Basics of Photogeology

The interpretation of Aerial Photographs for geological purposes is termed Photogeology. This a form of Remote Sensing. The primary objective of Photogeology is, to identify geological structures and rock types of an area and to prepare a geological map of the area. Photogeology is usually employed before field geological work in order to get a general idea of the geological features of the area. The observations and inferences obtained from Photogeology are then confirmed by using field data.

The analysis of aerial photographs in photogeology is done using the same elements of interpretation. "Tone" is used in the identification of rock type. Generally lighter tones indicate rock types like Quartzite. Tone is also used in the identification of the density of vegetation which in turn provides clues about the underlying rock. Usually dense vegetation can be seen in area where the rock Khondalite is present and in areas where Quartzite is abundant, very little or no vegetation is present.

By using a stereoscope to view the three dimensional topography of the area structural features such as ridges, valleys and slopes can be identified. Due to differences in hardness and the extent of weathering, certain rock types show characteristic structural features. For example, Marble and Hornblendebiotite Gniess are found at the bottom of valleys because they are highly prone to weathering. Rocks that are much harder and are resistant to weathering like Quartzite, Granite and Granitic Gneiss usually form ridges.

geological features in an aerial photograph

When viewing outcrops from above, for example in a ridge like structure, by looking at the texture on either side of the ridge, the direction of dip and strike can be inferred. Since the side where multiple layers of rock are exposed undergoes differential weathering that side become ragged and rough. This side is termed the escarpment face. The side on which a single layer of rock is exposed undergoes uniform weathering and therefore is relatively smooth. This side is the Dip slope. Therefore, generally the side of a ridge that has a relatively smooth texture indicates the direction of dip.

cross section of a ridge

The drainage pattern and shapes of marshes lakes etc also provide clues about the geological structure of the area. A drainage pattern like shown in the first diagram indicates a homogeneous and relatively flat rock while a pattern like in the second diagram indicates a highly jointed rock. In addition to this, the displacement of any feature along a line indicates a rock fault.

Once all possible features are identified form the aerial photograph, a geological map of the area is plotted. This map is referred and then the data is confirmed by using field data.

 

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.

Lens Stereoscopy

3D viewing is widely used in cinemas today. Technologies such as, Active Shutter, Polarization and Interference filter method are used for 3D projection. In all these methods the basic idea is to provide two different images to each eye like we naturally perceive the world. Then the brain does the rest of the work and combines those images to perceive depth. Although 3D vision is most common in movies at present, in the past it was used in Remote Sensing for military purposes. The instrument used for this purpose is the Stereoscope.

the stereoscope with an image

The Stereoscope consists of two lenses resting on a stand that focus on the two separate images of the stereoscopic image sheet. When looking through the two lenses, each eye sees a slightly different image that is meant for that eye and this results in the perception of depth - the image appears 3 dimensional. This is very useful in analyzing aerial photographs because it allows viewing the ground terrain in 3D.

test image for stereoscopic vision

Shown above is a test image that is used to check stereoscopic vision. The relative heights at which the symbols appear to be seen can be checked against the actual order of heights which are provided overleaf.

Stereoscope with a test image

While in modern remote sensing, stereoscopes have been replaced by more advanced computerized equipment that not only provide 3D views but also help manipulate images in 3D, the stereoscope helps understand the basics of stereoscopic vision. Lens Stereoscopy can also be considered as the foundation of all modern stereoscopic equipment.

An Introduction to Remote Sensing

Remote Sensing, like the name implies, means observing and gathering information of an object from afar. In this sense we humans perform remote sensing all the time with our eyes and ears, where eyes and ears are our remote sensing equipment. However when moving away from the basics and expanding the scope of observation, many factors become involved and remote sensing becomes a broad and detailed area of study. Incorporating all these ideas, a general definition for remote sensing can be given as,

"The art and science of obtaining information about an object without being in direct contact with the object"

Remote sensing is extensively used in observing the earth and the environment for various purposes. The most common application of remote sensing is done in mine development, disaster mitigation and large scale environmental studies. Depending on the application, the sensors and the type of radiation used, the method of recording and processing and the analysis of the data will differ.

The major components in remote sensing are,
- an energy source for illumination :- usually the sun or an artificial source of radiation.
- medium of travel for energy :- the atmosphere.
- sensor :- orbits in a satellite eg. Landsat, Quickbird
- receiver on earth :- gathers the data transmitted from the satellite.
- equipment for interpretation and analysis of data :- remote sensing software.

All remote sensing systems have four types of resolution. They are,
- Spatial resolution :- refers to the pixel resolution of the image produced.
- Spectral resolution :- refers to the range of EM radiation that can be detected. A broader range increases the scope of analysis.
- Temporal resolution :- refers to the orbital period of the satellite carrying the sensor. Faster the period, higher the resolution.
- Radiometric resolution :- refers to the differences of the intensity of radiation that can be distinguished by the sensor. Given as a bit range. A higher bit range can distinguish smaller variations.

Once all the above details are decided according to the application, remote sensing is performed. Then a "reflectance pattern" is produced from the gathered data. By comparing these patterns with the spectral signatures of known objects, the type and extent of the sensed objects on the earth can be inferred. In addition to various applications on earth, remote sensing is also used in other planets to gather data about surface conditions etc without landing on the planet.

Remote sensing in Mars. image form wikipedia