Although the masks really come into their own when used with Photoshop’s Alpha Channels (Chapter 15), you can also use the masks to apply quick and easy selections to a map without using channel operations. Just follow these instructions:
1. Open the base version of the map you wish to work on (you may wish to use a low resolution PICT version for purposes of experimenting, since this will provide faster results).
2. Open the corresponding ocean mask for the map. If you are working on the TIFF version of the map, select “Grayscale” from the Mode menu. Enter a value of “1” in the Size Ratio box in the dialog which follows, and click “OK” (if you are working on the PICT version, you can skip this step, because the low resolution versions are already grayscale).
3. Choose “Load Selection” from the “Select” menu. In the dialog box which opens, the “Source” pop-up menu will show the name of the map you are working on, while the “Channel” pop-up menu will display the mask you have just opened. If you have more than one mask open from the same map, their names will also appear in the menu. Click “OK.” The coastline will now appear on the map as “marching ants.” This indicates the selected area.
That’s it! Any action you now take will apply only to the selected portion of the map. To get an instant result, select “Adjust > Brightness/Contrast…” from the Image menu, and, with “Preview” checked, move the sliders.
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