## 6.1. CLASSIFICATION OF PHOTOGRAPHS

A number of systems have been used to classify photographs. The most common system is the one that separates photographs into terrestrial and aerial (Figure 6-1).

Terrestrial photos are taken on the ground, while aerial photos are taken from a platform in the air. Aerial photographs may be either vertical or oblique. Vertical photographs can be truly vertical, or slightly tilted (less than 3o from the vertical). Most aerial photos are tilted to some degree. Therefore, the use of the term vertical photographs in this chapter assumes truly vertical photographs, while in reality they might be tilted up to 3o. Oblique aerial photos are photographs purposely taken with an angle between 3 and 90o from the vertical. They can be low oblique (if the horizon is not visible) or high oblique (if the horizon is visible) (Figure 6.2, Low oblique versus high oblique aerial photographs). Terrestrial photos are usually oblique or horizontal, where the axis of the camera is tilted about 90o form the vertical.

Whether they are vertical or oblique, aerial photographs may be obtained in various formats and sizes depending on the purpose and the type of application they are used for. The format of most aerial photographs is square and sometimes rectangular depending on the camera but the most common format is 23 by 23 centimeters (9 by 9 inches). Vertical and oblique aerial photographs also have advantages over one another in respect to their usage.

### 6.1.1. Advantages of vertical over oblique aerial photographs

1. Vertical photographs present approximately uniform scale throughout the photo but not oblique photos. It follows that making measurements (e.g., distances and directions) on vertical photographs is easier and more accurate.
2. Because of a constant scale throughout a vertical photograph, the determination of directions (i.e., bearing or azimuth) can be performed in the same manner as a map. This is not true for an oblique photo because of the distortions.
3. Because of a constant scale, vertical photographs are easier to interpret than oblique photographs. Furthermore, tall objects (e.g., buildings, trees, hills, etc.) will not mask other objects as much as they would on oblique photos.
4. Vertical photographs are simple to use photogrammetrically as a minimum of mathematical correction is required.
5. To some extent and under certain conditions (e.g., flat terrain), a vertical aerial photograph may be used as a map if a coordinate grid system and legend information are added.
6. Stereoscopic study is also more effective on vertical than on oblique photographs.

### 6.1.2. Advantages of oblique over vertical aerial photographs

1. An oblique photograph covers much more ground area than a vertical photo taken from the same altitude and with the same focal length.
2. If an area is frequently covered by cloud layer, it may be too low and/or impossible to take vertical photographs, but there may be enough clearance for oblique coverage.
3. Oblique photos have a more natural view because we are accustomed to seeing the ground features obliquely. For example, tall objects such as bridges, buildings, towers, trees, etc. will be more recognizable because the silhouettes of these objects are visible.
4. Objects that are under trees or under other tall objects may not be visible on vertical photos if they are viewed from above. Also some objects, such as ridges, cliffs, caves, etc., may not show on a vertical photograph if they are directly beneath the camera.
5. Determination of feature elevations is more accurate using oblique photograph than vertical aerial photographs.
6. Because oblique aerial photos are not used for photogrammetric and precision purposes, they may use inexpensive cameras.

### 6.1.3. Advantages of aerial photography (vertical or oblique)

Aerial photographs have the advantage of providing us with synoptic views of large areas. This characteristic also allows us to examine and interpret objects simultaneously on large areas and determine their spatial relationships, which is not possible from the ground. Aerial photographs are also cost effective in interpreting and managing natural resources. They have played a significant role in map making and data analysis.

The basic geometry of a vertical aerial photograph is illustrated in figure 6.3. Incoming light rays from objects on the ground pass through the camera lens before they are imaged on the film in the focal plane. The distance between the lens and the focal plane is termed focallength. The x coordinate axis is arbitrarily assigned to the imaginary flight line direction on the photograph and the y-axis is assigned to a line that is perpendicular to the x-axis. These two axes usually correspond to the lines connecting the opposite fiducial marks recorded on each side of the print (i.e., positive image).

The photocoordinate origin, o, is considered to be the intersection of the two lines joining the fiducial marks and, therefore, can be assumed to coincide exactly with the physical center (i.e., the geometric center) of the photograph, called principal point. Points to the east side of y-axis have positive x coordinates and points to the west side have negative x coordinates. Similarly, points north of the x-axis have positive y coordinates and points south of the x-axis have negative y coordinates.

 Objectives  |  6.1  Classification of Photographs  |  6.2  Focal Length and Angle of Coverage