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I am using GIMP to create altitude profiles for the stages of a planned bicycle tour. Playing around on my first attempt, I created the profile below. (Note the smooth edge between the land and sky.)

enter image description here

After putting the project down for a day or so, I had forgotten my steps, but went ahead and tried to repeat the process for all stages. However, this time I could not achieve such a sharp, smooth edge between land and sky. (Note, particularly, the pixelation of the gentle rise at the beginning of the stage.) In the latter case, I have simply used feathering to smooth the edge of the land from green to alpha. The sky is a background layer.

enter image description here

I suspect what I did unwittingly in the first instance is to leave a lighter green on the edges of the land, which thereby produced a more blended transition. In the second instance, I have used a hard, dark edge of a single colour, then feathered it with alpha. Even with the feathering, the pixels are clearly visible.

Does anyone know of a good technique to create a smoother edge between stark contrasts of tone, without increasing the blur radius? (I do not want the edge to look fuzzy.) Any help would be greatly appreciated. Thank you.

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    What image did you start with? Is this from a photograph, or did you draw it? Why use GIMP for this? Personally, I'd be more inclined to use a vector image editor to make a diagram like this. Inkscape is free and Open Source like GIMP.
    – Billy Kerr
    Commented Feb 19, 2022 at 20:24
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    The image is small so there is a tendency to zoom it which make the pixels show because they are enlarged.
    – xenoid
    Commented Feb 19, 2022 at 20:33
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    @POD - If you don't want to manually redraw the mountains, you could get a half decent result by auto tracing the bitmap in Inkscape: See example
    – Billy Kerr
    Commented Feb 19, 2022 at 22:08
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    If you use the road profile from the Maps sidebar, it's all the more useful to use paths: you make the path on the thumbnail and then scale it up (or stretch it horizontally) with no loss of sharpness.
    – xenoid
    Commented Feb 19, 2022 at 22:43
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    You can get better resolution raster images by avoiding Google (which is terrible for bike routing BTW). RideWithGPS, Komoot, and many other tools (some free) will produce a better route and give you an elevation plot nearly the whole width of your screen. If you've got a GPX file of your route you can import it into any of those. My own crude tool for comparing routes used to as well, but I've switched to cumulative climb.
    – Chris H
    Commented Mar 3, 2022 at 11:56

2 Answers 2

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The best way to get something sharp and smooth is to use a path:

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Section zoomed in over your image:

enter image description here

That's only 78 points, takes a couple of minutes. Once you have the path you create a selection from it and bucket-fill:

enter image description here

Another good thing about a path is that if you have the data, you can make a CSV out of it, and have the CSV converted to a path.

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  • That produces a very clean line, thank you.
    – POD
    Commented Feb 20, 2022 at 4:59
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For those who might be interested, whilst the answer above solved the problem perfectly, it was rather time-consuming. A suggestion in the comments from my original post was to use Inkscape to trace path around the bitmap, then import the vector image into GIMP again as a bitmap. This produced a tidy image, but it tended to simplify the edge detail too much. The solution to this was to scale the image along the y-axis—I used a factor of 5—and those details were preserved. I then simply scaled the traced image the other way before import into GIMP.

enter image description here

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