5. Displacement and errors
In photography survey we try to represent 3 D surfaces
of the earth as 2D image
Object appear in photograph are geometrically
distorted, known as relief displacement.
Reasons for distortion
Movement of camera
Instability of aircraft
Variation in altitude, tilt and speed
Curvature of earth
Rotation of the earth
Spring 1998
Field Geography
6.
7. Crab
Opposite lines of photographs are not
parallel to flight line is Known as Crab
of photograph.
Spring 1998
Field Geography
9. Drift
When aircraft is swayed away from its
preplanned flight line then it is known as
Drift
Usually coursed by wind spirits
Spring 1998
Field Geography
12. PLANNING FOR AERIAL
PHOTOGRAPHY
Successful execution of any photogrammetric project
requires thorough planning prior to proceeding with the
work.
Planning, must be done by knowledgeable and
experienced persons.
One of the most important considerations that must be
addressed in planning
the decision of exactly what products will be prepared,
with their scales and
accuracies.
13. Elements of planning
Spring 1998
Field Geography
1. purpose of photogrammetry
Wide angle photograph (152mm focal length) is
required for topographic mapping
If area is highly wooded us f= 210 mm( standard
angle) to allow more visibility through trees
Generally 60% overlap with 15% side lap
14. 2 photo scale
Spring 1998
Field Geography
It depend on the accuracy level work
Type of the purpose of the photograph.
Some what depend on type of plotter
15. 3. Relief displacement
Spring 1998
Field Geography
Relief displacement decrease as flight high increase
Flight high increase as relief displacement decrease.
16. 4. Tilt
Spring 1998
Field Geography
Will cause overlap will be greater on one end than
other
Will increase side lap on one side and decrease on other
17. 5. Crab and drift
Spring 1998
Field Geography
Crab- angle formed b/n flight line and edges of photo
Drift error- Plane which is not stay on flight line.
Commonly coursed re-flight and gaps
18. 6. Flying height
Spring 1998
Field Geography
Determined after side lap and overlap
Factors affecting
1. scale, relief displacement, and tilt
2. precision of equipment used
Greater precision grater possibility flight height
7. direction and orientation of the terrain
Arrange to fly along ridge not across
19. Project planning
Spring 1998
Field Geography
Successful execution of any photogrammetric project
requires thorough planning
Must first determine the selection of products to be
prepared their scales and accuracies of areal photo, plan
metric map, Cadastral map, Digital elevation map (DEM).
20. flight planning
Spring 1998
Field Geography
Success of photogrammetry project depend on
acquisition of good quality pictures
Due to weather and ground condition, time frame for
photograph is limited
Refight are expensive and cause long delay on project
Mission must be carefully planned and executed
according to flight plane
where photo should be taken and specifications
24. Weather conditions
Spring 1998
Field Geography
Flight crew should interpret weather condition and
make sound decision on whether to fly or not fly
Ideally cloud free < 10% cloud cover acceptable
Cloud higher than the flight high might cause large
shadow on the ground.
Photos for industrial areas susceptible to atmospheric
haze, smog, dust, and smoke are taken after heavy
rains
High wind motion will difficulty in camera and aircraft
ordination.
26. Photo scale
Average photographic scale is one of the most important
variables that must be selected in planning Aerial
photographs.
Photo scale is determined by knowing the focal length of
the camera and flight height.
Sp = f / H
If average ground elevation is considered then,
Sp = f" / (H'-h')
27. Flying Height
Once camera focal length and required average photo
scale have been selected, required flying height above
average ground is fixed in accordance with the scale.
The flying height of the camera depends on the scale of
the photograph.
The Aerial photograph scale depends also on the
topography or changes in elevation.
Actual Flight Height = Flight Height + MSL
28. Overlaps
The overlap of photographs in the direction of flight line is
called forwarded overlap or End Lap.
A minimum of 55% to 65% with an average of 60% is
maintained while running a flight.
The overlap between adjacent flight lines is known as Side
Lap.
The side lap is maintained between 20% to 35% with an
average of 30%.
29. End Lap gain:
End lap gain, the distance between the centers of the
consecutive photographs along a flight path, it can be
calculated by using a formula
Gend = Sp x W (100-Oend)/100
Where; Gend = distance between exposure station
Sp = photo scale denominator
Oend = side lap (percent)
W = width of the exposure frame (inches or cms).
30. Cont…
Send = photo scale denominator
Oend = end lap (%)
W = width of exposure frame (inch or cm).
31. Side Lap gain
Side lap gain, the distance between the centers
of adjacent flight lines and calculated by using
following equation:
Gside = Sp x W [(100 – Oside)/100]
Where
Gside= distance between flight line centers (feet)
Sp = photo scale denominator
Oside= side lap (percent)
W = width of the exposure frame (inches or
cms).
33. Calculation of Number of strips and
photographs for a specified area.
For a complete aerial survey a complete photographic
coverage with requisite overlaps is required.
Once the sale of photo is decided the time interval for
successive exposure can be calculate considering the
velocity of aircraft.
Strips are spaced at calculated distances to ensure the
required side overlap between the adjacent flights.
34. a) Total area of project site = L x W
b) No of flight line required to cover the site area
= Total width / Side lap gain
c) No. of photo per line = Line length /End Lap
gain +1
d) Total No. of photos = No of flight line x No of
photo per line
e) Time interval for successive exposure can be
calculated considering the velocity of aircraft
35. e.g. if the photo size is 23cm x 23cm, the overlap is 60%,
scale of photo is 1:25,000 and speed of plane is 360 km/h.
Find the time interval for successive exposure.
Solution
End Lap gain = Sp x W (100-end)/100
=25,000 x 23 x 40/100 = 230000 cm = 2300m.
Speed of aero plane = 360km/hr = 360x1000/60x60
=1000 m/s.
Time interval = End Lap gain /Spaced of plane
2300m/100m/sec = 2.3sec.
36. Flight Planning for Aerial
Photography
We have a study area over Bahir dar town, 30 km wide
in east to west direction and 15 km along in the north –
south direction
Camera focal length 152.4 mm focal length, and the sine
size is 230 mm( 9*9( format is used)
The required scale is 1: 20,000 and the end lap and side
lap is 40 % and 20% respectively.
The plane speed is 300km/h
Spring 1998
Field Geography
37. Your assignment calculate each question 4 mark
Flight direction
Flight height
Ground coverage per sine or exposure
Over lap and side lap
If plane speed is 300 km/h calculate time b/n exposure
No of line
No of photo
Note;- each question is with brief explanation and
brief procedure.
