Analog vs. Digital Images:
Discovering the Difference Between Two Types of Images

Objectives:
Students will a) make an analog image of a section of the school grounds; b) they will digitize their analog image; c) they will compare an analog and a digital image.

Grade Level: 4-6

Time Duration: two to three 50-minute class periods

Subjects: language arts, geography, science, math, art

Skills:
measurement, drawing, calculating fractions, averaging coordinates

Framework: scale and structure, systems and interactions

Vocabulary:
scale, dimension, grid, map, pixel, analog, digital, digitize, resolution, fraction, aerial photography, image, average, vertical, biotic, abiotic, line, sample, integer

Materials:

Figure 1 is a page marked as a 20 cm by 20 cm grid. Reproduce it for each of your students.

Figure 2 is a schematic of the larger square that is to be laid out on the school yard. Each student grid page is represented as a numbered square in this schematic.

Background:
What is similar between how we see and how a camera sees? Does a computer "see" the way we see? How does a computer get information from a photograph? Many processes that scientists use today are mysterious until we understand them. Sometime this understanding is simple if taken in small steps.

Geography and information systems use both analog and digital imagery. A camera produces an analog image and imitates how the eye sees. This type of image shows a continuos change in color and shading. It is rich in detail providing a great deal of information. Scale is the ratio between the actual distance covered by a photo or map and the size that is reproduced. For example, if one meter of ground distance equals one centimeter on an aerial photo, the scale is 100:1.

A computer can record and manipulate data provided by an image once the image has been translated into digital form. This requires transposing a grid over the image. Each square in the grid is called a pixel. The value or shade of each pixel is then averaged so that each will represent one digital value . Each value used in a digital image can also be assigned a number thus becoming an array of integer values. The horizontal rows of pixels are called lines and the vertical columns are called samples. The pixel in the top left corner in the array is line 1, sample 1. The book "Basic Concepts of Computerized Digital Image Processing for Geologists" published and available from the U.S. Geological Survey for a small fee, is an excellent resource for understanding digital imagery. It contains a booklet and four posters representing different aspects of digital image processing.

Resolution in digital imagery refers to the size of the pixel in relation to the area of ground that it represents. The smaller the area represented by a pixel, the greater the resolution and the clearer the image. For example, if the area covered by a pixel is one kilometer by one kilometer then the resolution is one kilometer. If the area covered by a pixel is 5 x 5 meters then the resolution is five meters. In this activity, each square (or pixel) in the grid will represent a square meter on the ground. This is a resolution of one meter. If a greater resolution is desired, divide the grid into smaller pixels before doing the digitizing.

Students will each produce an analog type image of a section of the school grounds. When all of the individual squares are put together, a larger image square will be formed. When these are arranged, they will form a digitized image of the large square.

One meter of ground distance will be represented by a 5 cm square on paper so the scale will be 100:5 or 20:1. Students will better comprehend the principles of digital imagery, once they have undertaken the process of digitizing an analog image. They will gain direct experience using aerial perspective, coordinates, measurement, scale resolution, color analysis and digitizing.

Procedure:
Part 1. Creating an Analog Image:
Choose to map an area of the school grounds with variable landscape, i.e. an area that has a sidewalk, asphalt, dirt, bushes, grass, etc.

  1. Show the students photos as samples of analog imaging. Discuss the similarity between snapshots and how the eye sees.

  2. Proceed with the students to the area to be mapped. Students should take with them a piece of paper marked with a 20 cm x 20 cm grid, pencils, crayons and something to write on for drawing, and the marked string.

  3. Lay out the string on the ground to form a grid.

  4. Students draw the details of one grid square. Instruct students to do a vertical image on their paper, drawn as if they were looking straight down on their square.. The calibrations on their paper correspond to the red calibrations on the string making it easier to draw to scale.

  5. Students color their square as accurately as possible.

  6. Gather supplies and return to the classroom.

  7. Discuss. How are your pictures are like a photo? What could you do to make them more like a photo? How is your eye like the lens of a camera?

  8. Assemble all of the squares to make the large picture (use adhesive clay or some other non-permanent material) and view as a whole. What landmarks do you see? What parts of the image are biotic? Abiotic? How would this image look during another season? Does this image tell you anything about the local climate or current weather conditions? Would an image of this same area look the same if it had been created 100 years ago?

Assessment:
You are a city planner. A new shopping center is being proposed for a forested area in your community. An environmental advocacy group has written that building the mall on this site would hurt the local wildlife because the community has little remaining undeveloped land and this habitat is needed to provide food and shelter for existing wildlife. Can an aerial photo of your community help you plan your recommendation for the project? How?

Part 2. Translating an Analog Image into a Digital Image:
Photocopy another set of the 20 x 20 cm grid worksheets. Have students use rulers to divide each square on the analog image into quarters. Reproduce the same grid on the new squares. Students can then average the color of their squares and make a second solid colored square. When these are arranged together, they will form a digitized image of the large square.

  1. Return the individual squares to each student. Also, distribute the blank second handout. Have student connect the lines on both papers. Students should now have two 4 x 4 grids.

  2. Students choose the colors to represent each square in the colored picture. Have students look at each square and compare the area represented by each color within the square. Choose the color covering the most area to fill in the corresponding box in the second grid. If two colors are represented equally, choose one, basing the choice on which color would emphasize line. For example, if three squares are connected and divided in half by two colors, choose the same color for each so that there is not a checkerboard effect.

  3. Assemble the two series of squares into larger pictures. Compare the two. How are they alike and different? Which photo gives us the most information and why? Compare the fraction of each color used in each image. Which image is easiest to calculate?

Discussion:
If you had chosen one color to represent each of your 4 x 4 squares, how would the larger picture be different? How could you make the picture clearer? Besides colors, could we use anything else to represent each square?

Assessment:
You have taken a photo from a low flying aircraft. Information from the photo can be can be used in an emergency. You must use a computer to send either the photo or the required information to another location. How can you transfer the image data to the computer without using a video camera or a scanner?

Extensions:

  1. Assign a number to each color used and build a coordinate grid where data is recorded in numbers.

  2. Figure the fraction or percentage of the total square that is covered by each color or number.

  3. Refine the image by providing greater resolution. Photocopy onto acetate or trace onto tracing paper a grid divided into smaller squares. Provide a class set of this grid photocopied onto paper. Place the transparent grids over the analog images. Repeat the steps for color averaging and create a digital image on each of the grid papers.



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