Various type of photographs for aerial photogrammetry

This is a continuation of previous post about various type of photographs used in aerial photogrammetry.
Vertical photo for aerial photogrammetry
A vertical photograph for aerial photogrammetry is taken with the camera pointed as straight down as possible Allowable tolerance is usually + 3° from the perpendicular (plumb) line to the camera axis. The result is coincident with the camera axis. A vertical photograph has the following characteristics:

(1)   The lens axis is perpendicular to the surface of the earth.
(2)   It covers a relatively small area.
(3)   The shape of the ground area covered on a single vertical photo closely approximates a square or rectangle.
(4)   Being a view from above, it gives an unfamiliar view of the ground.
(5)   Distance and directions may approach the accuracy of maps if taken over flat terrain.
(6)   Relief is not readily apparent.

Three terms need defining here; they are Principal Point, Nadir and Isocenter.  They are defined as follows:

 1. Principal Point - The principal point is the point where the perpendicular projected through the center of the lens intersects the photo image.

 2. Nadir - The Nadir is the point vertically beneath the camera center at the time of exposure.

 3. Isocenter - The point on the photo that falls on a line half- way between the principal point and the Nadir point.

Low Oblique for aerial photogrammetry

 This is a photograph for aerial photogrammetry taken with the camera inclined about 30° from the vertical. It is used to study an area before an attack, to substitute for a reconnaissance, to substitute for a map, or to supplement a map. A low oblique has the following characteristics:

(1)   It covers a relatively small area.
(2)   The ground area covered is a trapezoid, although the photo is square or rectangular.
(3)   The objects have a more familiar view, comparable to viewing from the top of a high hill or tall building.
(4)   No scale is applicable to the entire photograph, and distance cannot be measured. Parallel lines on the ground are not parallel on this photograph; therefore, direction (azimuth) cannot be measured.
(5)   Relief is discernible but distorted.
(6)   It does not show the horizon.

High Oblique for aerial photogrammetry

The high oblique for aerial photogrammetry is a photograph taken with the camera inclined about 60° from the vertical. It has a limited military application; it is used primarily in the making of aeronautical charts. However, it may be the only photography available. A high oblique has the following characteristics:

(1)   It covers a very large area (not all usable).
(2)   The ground area covered is a trapezoid, but the photograph is square or rectangular.
(3)   The view varies from the very familiar to unfamiliar, depending on the height at which the photograph is taken.
(4)   Distances and directions are not measured on this photograph for the same reasons that they are not measured on the low oblique.
(5)   Relief may be quite discernible but distorted as in any oblique view. The relief is not apparent in a high altitude, high oblique.
(6)   The horizon is always visible.

Trimetrogon for aerial photogrammetry

This is an assemblage of three photographs taken at the same time, one vertical and two high obliques, in a direction at right angle to the line of flight. The obliques, taken at an angle of 60° from the vertical, side lap the vertical photography, producing composites from horizon to horizon.

Multiple Lens Photography for aerial photogrammetry : These are composite photographs taken with one camera having two or more lenses, or by two or more cameras. The photographs are combinations of two, four, or eight obliques around a vertical. The obliques are rectified to permit assembly as verticals on a common plane.

Convergent Photography for aerial photogrammetry: These are done with a single twin-lens, wide-angle camera, or with two single-lens, wide-angle cameras coupled rigidly in the same mount so that each camera axis converges when intentionally tilted a prescribed amount (usually 15 or 20°) from the vertical. Again, the cameras are exposed at the same time. For precision mapping, the optical axes of the cameras are parallel to the line of flight, and for reconnaissance photography, the camera axes are at high angles to the line of flight.

Panoramic for aerial photogrammetry: The development and increasing use of panoramic photography in aerial reconnaissance has resulted from the need to cover in greater detail more and more areas of the world.
(1)  To cover the large areas involved, and to resolve the desired ground detail, present-day reconnaissance systems must operate at extremely high-resolution levels. Unfortunately, high-resolution levels and wide-angular coverage are basically contradicting requirements.

(2)   A panoramic camera is a scanning type of camera that sweeps the terrain of interest from side to side across the direction of flight. This permits the panoramic camera to record a much wider area of ground than either frame or strip cameras. As in the case of the frame cameras, continuous cover is obtained by properly spaced exposures timed to give sufficient overlap between frames. Panoramic cameras are most advantageous for applications requiring the resolution of small ground detail from high altitudes.