Advantage: measurements and directions (azimuth) are more accurate and easy; scale is almost uniform on the photo; tall objects do not obscure short objects; easy to use for stereopair processing.
Lehman College, 2017
Oblique (high and low)
Advantage: cover larger area; better show texture (i.e. high/low objects) on the ground because of shadows and perspective; this also helps to assess relative height of the objects; does not require aircraft to fly directly over the object.
ORTHOPHOTO: corrected image with uniform scale. The process of correcting is called orthorectification; it removes distortion effects caused by terrain relief and camera tilting.
Height of objects can be calculated using Pythagoras Law:
tanα is a constant for the image, i.e. by knowing at least one height of any object on the image we can identify heights (H) of other objects as: H = tanα• L
To make linear measurements (e.g. L) on the image we need to know image scale that shows relationship between units of measurement on the image and on the terrain. In cartographic tradition scale is usually represented as a fraction, e.g. 1:2,000 or 1:300,000. This also can be written as 2K or 300K scales. Therefore in scale calculations we always need to keep nominator as 1.
The units of nominator and denominator should be the same. Because focal lens is usually expressed in mm we need to convert aircraft height into mm as well. This is easy since 1 m = 100 cm and 1 cm = 10 mm. Therefore 1 m = 1000 mm.
The aircraft altitude is the height above ground, not the absolute altitude above mean sea level
Example: if you use lenses with focal length 135 mm and the height of the flight is 700 m (conversion to mm makes it 700,000) then the image scale will be = 135 / 700000 = 1/ 5185 or 1:5185.
For aerial photo with well-defined Principal Point the object height can be calculated using this equation (of course units should be the same):
H = m / r * h,
where:
m – relief displacement
r – distance from the Principal Point to the top of the object