Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition

A Look (Optional) at How DRG Color Values Are Put Together

(Skip to Step 14 if you don’t want to do this section.) A DRG is simply a color picture made up of pixels (pixel elements). It may be interesting to you to see how this works. Each color used on the picture is made up of specific quantities of three primary colors: red, green, and blue.24 Imagine that you have three flashlights, one of each color, focused on a single spot (pixel) and that the amount of light coming from each flashlight can be controlled, with 0 indicating none and 255 indicating brightest. Of course, when all three lights are off, you get black. When all three lights are on at their brightest, white light is produced. These two conditions are represented in the table as Value 0 (black) and Value 255 (white).

____ 11. Click a brown pixel on the map. Its color index is 4. The Identify window Color field will tell you that the maximum value for any color value is 255, and that this color is made up of intensities 131 (red), 66 (green), and 37 (blue). (Incidentally, recall that you previously viewed the attribute table for the DRG. This Identify window represents one of the few cases where the values shown in the Identify window are not simply the values in the attribute table.)

____ 12. Right-click the yellow rectangular symbol (7) in the T/C, and click More Colors to open the Color Selector window. Notice that the Red, Green, and Blue (RGB) values are approximately 254, 234, and 0. Move the slider on ...