How do I optimize my maps for printing?

[Coriolis]

I'm preparing a small portfolio of 12 pieces to send to a publisher. This publisher wants physical prints, though they'll take image files in addition to them. I'm looking for methods and principles of getting these images to look as clear and as eye-catching as possible.

I've printed these out on a 600 dpi color printer, and I'm underwhelmed with the results. These are the problems I see when I print them out:
1. Most of them are too dark, even after I've lightened them. I'm having trouble finding the right level of brightness.
2. Contrast seems to heavy in some areas and too light in others. Many smaller labels become difficult to read for want of clarity, while the contrast on many of the icons have too much contrast, to the point where details of the parts in shadow can't be seen. Meanwhile, a lot of the lowland topographical bump mapping seems very subdued.
3. Most of the coastlines don't have nearly as sharp a distinction between ground and water as they do on the screen.

Here are some examples:
AssembledValleyv4_zpsf3203f6a.png
My most-recent map is a hidden mountain vale. Though the mountains that surround it are clear and bright, the land in the middle is far too dark on the printout, to the point that it's hard to see details and read labels.

FinalAssembly_zps2434a7f8.png
A post-apocalyptic map of Michigan's upper peninsula from the Rifts RPG. The borders help to make the coast stand out a bit more, but a lot of the details are hard to make out, many of the small labels are difficult to read, and the light/shadow contrast of the city icons is far too strong. The map prints out a bit dark, and the water loses a lot of the texture and bathymetry I put into the image file.

AtlantisMasterMap_zpsc5edbea0.png
A map of Atlantis from the Rifts RPG. The ocean prints as too dark, the swirls in the water are too difficult to see, the land doesn't stand out from the sea enough, the forest makes the land a bit too dark, and the icons suffer from the same exaggerated light/shadow contrast as in the one before.

RegnumAssembled_zps397aa54c.png
An older map I made about a year ago, this one actually prints out decently, but the ocean is still too dark and swallows the texture work I did on it. The coast doesn't pop as much as I'd like it to Finally, the whole map seems a bit too dark.

This grayscale map I made a while back is another example. It actually comes out looking pretty good, but the labels are harder to see.

Overall, I'm pretty satisfied with how these look on the screen, and most of them have plenty of pixels to print on 8 1/2 x 11 inch paper at 300 dpi (my standard is 3375 x 2625 pixels, a bit bigger for maps with frames), but I can't quite seem to make these look as sharp or as eye-catching as they look on the screen.

I use GIMP. Are there any tricks to making these print better? Should I work with color curves? The brightness/contrast tool? Or should I play with the saturation? I'd really appreciate some advice and pointers.

[BraveSirKevin]

A couple tips that should help immediately:
• If you're using an ink-jet printer then print on a photo paper. Plain papers are quite absorbant so the ink dots spread giving you dark, muddy colours with very little contrast. You can find photo papers in matte and gloss finishes.
• Familiarise yourself with the Unsharp Mask filter. It will punch up the contrast without messing up your colour values. Good use of this tool will even help get a decent result when printing to plain paper with an ink-jet printer.
• Check your images in grayscale. If there's not enough contrast in the gray, then there's definitely not enough in the colour version. Also, any area that's darker than 80% black in greyscale is going to be bumped up to almost full black when printing on an ink-jet printer. I had a look at your atlantis image and the ocean and forests are virtually identical in the grayscale and both are in the very high 80s. Your plains are in the mid 60s and only the mountains actually drop below 50. The reason your icons are popping excessively is that they actually have an appropriate amount of contrast on them while the rest of the image has almost no contrast at all.

More long term, if you're looking to get into cartography for print then there's a few things you really should take some time to learn about. Find some good tutorials on the art of trapping for print. You can probably find some useful stuff by looking up comic-book colouring techniques. Short version of what you need to know is:
• When illustrating for litho or gravure printing you should always work in CMYK from the start.
• 300dpi is good for the stuff you're doing, but if you have crisp inked linework that you want to preserve it's common to work at 600 dpi with a non-antialiased black channel.
• The percentages of each of the 4 ink colours should never add up to more than 300 (or 250 if the colour block is quite large).

[Coriolis]

Thank you very much for the help! The Unsharp Mask Filter was quite useful, especially once I read the manual and decomposed the colors first. I also played with the color curves, making them rise steeply in areas of high brightness concentrations. The results are much better on printer paper. I imagine they'd look even better on photo paper, but I'm a bit too cheap to do that yet.

I remastered all the maps, but the Atlantis map is a good representation. Here's my attempt at an enhanced version. I should also mention that I decided to do a revision and use the same border technique I used on the Upper Peninsula map.
AtlantisMasterMapCurveAndUnsharpAndBorders_zps38b3d56c.png

[BraveSirKevin]

There's certainly a lot more life in the new version! Good luck with the publisher!