Mapping Hacks

Hack on Base Maps in Your Favorite Image Editor #32

Chapter 3, Mapping Your World

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HACK

want to tweak this setting to see what you end up with. You should now

have two different images loaded, one of the satellite imagery and another of

the political boundaries.

Combining the Layers to Make a New Map

The first thing you’ll notice about the boundary layer—in the GIMP, any-

way—is that it’s lying on its side! That’s because GMT produces landscape-

oriented PostScript by default, but the GIMP assumes that all PostScript is

portrait-oriented. So the first thing to do is to correct this, by clicking on the

boundary layer, and then selecting “Image

➝ Transform ➝ Rotate 90 degrees

CW” from the drop-down menu. This should rotate the image such that it’s

now oriented correctly. Now execute Edit

➝ Selection ➝ Select All to select

the entire image, and then Edit

➝ Copy to copy it to the clipboard.

Next, click on the Blue Marble image and run Edit

➝ Paste to paste the

boundary image in as a floating layer, which should make the Blue Marble

image disappear beneath it. We need to alter the opacity of the boundary

layer so that the white parts become transparent, leaving only the black bor-

ders. In the GIMP, you can go to Tools

➝ Layer Dialog, which pops up a

dialog displaying both layers of the current image. Click on the upper of the

two, which should be the floating layer containing our political boundaries. ...