Photogrammetry

 Principle and types of Aerial Photographs

Photogrammetry

Photogrammetry involves art, science and technology.

It also collects the information by measuring. interpreting the photographic images and by using the patterns of recorded electromagnetic radiations. The images are captured through a device known as camera.

The photogrammetry is decided on the basis of.

1 Different types of images used

2 The exact location of the camera from the object

3 Aim of these images.

 Different types of photogrammetry.

Types of Photogrammetry

The various types of photogrammetry are as follows.

1 Terrestrial photogrammetry

2 Close range photogrammetry

3 Space photogrammetry

4. Stereo photogrammetry.

1 Terrestrial Photogrammetry

This type of photogrammetry deals with the photography carried out from the ground surface which is considered as a platform for the photographic instruments (ie., camera), The distance between the object and photographer in terrestrial photogrammetry is supposed to be greater than or equal to 300 mts.

2 Close Range Photogrammetry

If the distance between the object and the photographer is less than 300 mts, then the terrestrial photogrammetry is known as close range photogrammetry.

3. Space Photogrammetry

The type of photogrammetry deals with the photography which is carried out using an aerial camera on board aircraft. It involves the use of traditional analog photographs or the digital photographs which

Introduction to Photogrammetry

are been clicked from any airborne or spacebome platform. Hence, these pictures can be either vertical or oblique depending upon its characteristics and geometry. The space photogrammetry is also known as the aerial photogrammetry

4. Stereo Photogrammetry

This type of photogrammetry deals with the photographs of overlapping pairs which are measured and attained using a stereoscopic viewing device. produces a three dimensional view and generates an illusion which provides the photogrammetrist or the observer a relaxing view of the terrain. The stereo photogrammetry is carried out with the use of specially designed instruments such as stereometer, digital computers etc. It is required for generating a digital elevation model of real world by determining the elevations of h-measurements

Classification of Aerial Photographs

Aerial photographs are mainly classified into two, based upon the directions,

1. Vertical

2 Oblique.

Aerial photographs are those which are taken by means of an aerial camera fixed in an aircraft

1. Vertical Photographs

Vertical photograph is the one where the axis of camera is truly vertical. In some cases, the axis of camera may not be exactly vertical, tilting of axis may appear. The allowable tilt in the preparation of topographical maps may be in a range of 1 to 3°. These photographs may also be termed as vertical photographs.

2. Oblique Photographs

The axis of camera is purposefully tilted from 90° inclination. Oblique photographs are sub divided into two categories,

(a) High oblique

The photograph in which horizon appears 

(b) Low oblique.

Photograph on which horizon is tilted such that the horizon does not appear 

Types of Artificial recharge structures constructed for increasing ground water table in a watershed and identifying sites for locating these structures using RS and GIS techniques

 Check dams suits on hard rock and alluvial formation

Rain water harvesting structures suits on sand and gravel

Percoltion tanks suits on Highly fractured weathered rock

Water spreading 

Recharge through pits and wells suits on soil with impervious layer

Ditch and furrow suits on hard rocks

Steam augnentation suits on weathered formation rocks

Contour ploughing suits on cohesive soils.

Level IV land use and Land cover map using Remote sensibg anf secondary data

 Level-IV land use and land cover map involves large scale of aerial photographs with low altitude. The data captured is below 10,000 ft and scale is greater than 1: 20,000. Remote sensing involves the mapping of earth surface features derived from the classification of land use and mapping. All these earth features have special reflectance values. Based on these features, a map is prepared. The images are examined and converts the digital image into a pictorial one using visual interpretation techniques. These techniques include as follows,

Image Interpretation Process

In interpretation process, the image characteristics like colour, texture, size, shape, etc, helps to recognize the land features, usage, class etc. This is then finalised after the ground truth. Based on the ground information, modifications are done to final the classification of land cover and land use.

Production of GIS Output

Then the image is remoulded into a cartographic output. Through digitisation process the output has changed to softcopy resulting accuracy.