Distortions and displacement on aerial photograph

Distortions
VIDYASAGAR UNIVERSITY
GEOGRAPHY & ENVIRONMENTAL MANAGEMENT
PRESENTED BY - CHANDAN ROY
ROLL- 46
01-11-2018 Chandan Roy,Geo&EM 1

What is Distortion?
The shift in position of a landscape feature on a photograph that alters the perspective
characteristics of the image.
What is Displacement?
Displacement is any shift in the position of an image on a photograph that does not alter the perspective
characteristics of the photograph.

Principle points(PP) & Conjugate principl Points(CPP)- Not the only points of
interest on an aerial photo
Nadir & Isocenter:- also important, especially on oblique photos.
PP – physical or optical center of an uncropped photo (intersection of diagonal
fiducials)
1) If a photo is truly vertical, PP is directly beneath the aircraft
2) Not so on and oblique’s
3) Distortions due to lens imperfections radiate from the PP, but are typically very
slight….so they are ignored

Nadir is the gravitational center of the photo, and is directly beneath the plane
regardless of flight attitude (pitch & roll) at the time of film exposure.
1) If tilt is not too great, nadir will be on the photo
2) Displacement due to elevation differences are radial from Nadir
3) Displacement do to elevation differences can cause serious problems when trying to
measure distance and directions accurately
4) Displacement do to elevation differences is what allows us to see photos in stereo
and will provide us with 2 of the 3 techniques for heights from aerial photos.

Isocenter – is always on a line through the PP &
Nadir called the Principal Meridian, and is located half
way between these two points.
1. Displacement due to tilt is radial from Isocenter
2. Displacement due to tilt is zero on a truly vertical photos, but increases proportionally as tilt
angle increases.

TYPES OF DISTORTIONS:-
DISTORTION
GEOMETRIC RADIOMETRIC
PREDICTABLE
 SKEW
 PANAROMIC
UNPREDICTABLE
 ROLLING
 CRABING
 PITCHING
INTERVAL
 FILM FILTER
EXTERNAL
 SIGNAL STRENGTH
 OBJECT PROPERTY

Types of Distortion and Displacement
DISPLACEMENT AND ERROR:-
 In Photography survey we try to represent 3-D surface of earth as 2-D image.
 Object appears in photograph are geometrically distorted.(Known as relief displacement)
Reasons for Distortion
 Perspective view
 Movement of camera
 Instability of aircraft
 Variation in altitude, tilt and speed
 Curvature of earth
 Rotating earth
Usually negligible

LENS DISTORTION:-
O
X
Y
∆r
∆t
Lens Distortion deteriorates the positional accuracy of image points
located on the image plane . Two types of lens distortion exist: radial
and tangential lens distortion. Lanes distortion occurs when light
rays passing through the lens are bent, thereby changing directions
and intersecting the image plane at positions deviation from the
norm.
Radial vs. Tangential Lens Distortion
Lens distortion radiates from the PP
• Causes an image to appear closer to or from the pp
• Most serious near of the photo

TILT DISTORTIONS:-
Caused by displacement of the focal plane from a truly horizontal position by
aircraft motion(altitude).Image areas on the upper size of the tilt depict ground
features in smaller-
Roll, Crab , Pitch

Roll distortion-
• About its flight axis roll
• compensation
TYPES OF TILT DISTORTION-
Pitch distortion-
 Result in local scale change.
 can be ignored in most analyses

Crab distortion
Crab is the condition caused by the failure to orient the
camera with respect to the planned flight line.
 Caused by deflection of aircraft due to crosswind.
 Correction on the plane or by computer.
 Crab should not affect more than 5% of the photograph
Crabbing

 RELIEF DISPLACEMENT:-
Relief displacement is the shift in an object's image position caused by its elevation above a
particular datum. For vertical or near vertical photography the shift occurs radially from the nadir
point.
This effect is demonstrated in the diagram below.
Key facts about relief displacement:-
Even from great flying heights, tall objects can
exhibit image displacement
 PARAMETERS OF RELIEF DISPLACEMENT:-
 Perspective Radial displacement (r)
 Object height (h)
 Variation of flying height (H)
FORMULA:-
R.D.- h*r/H

PARALLAX:-
Parallax is a displacement or difference in the apparent position of an object viewed along two
different lines of sight, and is measured by the angle or semi-angle of inclination between those two
lines. Due to foreshortening, nearby objects show a larger parallax than farther objects when observed
from different positions, so parallax can be used to determine distances.

ORTHORECTIFICATION:-
Orthorectification is the process of removing the effects of
image perspective (tilt) and relief (terrain) effects for the
purpose of creating a planimetrically correct image. The
resultant orthorectified image has a constant scale wherein
features are represented in their 'true' positions. This allows
for the accurate direct measurement of distances, angles,
and areas (i.e. mensuration). Orthorectified images are
commonly used as in visualization tools such as Google Earth,
OSSIM Planet, ArcMap, WMS, etc.
Click here for more information01-11-2018 Chandan Roy,Geo&EM 14

THE GOAL OF THE ORTHORECTIFICATION
The goal of the ortho-rectification is to resample the pixels of an aerial photo to a
coordinate system that is interpreted in a selected horizontal surface (practically in
a level surface of the region of the air photo). This coordinate system should be
defined in the geographic information system, according to the above chapters
(e.g. a map projection plane).
Geo-referring the aerial photos, two different distortion effects should be
corrected:
The perspective distortion, which is the result of the geometry of the photograph
taking.
The distortion effect of the relief and/or the surface.

REFERENCES:-
• Jensen,JR.,2005, Introductory Digital Image Processing: A Remote
sensing Perspective, 3rd Ed., Upper Saddle River: Pearson Pub.
• Humbold University
• www.nrem.iastate.edu/class/assets/nrem345/Week4_ALL.pdf
• www.ngi.gov.za/index.php/technical-information/remote-sensing-
and-photogrammetry/rectification
• en.wikipedia.org/wiki/Distortion_(optics)

THANK YOU!!

Distortions and displacement on aerial photograph

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