How to have smooth terrain in maps?

[Speedyscout122]

Literally every time I make a height-map and go to test it, the areas where it gets higher or lower are all bumpy instead of having a nice smooth slope!! Help, how can I get them smooth? I do use the soften/blur tool in GIMP, and the smudge tool, on the heightmap, but it stays all lumpy. It's really annoying. So, anyone got advice?
It looks like this:

[SophyTheLioness]

How much is the width/height that you applied to your map? Is the heightmap 512 pixels in size or bigger?

[Vilivikhorith]

You could try using a larger heightmap to decrease bumps when rendered (especially if you're resizing the terrain larger in the Map Maker). Contrary to popular belief, you can make heightmaps larger than 513x513. However, they MUST be a power of two plus one, hence (512 + 1, 512 + 1), (1024 + 1, 1024 +1), (2048 + 1, 2048 + 1), et cetra. Note that it will also increase load times significantly, by about the same factor of a power of two used when loading the heightmap and generating terrain.

[SophyTheLioness]

You could try using a larger heightmap to decrease bumps when rendered (especially if you're resizing the terrain larger in the Map Maker). Contrary to popular belief, you can make heightmaps larger than 513x513. However, they MUST be a power of two plus one, hence (512 + 1, 512 + 1), (1024 + 1, 1024 +1), (2048 + 1, 2048 + 1), et cetra. Note that it will also increase load times significantly, by about the same factor of a power of two used when loading the heightmap and generating terrain.

Ah, adding to what Vili said! I think you need to edit the code of the map for anything bigger than 1024, but i am not certain since i've tried this one months ago and my memory is rather blurry. Don't take that as a fact, just as a note.
What i wanted to say, too, was that lag increases wildly if you decide to hop into a 2048 heightmap (At least in my case) I recommend using 1024, detailed enough and less laggy. 2048 is kind of... An excess? I tried it in a map that was plain except a bunch of random tests and even then it was a little laggy. Imagine how much my computer strained loading when i decided to add mountains, tunnels and other detailed stuff. Not a single object...
Here are black basic files with the heightmap sizes already set, they are all grayscale already.
(1025 px) (read explanation above)

(2049 px) (Yes thats the size it says it has. It works on FH, so don't question me, question the game LOL)