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. Classification of Photographs (from Paine,
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.
Figure 6-2. Low oblique versus high oblique aerial
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
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.
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.
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
Vertical photographs are simple to use photogrammetrically as a minimum
of mathematical correction is required.
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.
Stereoscopic study is also more effective on vertical than on oblique photographs.
6.1.2. Advantages of oblique over vertical aerial photographs
An oblique photograph covers much more ground area than a vertical photo
taken from the same altitude and with the same focal length.
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.
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.
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.
Determination of feature elevations is more accurate using oblique photograph
than vertical aerial photographs.
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).
Figure 6.3. Basic geometry of a vertical aerial photograph.
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.