Different types of Aerial photographs are acquired from sensors mounted on airborne platforms. These photographs are classified into types into various factors such as orientation of camera axis, scale of photograph, lens system used, film and filter combinations used in the photography etc. Aerial photographs classified using various factors are described in this article.
Based on orientation of camera axis photographs are classified into 3 types. 1. Vertical Photographs 2. Low Oblique Photographs and 3. High Oblique Photographs
These are the photographs which are acquired when the axis of camera is in vertical condition. Following are some of the important points related to vertical photographs.
- At a given height (flying height) these photographs cover relatively less amount of area compared to an oblique photograph.
- Relief is not readily seen in a vertical photograph.
- Scale is considered approximately uniform for flat terrains in case of vertical photographs.
- A vertical photograph can be used as a substitute in the absence of map.
However, capturing of vertical photographs is very difficult due to turbulence of aircraft and unstable conditions during the photography. Hence, most of the photographs are acquired with camera axis in slightly tilted position. Such photographs are called as tilted photographs. Tilt should not be more than 3 degrees for a photograph to be considered as vertical. In other words, tilted photographs with less than 3 degree tilt are also considered as vertical photographs.
All other photographs, where axis is tilted more are called as oblique photographs.
Low Oblique Photographs:
Photographs which are taken keeping the camera axis intentionally tilted are called as Oblique Photographs. If horizon is not visible in such photographs, they are categorized as low oblique photographs. Horizon is the area where earth and sky appears to be meeting in a photograph.
High Oblique Photographs:
These are oblique photographs, where horizon is visible in the photographs. These photographs are taken with high amount of tilt and cover larger area compared to vertical photographs. Scale is normally not uniform even in case of a plain terrain for oblique photographs.
Types of Photographs Based on Scale:
Large Scale Photographs:
Based on flying height of airplane, the amount of area captured by a camera varies. If flying height is less, the camera covers less area, but objects are seen in bigger dimension. Hence, the representative fraction (ratio of photo distance to ground distance) is also of higher value in low altitude photography. Such photographs are called large scale photographs.
Small Scale Photographs:
If the flying height is more, in such situation, large area is covered in a single photograph. However, the ratio of size of object in the photograph to ground dimension becomes smaller. Such photographs are called as small scale photographs.
Types of Photographs Based on Lens System:
Based on types of lens system and no of lens arranged in a camera, photography is classified into many types. A 2 – lens system makes use of 2 cameras attached together for photography. In the same fashion a 3-lens system is used for capturing areas from one horizon to another horizon. This 3-lens system is also called as trimetrogen system and is used in worldwar-II for mapping enemy territory. Multi-lens systems is the term broadly used for denoting photographic systems with more than 2 cameras.
Types of Photographs Based on Film and Filter Combinations:
Based on different types of films and filters used while capturing, photographs are classified as below.
- Panchromatic Photograph – A panchromatic film is used in the camera. It captures all visible wavelengths of energy for photography. The resultant photograph is a gray scale (black and white) image. These photographs are used for conventional applications such as map study, reconnaissance etc.
- Color Photograph – The film used in this camera system is capable of capturing and recording various visible bands separately. The photograph is a color image and is used for interpretation of various objects in the study area.
- Infra-red Image -An infra-red film is placed in the camera. This film responds and records only infra red energy coming from the objects. The images produced are gray scale normally. These images are used in studies such as vegetation and water body delineation etc.
- Color-Infra red Imagery – The film used in this camera is capable of recording both visible and infra red energy. The photographs produced are used for vegetation studies, water body mapping, urban applications etc.
- Thermal infra red imagery – Thermal infra red energy is captured for preparing photographs. The photographs are used in temperature studies.
- Radar Imagery – Radar waves or microwaves are captured in this photography. These images normally contain lot of noise and require radiometric correction. These images are used in tecto-morphic studies, weather applications etc.
- Spectra-zonal imagery – The images are acquired in selected portions of electromagnetic spectrum.
The above are many types of aerial photographs classified based on various factors. These photographs differ in various properties such as spatial, spectral and radiometric resolutions and are used in different type of applications.