PHOTOGRAMMETRY an introduction

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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
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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…
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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!!
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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 3dimensional 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 lensessetting:
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
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They have stable and precisely known internal
geometries and very low lens distortions.
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The principal distance is constant, which means, that the
lens cannot be sharpened when taking photographs.
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The image coordinate system is defined by (mostly) four
fiducial marks, which are mounted on the frame of the
camera.
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Aerial metric cameras are built into aero planes mostly
looking straight downwards.
Stereometric camera
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The overlapping area of these two photographs (which are called a
„stereopair“) can be seen in 3D, simulating man's stereoscopic vision.
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In practice, a stereopair can be produced with a single camera from
two positions or using a stereometric camera.
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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).
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Both cameras have the same geometric properties. Since they are
adjusted to the normal case, stereopairs are created easily.
Amateur cameras
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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
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Only useful for plane (2D) objects.
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Obliquely photographed plane objects show perspective
deformations which have to be rectified.
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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:
1.Paper
strip method
2. Optical rectification
3.Numerical rectification
4.Monoplotting
5. Digital rectification
1.Paper strip method
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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 rectification
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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.Monoplotting
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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
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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
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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
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A computer manages the relationship between image- and realworld 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
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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
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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.
!!Thank You!!
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