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Book Chapter

Aliasing for the Layperson

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Published:
January 01, 2004

Abstract

This chapter consists of a slight modification of a paper by Lines et al. (2001) published in the Canadian Society of Exploration Geophysicists Reporter. It is included in this text with permission.

Examples of aliasing can be observed in western movies by watching the motion of stagecoach wheels. As the stagecoach starts to move, we observe its wheels rotating in the expected forward direction. As the stagecoach speeds up, we see that the wheels appear to rotate in the opposite direction to the initial one. We realize that this phenomenon happens because the movie consists of a number of frames, or “digital samples.” With the wheels' acceleration, the digitized succession of frames will show the spokes appearing to move opposite to the actual direction of rotation. This occurs because the movie camera has undersampled the wheels' motion. Similar effects in real life occur when one watches helicopter blades as they speed up. Our brains sample the apparent motion too slowly to detect the actual motion, and the blades may appear to have a slower rate of rotation than the true speed.

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Contents

Geophysical Monograph Series

Fundamentals of Geophysical Interpretation

Laurence R. Lines
Laurence R. Lines
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Rachel T. Newrick
Rachel T. Newrick
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Society of Exploration Geophysicists
Volume
13
ISBN electronic:
9781560801726
Publication date:
January 01, 2004

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