Photogrammetry - Aerial and Terrestrial

Photogrammetry

Photogrammetry is the branch of surveying in which measurements are made from photographs.

Merits

• This is a very quick and accurate method of surveying in which the ground observations are almost totally eliminated.
• This is very accurate method if true interpretations of photographs are made.
• It also provides means to develop a Contour map.

Demerits

• This method requires fair weather conditions.
• The instrument is very expensive and staff should be highly qualified and experienced to make full use of this method.

Types of Photogrammetry

There are Two main Types of Photogrammetry - Aerial Photogrammetry and Terrestrial Photogtammetry.

1 - Aerial Photogrammetry

In this photographs are taken from specially manufactured plane. The characteristics of this procedure are following: The plane is made to fly along the center of longitudinal strips marked with the help of clearly visible ground monuments. The speed of aircraft being known the camera speed is adjusted accordingly to provide the requisite transverse and longitudinal overlap between successive photographs.

The speed of the aircraft, its height and specification of the camera are already known. The photographs are then developed in laboratory with each photograph being placed in its proper position and by cutting the overlapped edges. This will provide a base map on the basis of actual photographs which can be processed further for particular requirements. The scale of photographs can be established by distances on the ground between two points and this dimension on the graph. The contours can be drawn by putting the photographs under the stereo plotter.

Stereo plotter is an optical device which gives three dimensional view of plane photographs.

2 - Terrestrial Photogrammetry

In this type, the photographs are taken from elevated ground stations. Further development of these photographs will take into account the elevations of camera and tilt of the axis of photograph. This method is very similar to previous one except that the camera is in stationary position. The camera used in this method is called photo-theodolite as it will require the same features as theodolite. This type of photogrammetry is much cheaper and can be carried out by individual surveying firms also.