Scientific Visualization: 2D Area Rendering

**NC Scientific
and Technical Visualization Objectives:**

Level II 3.01 B. The student will identify sources of data and applications for 2-D area rendering.

**NC
Earth and Environmental Science Goals and Objectives (objectives from 1994
revision):**

5.2 Demonstrate knowledge of current
research in earth/environmental science.

6.1 Demonstrate knowledge of mapping
skills and location of points on earth.

**Materials**

pencils, graph paper, colored pencils, long strips of paper

Students will solve the problem of devising a coding scheme suitable for encoding and transmitting map data. They will have to work together to discover and agree on a code and transmission protocol that will work. Do not give the students a transmission protocol. They will develop a need for it as they try to solve their transmission problem. They will have to agree on at least the following: whether 0 or 1 codes for land, the exact width and length of their map, which corner and which direction they will start with. It is best to have the first map students work with be small and simple. The 1st picture below is a sample simplified island map, suitable for the first iteration. The coding scheme is illustrated in the second box. A one was used when a box had any land at all, otherwise a zero was used. On a long strip if we start in the upper left and move across this would appear as

0000000000000111000000111100000111110000011111110001111111100111111110011111111000001110000000000000

The third box illustrates the result. In order to decode the long
strip correctly a group would need to know that the map was 10 by 10, the
code starts in the upper left and moves left to right, and that 1
coded for land.

Rather than explaining to students how to do this activity, try allowing them to figure it out. It will help them to have materials and sample maps while they discuss their coding schemes. If the students are stuck there are several ways the problem can be made easier. In the first iteration, they can use graph paper instead of the long strips to transmit the numbers - then all they have to decide on is whether ones or zeros are shaded. The first maps passed out can also be stylized designs (like the result on the right above) so that students aren't confused by attempting to transmit information about darkness of shading or information about which part of a box is shaded. The problem can be made easier by giving all groups small graph paper squares that are cut out to be exactly the same size or harder by giving them larger sheets of different sizes. The first maps can be teacher produced, but students will enjoy producing their own. Given a State map different groups can be assigned to produce an encoded map of a particular county or island or Outer Bank island. The other group will then have to decode the map and figure out which county (out of a limited number of possibilities) or island the map represents.

**Discussion**

After completing this activity the teacher should ask how the map coding would be affected by using graph paper with smaller squares - if each small square was divided into 4 smaller squares or 9 smaller squares. This leads to the concept of a pixel and the relation of pixels to image resolution. A pixel is a picture element of an image. If you keep zooming in on an image in an image processing program you eventually see that the image is made up of many tiny boxes -each with one color. Each box is a pixel. In this example each graph paper square represents one pixel. If students are not already familiar with this have them try zooming in on an image using your image processing software. Just as the students had to choose whether boxes that were partially shaded should be coded with ones or zeros, a satellite sends an average value for each area it samples. The smaller the sampling area for a pixel, the higher the resolution of the image.

Then ask the students to transmit maps with color coding of different types of features. (For example dark green for forest, yellow for sand, light green for grazing land, red for grain crops etc) Now they will have to agree on a more complicated coding scheme. The groups receiving the maps might choose to use different colors to represent the data they receive. This is like changing the LUT, it doesn't change the underlying data that was transmitted only the display which can be manipulated to emphasize the features of interst. Depending on the mathematical sophistication of the class you may want to explain that since digital transmission is only of ones and zeros all other numbers must be converted to binary before transmission. Ask students to discuss how true color, false color, and puedo color images would differ in this process. Remember in true color images the image uses red, blue, and green light to represent the red, blue, and green light recorded by the sattelite sensors. False color uses red blue and green to display information from other usually not visible light parts of the spectrum and psuedo color uses color to represent different values of one measured quantity such as temperature. So a map that uses color for altitude would be psuedo colored.

Other concepts which can be discussed here include: scale and
different error checking schemes (checksum). For example the map
can appear larger but not convey any more detail if there are the same
number of larger squares on the output map. Regardless of the size
of the output map the scale is determined by the size of the squares that
each pixel represents.

**Step by Step**

- Prepare graph paper squares with simplified maps in advance
- Put students in an even number of groups of 2 or 3 students.
- Pair the groups.
- Pass out an example map and strips of paper to each group.
- Explain the problem. Tell students that each group will be given a map. They will have to transmit the contents of the map to their partner group using only the long sheets of paper and 1s and 0s. No punctuation or grouping symbols allowed. They can meet in advance to decide on their coding sheme and transmission protocol.
- Have group pairs separate.
- Give each group a map to transmit and a paper strip. Have them exchange strips when ready.
- Each group should then decode the strip they received into a map.
- Groups should compare their maps to the originals.
- Follow discussion outlined above.

**References**

This activity was adapted for use of Scientific Visualization classes
from the "No Class is an Island" and "Gaia Challenge" Activities
from
__The Gaia Crossroads Project Guidebook to Using Satellite Imagery
In the Classroom and Community.__ This guidebook is an excellent
source of information about using remote sensing images in the classroom.
The "No Class is an Island" activity was originally developed by
Larry Ryan for Maine Center For Education Project SEED and the "Gaia Challenge"
activity was created by Dean Meggison at Kennebunk High School
in Maine. For more information check out the Gaia Crossroads Project
website at http://www.bigelow.org/~gaia/

last update 4/6/99