To add to what Pananacakes said - The height map itself isn't anything mysterious. Its just a bitmap that shows the sea as black, and the tops of the tallest mountains as white, with all the altitudes in between as blended shades of grey, rising from very dark at the bottom of the scale, towards white near the top of the scale.

You can make a really simple one and erode it in something like Wilbur to make it more realistic.