The GLOBE Program

Global Learning and Observations to Benefit the Environment

Below you will find two hyperlinks to the Thematic Mapper™ images of the Garfield Community Magnet School's GLOBE Site. The images were produced using Bands 3, 2, 1 (natural color) and Bands 4, 3, 2 (infrared. The images are 512 x 512 pixels in size (about 15 x 15 kilometers) with the Garfield Community Magnet School located in the center of the image. The images includes the following TM bands 9channels):

TM1 - blue visible light

TM2 - green visible light

TM3 - red visible light

TM4 - near-infrared energy

TM5 - mid-infrared energy

Unsupervised Classification "Clustering"

Each pixel in your LandSat TM image contains a wealth of information about the surface materials that

reflected light from that pixel to the satellite sensors. Each pixel contains a value, from 0 to 255, for

each TM band supplied with your image. If, for instance, your image contains data for five bands, then

each pixel contains five pieces of data, each ranging from 0 to 255, as shown in the sample pixel diagram to the right.

 

 Land Sat Pixel

	Band I Blue -----------39Band 2 Green ---------53Band 3 Red -----------25Band 4 Near IR___129 Band 5 Mid IR_____ 46
				30 m
		30 m

This means that your image could contain 256' (that's approximately 1. 1 billion) different possible

spectral combinations. Each of these combinations does not represent a different type of land cover;

most of these variations represent very small and, to us, "unseeable" differences in surface reflectance.

 

In most instances, your computer monitor will be displaying only 256 different colors, hence only 256

different pixels. Even set to "thousands" of colors, only a small part of the many different pixels can be

displayed. Even if a monitor could display all the different possible pixels, your eyes simply could

recognize only a small number of differences in their appearance.

 

Because there is a limited number of different land cover types (the Modified UNESCO Classifications

scheme, MUC, contains about 130 different types), and no GLOBE study site will have all of those

different land cover types, it is necessary to group pixels together into a smaller number of closely

related "classes." This process, whereby pixels with similar spectral characteristics are grouped, is

done in two different ways, by a supervised and unsupervised classification.

 

In a supervised classification, you "train" the software to recognize that certain types of pixels represent

specific land cover types. This is done on the basis of your knowledge of your own area, and field work

you may do. The software then classifies the pixels of your image into the groups you have specified.

The MultiSpec tutorial provided with your GLOBE materials contains a section on supervised classification.

For further information contact: Peter Orio, GLOBE Certified Teacher

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