Hawai'i Space Grant Consortium, Hawai'i Institute of Geophysics and Planetology, University of Hawai'i, 1996

Digital Images
based on activity sheet by Greg Smith
Teacher Page
Purpose

To simulate, by hand, how a computer may process digital images into color pictures and to learn how a planetary geoscientist interprets these pictures.

Brief Background

Remote Sensing
Remote sensing instruments provide information about a planet without requiring us to touch the planet. What kind of information can be derived from remote sensing data? The answers range from global to detailed. Geoscientists use remote sensing to study large, landform and atmospheric features as well as detailed, rock chemical compositions. The derived information depends on the wavelength(s) of radiation being measured by the instrument, the resolution of the instrument (how small a feature can be distinguished), and how much electromagnetic radiation (energy) it takes for the instrument to detect a feature.

Use the following link to access NASA's Internet reference pages on Electromagnetic Spectrum from The Observatorium.

LINK TO: Electromagnetic Spectrum.

Use the next link to access NASA's Mission to Planet Earth, Spaceborne Imaging Radar-C Internet pages on Remote Sensing. This resource contains a history of remote sensing tools, a discussion on the uses of remote sensing, a discussion on scale, and a table of measurements derived from remote sensing.

LINK TO: What is Remote Sensing?

Digital Image Processing
When a remote sensing instrument detects energy (light), the intensity of the light is translated into a number that a computer can read. On orbiting spacecraft, these numbers are coded into radio pulses and sent to receiving antennas on Earth. Computers then interpret these signals and convert them into arrays of numbers. Each number, in the rows and columns of the digital image, is assigned a unique brightness value (gray value) or color. Thus, the numbers become picture elements which together form a whole picture.

A list of numbers with their corresponding brightness values (or color values) is called a palette. For example, the following list shows six values from part of a gray-scale palette; a common range of values is 0 to 255.

numberbrightness
0black
25very dark gray
50dark gray
75gray
120light gray
255white

Instead of gray-scale palettes, images are also processed in color for more emphasis.

This Activity

Students are provided with a scenario in which, as space explorers, they are responsible for processing and interpreting a digital image in order to explore the surface of a planet they've encountered. They are asked to "color by numbers" to render the picture elements into a whole scene.

In this image, pixel values represent elevation data for the island of Oahu. Highest areas are shown as number 1, or the color red. Lowest areas as shown as number 5, or the color blue. Details of the elevations on Oahu are difficult to see because of the large pixel size. A higher resolution image (smaller pixels) would provide more detailed elevation information.

Students are asked to identify the scene and to derive information about the surface. Their interpretations may go beyond mere descriptions of elevation; however, in this simple exercise, no other data can be derived confidently. Interpretations of this image should be limited to elevation.

Of course, other information could be obtained about the planet using a different type of remote sensing instrument. The students are asked to consider what other data could be gathered remotely (see the Wrap-Up section below).

Preparation

Review and prepare materials listed on the student sheet. Colored pencils may be replaced with crayons or markers.

In Class

Distribute activity sheets to each student to color and interpret.

Wrap-Up

Why is the outline of the island so distinct?
Discuss other data that may be gathered remotely, such as vegetation cover, mineral resources, river drainage patterns, heat measurements, moisture content, rainfall, atmospheric data, wind speeds and directions, chemistry, etc.
Review the benefits of using remote sensing to learn about distant planets as well as inaccessible or hazardous places here on Earth.


Go to Digital Image activity sheet.

Go to Digital Images Student pages.

Return to Remote Sensing Activity Index.

Return to Hands-On Activities home page.
http://www.spacegrant.hawaii.edu/class_acts/