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Aerial Photogrammetry:
Session-2
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Flight planning
Success of photogrammetric project depends on
acquisition of good quality pictures
Due to weather and ground conditions, time frame
for photography is limited
Reflights are expensive and causes long delays on
project
Mission must be carefully planned and executed
according to flight plan
Consists of flight map, (where photos should be taken)
and specifications
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Stereophotography
Adjacent but
overlapping aerial
photos are calledstereo-pairs and are
needed to determine
parallax and stereo/3D
viewing
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Overlapping Stereophotography
Endlap - ~60% Common area covered
by two successive photos
of the same flight line
Sidelap - ~20-30%
Common area covered bytwo adjacent flight
lines
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Required Data for Flight planning
Project area boundary
Camera focal length3.5, 6, or 12
Photoformat sizestandard is 9 or 23 cm
Photoscale
Overlap requirements (in percentage)
percentage of endlap or sidelap Least number of flight lines (To be more
Economical)
Least number of exposures
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Flight planning computations
Flying height
Distance between exposures or Airbase
Distance between flight lines Total number of exposures
Flying height above mean sea level of each
flight line Total time needed for photography
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Flying height
H = f* s
Where:
f = focal lengths = scale of photograph (i.e. 1cm = s metres)
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Distance between exposures
L = (1Pl) s l
Where:
L = actual ground length covered by each photograph
Pl = percentage longitudinal lap
s = scale of photograph
l = length of photograph in the direction of flight
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Distance between flight lines
W = (1Pw) s w
Where:
W = actual ground width covered by each photograph
Pw = percentage side lap
s = scale of photograph
w = width of photograph normal to the direction of flight
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Number of photographs
N = A/a
Where:
A = total area to be photographed
a = L * W
L = actual ground length covered by each photograph
W = actual ground width covered by each photograph
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Interval between exposures
T = 3600 L/V
Where:
T = total interval between the exposures
L = actual ground length covered by each photograph
V = ground speed of the airplane (km/hour)
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Map vs. Photo Projection
Systems
Maps have a orthographic or planimetric
projection, where all features are located in
their correct horizontal positions and aredepicted as though they were each being
viewed from directly overhead. Vertical
aerial photos have a central or perspectiveprojection, where all objects are positioned
as though they were viewed from the same
point.
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Maps vs. Aerial Photos
Maps: Scale is constant
No relief displacement
Photos: Scale varies with elevation
Relief displacement
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Stereoscopic Parallax
The displacement ofan object caused by a
change in the point of
observation is called
parallax.
Stereoscopic parallax
is caused by taking
photographs of thesame object but from
different points of
observation.
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Mirror Stereoscope
Stereoscope is an
instrument used for
viewing stereopairs.
To assist in presenting
the eyes the images ofa pair of photographs
so that the relationshipbetween convergence and accommodation is the
same as would be in natural vision. To magnify the perception of depth
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1. Preparing the Photographs
a. Two photographs of the same area taken from
slightly different positions are required for stereo viewing in astereoscope.
b. Vertical aerial photographs are best with at least
60% overlap.
2. Arranging the Photographsa. Ensure that the photographs are arranged in the
same sequence in which they were taken. The overlap area
should be towards the inside.
b. Using the stereoscope, mark the principle points ofadjoining photographs on each photo.
c. Set up the Mirror Stereoscope so that the vertical
displacement between the left and right string images is
eliminated.
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3. Viewing the Stereo-Pairs
a. To view different areas of the image.b. You may also use the binocular viewer to magnify
the image.
4. Using the Vegetation Graticule
a. Place the relevant part of the graticule (thatcorresponds to the scale of the photograph) over the area you
wish to measure. The graticule only needs to be placed over
one of the photos.
b. Determine whether there is Tree, Grass or Shrub ateach point on the graticule and record it on the recording
sheet.
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Stereoscopic parallax
Line of Flight
Note the displacement between the top and base ofthe storage towers in this photo stereo-pair
top
bottom
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Absolute stereoscopic parallax PP = Principal point = center of photo
CPP = Conjugate principal point = adjacent photos PP
Absolute stereoscopic parallax the average photo base
length = average distance between PP and CPP
PP PP
Photobase
CPP
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Differential parallax Differential parallax - the difference between the
stereoscopic parallax at the top and base of the object.15.2 mm
13.5 mm
dP = 15.2mm13.5mm = 1.7 mm
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Stereoscopic Instruments
Parallax wedge - simplest device for
determining differential parallax
Parallax bar - movable floating mark canplaced at base and tops of objects to
measure differential parallax
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Computing height using stereoscopic
parallax
h = (H) * dP / (P + dP)
where h = object height
H = flying height
dP = differential parallax
P = average photo base length
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Stereoscopic Plotting Instruments
Stereoplotters - precision
instruments designed to
duplicate the exact relative
position and orientation of
the aerial camera at thetime of photo acquisition
to recreate the stereo-
model. A floating mark
can be used trace specific
elevations. Relief
displacement is removed
creating a planimetric
map.
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Mosaics
An aerial mosaic is an assemblage of two or more
individual over lapping photographs to form a
single continuous picture of an area . Theassembly is made by cutting and piecing to
gather part s of photographs , being careful to
make common images coincide as closely as
possible at the match lines between adjacentphotos .
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Photographic coverage
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Mosaic advantages
Show relative planimetric qualities , whereas
objects on maps which are shown with symbols
must be limited in number . Mosaic of large areas can be prepared in much
less time and at considerably lower cost than
maps
it is easy to understand and interpreted by people
without photogrammetry or engineering back
ground .
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Uses of Mosaic
Mosaic are widely used . Their value is perhaps
most appreciated in the field of planning , both in
land-use planning and in planning forengineering project .
Use to study geologic features
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Photo interpretation
1. Examine and identify the characteristics of the
photographic images.
2. Make measurements to get dimensions of theobject using scale of the photographs.
3. Analyse and evaluate the interpreted item.
4. Verify the interpreted item in the field
5. Tabulate the results.
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