Arial Photo Platform

In his book on aerial photo platform interpretation, Paine presents a dichotomous key for classifying aerial photo platform.
We can define vertical aerial photo platform as an aerial photo platform taken from an aerial photo platform (either moving or stationary) wherein the camera axis at the moment of exposure is truly vertical. In actuality, vertical airphotos with less than 3* tilt are considered vertical (for most aerial photo platform interpretation purposes); while those with more than 3* tilt are considered oblique. There are two basic types of oblique aerial photo platform photography. These two types are: 1. High angle oblique; and 2. Low angle oblique. In a high angle oblique, the apparent horizon is shown; while in a low angle oblique the apparent horizon is not shown. Often because of atmosphere haze or other types of obscuration the true horizon of an aerial photo platform cannot really be seen. However we often can see a horizon in an oblique aerial photo platform. This is the apparent horizon.

The basic advantages of vertical air photos are:

1. The scale is essentially constant;
2. Measurements of directions are easier than on an oblique photograph. Directions can also be measured more accurately;
3. Within limits a vertical aerial photo platform photograph can be used as a map (if grids and marginal data are added)
4. Vertical aerial photo platform photographs are often easier to interpret than oblique aerial photo platform photographs and are better for stereo (there is no masking)

The advantages of an oblique aerial photo platform photograph include:

1. Given a constant altitude and camera you can cover a much larger area on a single aerial photo platform
2. The view of some objects is more familiar to the interpreter
3. Some objects not visible on vertical photos may be seen on oblique.
(Paine talks about clearance and cloud cover; but that’s a tricky one (too cloudy for vertical but maybe enough clearance for an oblique).

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 aerial photo platform image.
2. Nadir- The Nadir is the point vertically beneath the camera center at the time of exposure.
3. Isocenter- The point on the aerial photo platform that falls on a line half-way between the Principal point and the Nadir point.

On a true vertical aerial photo platform photograph all three of these would be at the same point. There is no such thing as a true vertical aerial photo platform photo. All aerial photo platforms have some degree of tip or tilt.

These points are important because certain types of displacement and distortion radiate from them. It is the Isocenter of the aerial photo platform from which tilt displacement radiates.

Lets consider the viewing perspective of a map. On a map objects and features are both planimetrically and geometrically accurate. That is objects are located on the map in exactly the same position relative to each other as they are on the surface of the Earth, except with a change in scale. This is due to the fact that maps use an orthographic projection (i.e. using parallel lines of site) and constant scale to represent features.

Aerial photo platform photographs on the other hand are created using a central or perspective projection. Therefore, the relative position and geometry of the objects depicted depends upon the location from which the aerial photo platform was taken. Because of this we get certain forms of distortion and displacement in Air Photos.

There are basically four types of distortions and three types of displacement.

Types of distortion include:

1. Film and Print Shrinkage
2. Atmospheric refraction of light rays
3. Image motion
4. Lens distortion

Types of displacement include:

1. Curvature of the Earth
2. Tilt
3. Topographic or relief (including object height)

The effects of film shrinkage, atmospheric refraction and the curvature of the Earth are usually negligible in most cases- the exception is precise mapping projects. Of these, lens distortion is usually the smallest. So displacement is typically the largest problem/effect impacting our analyses.

Both distortion and displacement cause changes in the apparent location of objects in photos. The distinction between the types of effects caused lies in the nature of the changes in the photos.

Distortion- Shift in the location of an object that changes the perspective characteristics of the aerial photo platform.

Displacement- Shift in the location of an object in an aerial photo platform that does not change the perspective characteristics of the aerial photo platform. (The fiducial distance between an object’s image and it’s true plan position which is caused by change in elevation.)

These types of phenomena are most evident in terrain with high local relief or significant vertical features. These are problems/effects associated with:

1. Lens distortion
2. Tilt Displacement
3. Topographic Displacement

Lens Distortion- Small effects due to the flaws in the optical components (i.e. Lens) of camera systems leading to distortions (which are typically more serious at the edges of photos). Car windows/windshields, carnival mirrors are probably the best known examples of this type of effect. These effects are radial from the principal point (making objects appear either closer to, or farther from the principal point than they actually are); and may be corrected using calibration curves.

Tilt Displacement- A tilted photograph presents a slightly oblique view rather than a true vertical record. All aerial photo platforms have some tilt. The perfect gyro stabilization unit, like the perfect lens, has yet to be built. Tilt is caused by the rotation of the aerial photo platform away from the vertical. This type of displacement typically occurs along the axis of the wings or the flight line. Tilt displacement radiates from the isocenter of the aerial photo platform and causes objects to be displaced radially towards the isocenter on the upper side of the tilted aerial photo platform and radially outward on the lower side. If the amount and direction of tilt are known then the aerial photo platform may be rectified.

Topographic Displacement- This is typically the most serious type of displacement. This displacement radiates outward from Nadir. Topographic displacement is caused by the perspective geometry of the camera and the terrain at varying elevations. Topographic displacement is not necessarily bad as it allows stereo viewing, height measurement and Topographic mapping.