tiny diamond square algorithm
Java, 1017 bytes
Input is a space separated list like so: l s1 s2 s3 s4 h a b.
Output is an 2d-array containing the numbers.
Program:
import java.util.*;import static java.lang.Math.*;class C{public static void main(String[]a){int b=a.length,d=0;float[]c=new float[b];for(;d&lt;b;){c[d]=Float.parseFloat(a[d++]);}e=(int)(pow(2,c[0])+1);f=new float[e][e];f[0][0]=c[1];f[0][e-1]=c[2];f[e-1][0]=c[3];f[e-1][e-1]=c[4];g=c[5];float h=c[6],i=c[7];s(0,0,e-1,e-1,h,i);System.out.print(Arrays.deepToString(f));}static int e;static float[][]f;static float g;static void s(int q,int r,int s,int t,float h,float i){if(s-q&lt;2|t-r&lt;2|q&lt;0|r&lt;0|s&gt;=e|t&gt;=e)return;float o,p;int m=(q+s)/2,n=(r+t)/2;f[m][n]=(float)(a(q,r,s,r,q,t,s,t)+random()*(i-h)-h);d(m,r,m-q,o=h*g,p=i*g);d(q,n,m-q,o,p);d(m,t,m-q,o,p);d(s,n,m-q,o,p);}static void d(int x,int y,int e,float h,float i){float o,p;f[x][y]=(float)(a(x,y-e,x+e,y,x,y+e,x-e,y)+random()*(i-h)-h);s(x-e,y-e,x,y,o=h*g,p=i*g);s(x,y-e,x+e,y,o,p);s(x-e,y,x,y+e,o,p);s(x,y,x+e,y+e,o,p);}static float a(int...j){float k=0,l=0;for(int d=0;d&lt;j.length;d+=2){if(j[d]&lt;0|j[d+1]&lt;0|j[d]&gt;=e|j[d+1]&gt;=e)continue;l++;k+=f[j[d]][j[d+1]];}return k/l;}}

Program that is indented and displays the map:
import java.util.*;
import java.awt.image.*;
import java.awt.*;
import javax.swing.*;
import static java.lang.Math.*;

class D{
    
    public static void main(String[]a){
        int b=a.length,d=0;
        float[]c=new float[b];
        for(;d&lt;b;){
            c[d]=Float.parseFloat(a[d++]);
        }
        e=(int)(pow(2,c[0])+1);
        f=new float[e][e];
        f[0][0]=c[1];
        f[0][e-1]=c[2];
        f[e-1][0]=c[3];
        f[e-1][e-1]=c[4];
        g=c[5];
        float h=c[6],i=c[7];
        s(0,0,e-1,e-1,h,i);
        showImage(f);
    }
    
    static int e;
    static float[][]f;
    static float g;
    
    static void s(int q,int r,int s,int t,float h,float i){
        if(s-q&lt;2|t-r&lt;2|q&lt;0|r&lt;0|s&gt;=e|t&gt;=e)
            return;
        float o,p;
        int m=(q+s)/2,n=(r+t)/2;
        f[m][n]=(float)(a(q,r,s,r,q,t,s,t)+random()*(i+h)-h);
        d(m,r,m-q,o=h*g,p=i*g);
        d(q,n,m-q,o,p);
        d(m,t,m-q,o,p);
        d(s,n,m-q,o,p);
    }
    
    static void d(int x,int y,int e,float h,float i){
        float o,p;
        f[x][y]=(float)(a(x,y-e,x+e,y,x,y+e,x-e,y)+random()*(i-h)+h);
        s(x-e,y-e,x,y,o=h*g,p=i*g);
        s(x,y-e,x+e,y,o,p);
        s(x-e,y,x,y+e,o,p);
        s(x,y,x+e,y+e,o,p);
    }
    
    static float a(int...j){
        float k=0,l=0;
        for(int d=0;d&lt;j.length;d+=2){
            if(j[d]&lt;0|j[d+1]&lt;0|j[d]&gt;=e|j[d+1]&gt;=e)
                continue;
            l++;
            k+=f[j[d]][j[d+1]];
        }
        return k/l;
    }
    
    public static void showImage(float[][] f){
        float maxHeight = Float.MIN_VALUE;
        float minHeight = Float.MAX_VALUE;
        for (float[] row : f){
            for (float height : row){
                if (height &gt; maxHeight){
                    maxHeight = height;
                }
                if (height &lt; minHeight){
                    minHeight = height;
                }
            }
        }
        int e = f.length;
        BufferedImage image = new BufferedImage(e, e, BufferedImage.TYPE_INT_RGB);
        for (int x = 0; x &lt; e; x++){
            for (int y = 0; y &lt; e; y++){
                Color color = Color.getHSBColor((float)((f[x][y] - minHeight)/(maxHeight - minHeight)), 1, 1);
                image.setRGB(x,y,color.getRGB());
            }
        }
        JFrame frame = new JFrame(&quot;Picture&quot;);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(new JComponent(){
            
            @Override
            public void paint(Graphics g){
                g.drawImage(image, 0, 0, getWidth(), getHeight(), null);
            }
            
        });
        frame.setVisible(true);
        frame.setBounds(0,0,e,e);
    }
    
}

Here is a function in Java to show a map:
public static void showImage(float[][] map){
    float maxHeight = Float.MIN_VALUE;
    float minHeight = Float.MAX_VALUE;
    for (float[] row : map){
        for (float height : row){
            if (height &gt; maxHeight){
                maxHeight = height;
            }
            if (height &lt; minHeight){
                minHeight = height;
            }
        }
    }
    int size = map.length;
    BufferedImage image = new BufferedImage(size, size, BufferedImage.TYPE_INT_RGB);
    for (int x = 0; x &lt; size; x++){
        for (int y = 0; y &lt; size; y++){
            Color color = Color.getHSBColor((float)((map[x][y] - minHeight)/(maxHeight - minHeight)), 1, 1);
            image.setRGB(x,y,color.getRGB());
        }
    }
    JFrame frame = new JFrame(&quot;Picture&quot;);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.add(new JComponent(){
        
        @Override
        public void paint(Graphics g){
            g.drawImage(image, 0, 0, getWidth(), getHeight(), null);
        }
        
    });
    frame.setVisible(true);
    frame.setBounds(0,0,size,size);
}

All of these images have a size of 7. The 4 seeds are 5, 10, 15, and 20.
a and b are -10 and 10 respectively.
Roughness starts at .1 and increments by .1 up to 1.

Terrain generating code coming soon!!!
Images coming soon!!!