Stereoscopic Analysis of Aerial Photographs

Aerial Photographs were the primary source of visual information used in Remote Sensing before satellite imagery became widely available. Stereoscopic analysis of aerial photographs were widely used for reconnaissance purposes during the world war. Apart from this, interpretation of aerial photographs have proven to be useful in various field of study such  as geology, surveying, mine planning and development and even environmental studies.

aerial photographs, stereoscopes and a map.

A typical aerial photograph usually contains, information such as the flight line, time of capture, altitude and the photo number. The flight line and photo number becomes useful when locating the photographed area on a map. The scale of the photo is also an important detail. The scale can be obtained by dividing focal length of the camera by the vertical distance from the imaged ground, to the camera. Aerial photographs can be panchromatic(black and white), colour, or in any other band of the electromagnetic spectrum. These details should be selected depending on the scope and objective of the image analysis.

In the process of interpreting aerial images, several interpretative elements are used in order to make the task of identifying and interpreting objects in aerial photographs easier and systematic. These elements of interpretation are,

Shape - The general outline or form of the object. regular and irregular shapes can help identify natural and artificial object.

Tone/Colour - It refers to the relative brightness or colour of an image. While in colour images, objects can be identified by it associated natural colour alone, in panchromatic images identification is slightly more difficult with varying shades of grey.

Size - Refers to the physical extent or the dimensions of the object. It is often useful to assess sizes of objects relative to each other.

Texture - Refers to the frequency of tonal change in an area of an image. can be described as smooth, coarse etc. For example; water appears smooth while a forest canopy shows a rough texture.

Shadow - shadows provide valuable information about the height of an object while also being a tool of identification of certain features in monotonous areas. For example, shadows can help identify lamp posts, water towers which would otherwise be misinterpreted because of its plan view. Shadows can also help identify sand dunes in a desert.

Pattern - Pattern is the spacial arrangement of objects on the landscape. can be describes as regular, ordered, irregular symmetric etc. Can help distinguish agriculture from natural vegetation.

Association - refers to the fact that certain features and activities are always related to the presence of certain other features and activities. This association of one object to another helps the process of identification and also can provide additional information of the area under analysis. For example, a populated area indicates the presence of roads and other infrastructure and an area with fuel tanks, chimneys etc could imply that the area under observation is a power station.

Identifying most of these features using two dimensional images is a very difficult task and can lead to confusion.The identification of the objects in a photograph using the above elements becomes very easy in three dimensions. In order to view aerial images in 3D, a modified version of the simple lens stereoscope is used. 

stereoscope

 This stereoscope which is used in aerial photograph interpretation, consists of a set of mirrors that reflect the aerial photographs laid on the table into the two eyepieces. The photographs used here are overlapping each other and by viewing a two slightly displaced images through each eye, the 3D effect is created.

While proving clues about an objects height, a three dimensional view of an area can also avoid confusion created by similar textures and tones. For example, mountainous terrain without significant vegetation can appear as a flat terrain if not for the stereoscopic view. It also helps to follow the flow path of streams and rivers where subtle tone changes or other disturbances have hindered a direct view.