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Wilbur Erosion Tutorial
Wilbur is great basic terrain height map editing tool to create ArmA 2 terrains.
Filter → Noise → Fractal Noise
Filter → Fill → Fill Basins, use default -1 value
Filter → Mathematical → Span, Low = 1000, High = 2000
Filter → Noise → Percentage Noise, 5%
Filter → Fill → Fill Basins
Filter → Erosion → Incise Flow; blur 2, amount 3.5, flow exponent 0.35, effect blend 0.25
Filter → Fill → Fill Basins
Filter → Erosion → Incise Flow
That makes quite nice looking eroded terrain.