2. Some Basic Principles

Parallax

A little test: Stretch one arm and hold a spoon in your hand. Close one eye and look where the spoon is located in its environment. Close this eye and open the other.

What do you observe?

This is what happened... the parallax

left eye right eye

The left image is seen by the left eye, the right image is seen by the right eye. When you alternately close one eye, it looks as if the spoon is jumping from one location to the other.

Question: On the background you see a bottle of water, which also changes position. Compare its parallax with the parallax of the spoon. What's your conclusion?

Answer ↓ ↑

The parallax of the bottle is much smaller than the parallax of the spoon.
Conclusion: the parallax of closer objects is bigger than the parallax of distant objects.

Parallax in remote sensing

The same phenomenon appears when an airplane (or a satellite) is flying over an area and taking photographs from certain features (e.g. buildings) with an interval of a few seconds (thus from a different location).

Have a look at the images below.

Caïro.
Source: Ghent University

The IKONOS satellite took the following images of these high buildings in Caïro (Egypt).

Caïro, seen with the satellite IKONOS.
Source: Ghent University

You get two images of the same building, but each one taken from a different position. The apparent displacement of the building caused by the change in the point of observation is the parallax.

NOTE: We can use the parallax to obtain information about the height of features in the image.