Geography Reference
In-Depth Information
All layers
{Named layer}
{Additional named layer}
{Group name}
{Grouped named layer}
{Additional grouped named layer}
{Additional named layer}
{And yet another named layer}
{And so on}
10.
Select COLE_DRG.TIF in the “Identify from” text box. Click a pixel on the map and note its Color
Index. Match this number with the value in the T/C and note the adjacent color symbol. Select
a couple more pixels. Altogether, this should give you some idea of how a DRG TIFF image
is put together. For a more detailed description do Steps 11 through 13 below. If you aren't
interested in this topic, close the Identify window and skip to step 14.
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.
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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.)
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You may have learned that the primary colors were red, yellow, and blue and that you could use these to make other
colors. You perhaps verified this by mixing paints that went on paper. Actually, these paints absorbed wavelengths of
light for certain color ranges. Paints give off no light on their own. A paint that absorbs red and blue wavelengths from
the white light that falls on it and reflects the rest appears as yellow. “Red, yellow, blue” is a subtractive or reflective
color model. Light is only reflected, not generated. However, when you look at a computer monitor, you are seeing
colors from an emissive or additive color model. Light is being generated by the interaction of the particles coming
from the electron gun and the special phosphors on the inside of the monitor screen (or by some more modern whiz-
bang technology that generates colored light). Here, the best combination of pure colors to generate the visible spec-
trum was found to be red, green, and blue. These aren't the only color models. For example, most color printers use
magenta, yellow, and cyan (plus black separately) as their primary colors.
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