| Photogrammetry |
Photogrammetry is the art, science, and technology of obtaining reliable information about physical objects, the environment and terrain, through processes of recording, measuring, and interpreting Aerial Stereo Photographs or other images.
How do they do it?
Aerial photographs are taken with a very precise camera mounted in an aircraft or other aerial platform. The photographs are taken at intervals along a run so that the photographs overlap by about two thirds. Each run also overlaps the run next door.
The precise locations of control points visible in the photogrpahs are measured on the ground so tha the photographs can be oriented and scaled in relation to the true dimensions on the ground. In some modern cameras, a precise Global Positioning Systems (GPS) unit is integrated with the shutter to give the three dimensional co-ordinates of control points in the photographs themselves without ground survey.
Other adjustments have to be made to the photographs to account for the tilts of the aircraft as it flies along and the bluring that would occur due to the forward motion of the aircraft at low altitudes.
The photographs are exposed on special
high resolution film and film processing is precisely controlled to minimise
distortion of the terrain. Format of the photographs is usually 229x229 mm
What happens next?
When the control has been computed and sufficient points of known co-ordinates are available, two overlapping photographs are placed in a photogrammetric plotter. Through a train of complex optics, a photogrammetrist can look at the area where the overlapping portions are superimposed on each other and see that area of the terrain in three dimensions.
By optically placing a marker, known as a floating point, on the surface as he sees it, he has located the three dimensional co-ordinates of that point. He can then move the floating mark along a feature such as a road, river etc and, at the same time, a pantograph on a table beside the instrument will move a pencil to record that feature on a piece of paper, thus creating a map.
Modern equipment tends to be totally computerised and will record the location of each position of the floating mark as three dimensional co-ordinates in a database from which maps or digital terrain models can be drawn at any scale.
In more recent equipment, the computer
even carries out much of the observing function of the operator and may create
digital terrain models or maps automatically.
Who can do photogrammetric surveys?
Photogrammetry is a specialist skill that only a small number of surveying firms have. In addition, the high capital cost of the equipment required means that only a few firms carry out this type of work.
However, most surveyors have an understanding
of the principles of photogrammetry and are able to advise when this technology
might be applicable to a particular project and network with the specialist
consultant.
What are the advantages of aerial survey?
The principal advantage of aerial survey and photogrammetry inlcude
The maps produced by photogrammetry
are of a very high accuracy due to the advanced technology used.
When you might need an aerial survey
Aerial photography and photogrammetry are used for rmapping and other purposes in the following situations:-
What other services might you need at the same time?
Usually, the photography, mapping and digital terrain models are part of a broader project which might also need:-
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