Aerial Photgrammetry

7/30/2019 Aerial Photgrammetry

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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


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


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.

Aerial Photgrammetry

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