OpenGL heightmap terrain and normals with low resolution - c++

Question

I'm did a basic terrain generation with a heightmap images in openGL, work fine, but the terrain vertices and normals are too low resolution, as in the follow images:
Image of the normals
Image of the vertices in terrain
The terrain mesh generation doesn't metter here, it's just a basic triangles generation, so let's go to the normals and heightmap generation part:
//Calculate the height of the vertex in the terrain
int Terrain::GetPixelHeight(unsigned char* data, int imageWidth, int x, int y)
{
if(x < 0 || x >= 256 || y < 0 || y >= 256) //If coordinates passes the image limit, just return 0
return 0;
else
{
unsigned char* pixelOffset = data + (x + imageWidth * y) * 1; //Get the image
unsigned char r = pixelOffset[0]; //Get R value
unsigned char g = pixelOffset[1]; //Get G value
unsigned char b = pixelOffset[2]; //Get B value
float height = (float)r + (float)g + (float)b; //Put values together to calculate the height
return height / 40; //Return the height with smooth in 40
}
}
Now the normals calculation:
//Calculate the normals based in the terrain height
float heightL = GetPixelHeight(data, y, x-1, z);
float heightR = GetPixelHeight(data, y, x+1, z);
float heightD = GetPixelHeight(data, y, x, z-1);
float heightU = GetPixelHeight(data, y, x, z+1);
glm::vec3 normalVector = glm::normalize(glm::vec3(heightL - heightR, 2.0f, heightD - heightU));
normals.push_back(normalVector.x); normals.push_back(normalVector.y); normals.push_back(normalVector.z);
Shaders are basic too, I can guarantee there are no problems with them, because is the same shaders of the others objects in game, and this objects doesn't have problems with light calculation, so i'm think the problem is the normals calculation with the heightmap.
PS: Terrain size is the same of the image, 256x256, the image just have black and white channels.