How can I make a smoother my heightmap? (Perlin noise, openGL)

Question

I generate 2D perlin noise and I'm doing it terrain. But I have a problem with this. It's too gradual.

I tried x,z coordinates are divided or multiplied but doesn't work. How can I solve this problem without tessellation shader?

EDIT:

fragment shader code:

#version 430

in vec3 pos;
in vec2 text;
in vec3 norm;

layout(binding=3) uniform sampler2D texture_1;
layout(binding=4) uniform sampler2D texture_2;
layout(binding=5) uniform sampler2D texture_3;

vec3 lightPosition = vec3(-200, 700, 50);
vec3 lightAmbient = vec3(0,0,0);
vec3 lightDiffuse = vec3(1,1,1);
vec3 lightSpecular = vec3(1,1,1);

out vec4 fragColor;
vec4 theColor;

void main()
{

    vec3 lightVector = normalize(lightPosition) - normalize(pos); 
    float cosTheta = clamp(dot(normalize(lightVector), normalize(norm)), 0.5, 1.0);

    if(pos.y <= 120){
        fragColor = texture2D(texture_2, text*0.05) * cosTheta;
    }
    if(pos.y > 120 && pos.y  <  150){
        fragColor = (texture2D(texture_2, text*0.05) * (1 - (pos.y-120)/30) +  texture2D(texture_3, text*0.05) * ((pos.y-120)/30))*cosTheta;
    }
    if(pos.y >= 150)
    {
        fragColor = texture2D(texture_3, text*0.05) * cosTheta;
    }
}

vertex shader code:

#version 430

in layout(location=0) vec3 position;
in layout(location=1) vec2 textCoord;
in layout(location=2) vec3 normal;

out vec3 pos;
out vec2 text;
out vec3 norm;


uniform mat4 transformation;

void main()
{   
    gl_Position = transformation * vec4(position, 1.0);
    norm = normal;
    pos = position; 
    text = position.xz;
}

Answer 1

It looks like you're using an 8bit heightmap which only gives you 256 different elevations. You could try using 16bit greyscale or encode the height in rgb.