heightmap using SDL_image

Hi, i want to load OpenGl heightmap using SDL_image. But i don't know how get the pixel color. Please help me, and tell how can I get that pixel color, please give me some code sample.

SDL_Surface *Surface;

int height = /* heightmap height */; 
 
for(int y = 0; y < Surface->h; y++)
{
    for(int x = 0; x < Surface->w; x++) 
    { 
       int color = pixel color(0 - 255) at coordinate(x, y); 
                                                   //DON'T KNOW HOW TO GET IT???
       float h = color / (255.0f / height);
    }
}