PHOTOGRAMMETRY an introduction…

Photogrammetry is the technique of measuring objects (2D or 3D) from photographs

Its most important feature is the fact, that the objects are measured without being touched.

PHOTOGRAMMETRY

objects are measured WITHOUT TOUCHING.

It is a REMOTESENSING technique.

It is a close range method of measuring objects.

It is a 3-dimensional coordinate measuring technique that uses PHOTORAPHS as the fundamental medium for measurement.

Brief History of Photogrammetry 1851: French officer Aime Laussedat develops

the first photogrammetrical devices and methods. He is seen as the initiator of photogrammetry.

1858: The German architect A. Meydenbauer develops photogrammetrical techniques for the documentation of buildings and installs the first photogrammetric institute in 1885 (Royal Prussian Photogrammetric Institute).

1885: The ancient ruins of Persepolis were the first archaeological object recorded photogrammetrically.

1889: The first German manual of photogrammetry was published by C. Koppe.

History cont… 1911: The Austrian Th. Scheimpflug finds a way to

create rectified photographs. He is considered as the initiator of aerial photogrammetry, since he was the first succeeding to apply the photogrammetrical principles to aerial photographs

1913: The first congress of the ISP (International Society for Photogrammetry) was held in Vienna.

1980ies: Due to improvements in computer hardware and software, digital photogrammetry is gaining more and more importance.

1996: 83 years after its first conference, the ISPRS comes back to Vienna, the town, where it was founded.

!!PRINCIPLE!!

The main principle is “TRIANGULATION”.

Eyes use the principle of TRIANGULATION to gauge distance (depth perception).

TRIAGULATION is also the principle used by theodolites for coordinate measurement.

TRIANGULATION

By taking photographs from at least two different locations, so-called "lines of sight" can be developed from each camera to points on the object. These lines of sight (sometimes called rays owing to their optical nature) are mathematically intersected to produce the 3-dimensional coordinates of the points of interest.

Is This a real CAR ? ?

NOW YOU CAN TELL !!

Photography the first part…

Photography - The First Part of Photogrammetry

Taking photographs is, of course, essential for making a photogrammetric measurement. To obtain the high accuracy, reliability and automation the system is capable of, photographs must be of the highest quality.

The three main considerations for good photography are:

1. Field of View 2. Focusing 3. Exposure

Metrology - The Second Part of Photogrammetry

Photogrammetry can be divided into:

1.Depending on the lenses-setting:

A.. Far range photogrammetry (with camera distance setting to indefinite)

B. Close range photogrammetry (with camera distance settings to finite values).

2.Another grouping can be : A. Arieal photogrammetry (which is mostly far

range photogrammetry) B. Terrestrial Photogrammetry (mostly close

range photogrammetry).

Short descriptions of photogrammetric techniques

Photographing Devices

1.A photographic image is a „central perspective“. This implies, that every light ray, which reached the film surface during exposure, passed through the camera lens (which is mathematically considered as a single point, the so called „perspective center“).

2. In order to take measurements of objects from photographs, the ray bundle must be reconstructed.

3. The focal length is called „principal distance“, which is the distance of the projection center from the image plane's principal point.

4. The internal geometry of the used camera (which is defined by

the focal length, the position of the principal point and the lens distortion) has to be precisely known.

Metric cameras They have stable and precisely known

internal geometries and very low lens distortions.

The principal distance is constant, which means, that the lens cannot be sharpened when taking photographs.

The image coordinate system is defined by (mostly) four fiducial marks, which are mounted on the frame of the camera.

Aerial metric cameras are built into aero planes mostly looking straight downwards.

Stereometric camera The overlapping area of these two photographs (which are called a

„stereopair“) can be seen in 3D, simulating man's stereoscopic vision.

In practice, a stereopair can be produced with a single camera from two positions or using a stereometric camera.

A stereometric camera in principle consists of two metric cameras mounted at both ends of a bar, which has a precisely measured length (mostly 40 or 120 cm).

Both cameras have the same geometric properties. Since they are adjusted to the normal case, stereopairs are created easily.

Amateur cameras

The photogrammetrist speaks of an „amateur camera“, when the internal geometry is not stable and unknown, as is the case with any „normal“ commercially available camera.

Photographing a test field with many control points and at a

repeatably fixed distance setting (for example at infinity), a „calibration“ of the camera can be calculated.

They can only be used for purposes, where no high accuracy is demanded.

The precision will never reach that of metric cameras.

Photogrammetric Techniques

Mapping from a single photograph

Only useful for plane (2D) objects.

Obliquely photographed plane objects show perspective

deformations which have to be rectified.

To get good results even with the simple techniques, the object should be plane (as for example a wall), and since only a single photograph is used, the mappings can only be done in 2D .

Mapping from a single photograph

Some Common Techniques:

11.Paper strip method.Paper strip method22. Optical . Optical rectification rectification 33.Numerical .Numerical rectification rectification 44.Monoplotting.Monoplotting55. Digital rectification. Digital rectification

1.Paper strip method1.Paper strip method This is the cheapest method, since only a ruler, a

piece of paper with a straight edge and a pencil are required.

Four points must be identified in the picture and in a map .

The paper strip is placed on the image and the intersections with the lines are marked

The strip is then placed on the map and adjusted such that the marks coincide again with the lines.

After that, a line can be drawn on the map to the mark of the required object point. The whole process is repeated from another point, giving the object-point on the map as intersection of the two object-lines.

2. Optical rectification2. Optical rectification

Is done using photographic enlargeners. At least four control points are required,

not three on one line. The control point plot is rotated and

displaced until two points match the corresponding object points from the projected image.

After that, the table has to be tilted by two rotations, until the projected negative fits to all control points.

Then an exposure is made and developed.

3.Monoplotting3.Monoplotting The coordinates are here transformed into a

3D coordinate system. First, the orientation elements, that are the

coordinates of the projection center and the three angles defining the view of the photograph, are calculated by spatial resection.

Then, using the calibration data of the camera, any ray, that came from the archaeological feature through the lense onto the photograph can be reconstructed and intersected with the digital terrain model.

Stereophotogrammetry Stereopairs are the basic requirement, here.

These can be produced using stereometric cameras. If only a single camera is available, two photographs can be made from different positions, trying to match the conditions of the „normal case“.

While taking the photographs, the aeroplane flies over a certain area in a meandric way, so that the whole area is covered by overlapping photographs.

The overlapping part of each stereopair can be viewed in 3D and consequently mapped in 3D using one of following techniques:

Analogue Method Two projectors, which have the same geometric

properties as the used camera project the negatives of the stereopair.

Their positions then exactly rotated into the same relationship towards each other as at the moment of exposure ,After this step, the projected bundle of light rays from both photographs intersect with each other forming a (three dimensional optical) „model.

At last, the scale of this model has to be related to its true dimensions and the rotations and shifts in relation to the mapping (world) coordinate system are determined.

Analytical Method A computer manages the relationship between image-

and real-world coordinates. After restoration of the "inner orientation", where the

computer may now also correct for the distortion of the film, both pictures are relatively oriented.

Then, the absolute orientation is performed, where the 3D model is transferred to the real- world coordinate system. Therefore, at least three control points are required.

The analytical plotter uses the computer to calculate the real-world coordinates, which can be stored as an ASCII file or transferred on-line into CAD-programs. In that way, 3D drawings are created, which can be stored digitally, combined with other data and plotted later at any scale.

Mapping from several photographs

This kind of restitution, which can be done in 3D, has only become possible by analytical and digital photogrammetry.

Here, mostly more than two photographs are used. 3D objects are photographed from several positions.

The photographs can be taken with different cameras (even „amateur“ cameras) and at different times (if the object does not move).

Technique

Only analytical or digital techniques can be used. During all methods, first a bundle adjustment has

to be calculated. Using control points and triangulation points the

geometry of the whole block of photographs is reconstructed with high precision.

Then the image coordinates of any desired object-point measured in at least two photographs can be intersected. The result are the coordinates of the required points. In that way, the whole 3D object is digitally reconstructed.

Conclusion

Photogrammetry can also be thought of as the sciences of geometry, mathematics and physics that use the image of a 3D scene on a 2D piece of paper to reconstruct a reliable and accurate model of the original 3D scene. With this in mind it is easier to understand the current expanded definition, which, includes the science of electronics by using video and other synthetic means of reproducing 2D images of 3D scenes. And, these images are also used to reconstruct reliable and accurate models of the captured 3D scenes.

AM GREATLY INDEBTED FOR MY GUIDES Mr.CHINMYA DAS, Mr.B.K SATHPATHY AND OTHER TEACHERS FOR THEIR VALUABLE GUIDANCE TO COMPLETE THIS SEMINAR AND LASTLY I AM THANKFULL TO ALL MY FRIENDS FOR THEIR HELPING HANDS TOWARDS THE SUCCESSFULL PRESENTATION OF THIS SEMINAR.

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