Right! It's a satellite image of Charleston! As you know, Landsat satellites record electromagnetic energy (that's sunlight, pardner!) reflected from the Earth. Since different Earth materials (trees, water, etc.) reflect different amounts of energy, you can use the reflectance values to figure out what they are. And, on computer screens, you can display the different classes or categories in different colors. In this case:

Black = Fresh Water Blue = Sea Water Purple/Red = Urban Areas Light Green = Small Vegetation Dark Green = Forested Areas

Each pixel represents an area of on the Earth's surface that is thirty meters by thirty meters. By adding up the number of pixels in a classified image, you can calculate the area of objects in a scene. For example, if you count the number of green pixels you can calculate the total area covered with vegetation.

Don't know what a pixel is? Click to find out.

Since this area has already been classified, let's figure out the area occupied by each class.