Noel Stoutenburg writes:
> I have a topographical map, and I want to apply a color gradient which 
> follows the contour lines of the map, and blends from a darker hue at 
> the lower contour to a lighter hue at the next higher contour. The 

Have you used Shaped gradients? I think they'll help a lot.

If you haven't used them, try selecting one contour area, then
use the Gradient tool with Shape=Shaped, and drag from one edge
of the contour to the other. Neat, huh? But it's not quite what
you want, because it goes from white to black to white again, not
from white at one contour to black at the next.

Here's a way I found to do that.  It may still take a lot of steps,
but I think you'll get a better result than the smudging/airbrushing
method you mentioned:

Start with an image that has just the contour lines -- e.g.
black contour lines on a white background.

Use the magic wand tool and select the area within one contour
(the white area between two sets of black contour lines).

Save this selection somehow (e.g. switch to Quickmask mode, Copy, then
Paste and make it a new layer and turn visibility off on that layer;
or save it as a channel in the Channels dialog).

Still in magic wand, switch to Add mode and add the area in the next
contour up. (Or down, your choice.)

You may also need to select the line between the contours. If it's
antialiased, it may be faster to use quickmask and the paintbrush
rather than magic wand here. Or you may not need to select it at all.

Make a new layer (where you'll be drawing the gradient).

On the new layer, make a layer mask. Copy that original selection
you made, of a single contour area, and paste it into the mask.
Then click on the layer preview. Now you'll be drawing into the
new layer, but you'll only see the part corresponding to the current

In the Gradient tool with Shape=Shaped, drag across your contour
to make the shaped gradient.

I ended up with something like this:
You can clean up the edges of the layer mask as needed (those
white edges between the blue edge and the black contour line).
The important thing is that the shaped gradient gives you a nice fade
in the right directions without your needing to airbrush anything.

    "Beginning GIMP: From Novice to Professional":
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