Types of aerial photograph, scale relation

Types of aerial photographs

1. By camera axis:
   • Vertical: the camera axis is vertical (or tilted by less than 3°, unintentionally); used for mapping.
   • Tilted: the axis is unintentionally tilted by more than about 3°.
   • Low oblique: the axis is intentionally inclined, but the horizon does not appear in the photograph.
   • High oblique: the axis is inclined so much that the horizon appears in the photograph.
2. By scale: large, medium and small scale photographs.
3. By film: panchromatic, infrared, colour and false-colour photographs.

Scale of a vertical photograph

        O (lens)
       /|\
    f / | \
     /  |  \
   a----n----b   photo
   -----------
   A----N----B   ground (H - h below O)

The scale is the ratio of a distance on the photograph to the corresponding distance on the ground. By similar triangles $Oab$ and $OAB$:

$$S = \frac{ab}{AB} = \frac{f}{H-h}$$

where $f$ is the focal length, $H$ the flying height above datum, and $h$ the elevation of the ground above datum. For flat ground at datum, $S = f/H$. For undulating terrain, an average scale uses the mean elevation $h_{av}$: $S = f/(H-h_{av})$. The scale also equals $S = \dfrac{\text{photo distance}}{\text{map distance}}\times\text{map scale}$.