amount of post-processing.

 

The Role of Geometry – Principal Points and Isocenters

In order to provide oblique images of the highest quality, our photographers not only have to have a thorough knowledge of advanced photography techniques, but need to be familiar with the  geometry involved as well, an area most land-based photographers are unfamiliar with. Our staff members are well versed in the concepts of principal point, focal length, field of view, photo scale, and isocenters. They understand why high-angle photos or videos are best for some assignments and why low-angle photographs and videos provide superior results in others. They know when a certain amount of distortion can be tolerated, and may even be welcomed, and when it cannot. They know how to calculate the point of least distortion, a calculation that often has to be performed on the spot.

In addition to the geometry involved, the photographer needs to take into account the elements of size, shape, shadows, and sometimes tones and colors. Often these elements will determine what time of day is best to shoot, especially to obtain oblique images and they can be a critical factor in scheduling aerial photograph assignments.

Aerial photography and videography is not a matter of point and shoot, although there are those who follow that practice. There is a time and place for point-and-shoot photography, and there are certainly assignments where a great deal of previous preparation or post-processing is not required. This is one reason why we make every effort to determine as accurately as possible what our clients’ requirements are and what is expected of us.

 

Sources:

http://web.pdx.edu/~jduh/courses/geog493f09/Students/W2_Basics%20of%20aerial%20Photography%20Geometry.pdf

http://en.wikipedia.org/wiki/Areial_photographic_and_satellite_image_interpretation