 |
| Stereo plotter |
Photogrammetry is defined as the science of obtaining reliable measurements, by the use of photographs, in order to determine characteristics such as size, shape and position of photographed object.
Techniques
a.Arial using photographs taken from the air or from space with the camera usually pointing vertically downwards.
a.Arial using photographs taken from the air or from space with the camera usually pointing vertically downwards.
b.Terrestrial using photographs taken on the ground with the camera usually pointing in a horizontal direction.
c.Industrial & Scientific an adaptation of terrestrial techniques to small distances, the camera set at any suitable angle.
 |
| Photogrammetric coordinate system |
The term air survey is used to describe survey techniques using photographs taken from the air or from the space
 |
| Stereo Plotter |
Photographic Interpretation
 |
| A strip of aerial photographs |
Photographic interpretation is concerned with the examination of photographs in order to identify objects, and is an essential part of air survey.
Survey Applications of Photogrammetry
a.Topographical Mapping
–The plotting of map detail and contours is normally carried out using aerial photogrammetric methods
–These methods are used for both original survey and revision, and replace classical ground methods except where the task is so small that flying is uneconomical
–The technique needs a certain amount of ground surveyed control, but this requirement is being continually reduced with the improvement of aerial triangulation techniques to provide supplementary control.
–A final Field check is necessary.
b.Large Scale Plans
–Large scale surveys can be produced accurately and quickly by air survey methods, but require more field checking in addition to the provision of ground control
–For large tasks, such as road building and major constructions, air survey methods are quicker and cheaper than ground methods
–Profiles for determination of earthwork quantities and other data useful to Civil Engineer may be simply obtained from the same photographs.
 |
| A stereo pair of aerial photographs |
 |
| Stereo plotter |
c.Cadastral Plans
–Similar advantages may be gained by an air survey for cadastral purpose as are provided in the production of large scale plans.
–As the accuracy of the cadastral plan is related to the value of land, the traditional ground method trends to be slower, costly but very accurate
–In most cases, the accuracy of a well planned air survey is sufficient for cadastral purposes, and this method is used in many countries
–Where boundaries are related to described features, or land is very valuable, the additional accuracy of ground survey may be necessary.
 |
| Basic concept of stereo plotter |
d. Land use maps
– Air survey techniques may be used not only to define the extent of an area, but also to identify its use and measure the yield of a crop
–Forestry is a typical application where, by plotting the limits of timber and measuring tree heights, an accurate estimate of yield may be given.
–The use of special films, such as color and infra-red will provide additional information about the quality of the crop.
e. Hydrographic Maps and Charts
–Air survey techniques are particularly valuable in the accurate plotting of coastlines, sandbanks and small islands where the changing tide is a problem for ground methods
–The use of special film will again add more information, either by clearly delineating water limits or by extra penetration in shallow water
f. Exploration and Reconnaissance
–Information may be gained about areas to which access is restricted by employing air survey techniques
–In the case of military reconnaissance, a high flying or unmanned aircraft can obtain photograph that will provide data for an accurate survey
–For explorers, and area can be mapped before the first entry, either from photographs obtained by aircraft as in the case of Moon or Mars, or by specially equiped satellites