Terrain from float texture

This vertex shader uses 2D float texture to make terrain. N×N points (x,y) of the aPos attribute make (0,0, 1,1) square. They are used to get the z coordinate from texture too.

<script id="shader-vs-show" type="x-shader/x-vertex"> 
  attribute vec2 aPos;
  uniform mat4 mvMatrix;
  uniform mat4 prMatrix;
  uniform float d;
  varying vec4 color;
  uniform sampler2D uTexSamp;
  const vec4 dirDif = vec4(0., 0., 1., 0.);
  const vec4 dirHalf = vec4(-.4034, .259, .8776, 0.);
void main(void) {
   float h = texture2D(uTexSamp, aPos ).r;
   gl_Position = prMatrix * mvMatrix * vec4(aPos, h, 1.);

   vec3 aNorm = vec3(texture2D(uTexSamp, vec2(aPos.x + d, aPos.y)).r -
     texture2D(uTexSamp, vec2(aPos.x - d, aPos.y)).r,
     texture2D(uTexSamp, vec2(aPos.x, aPos.y + d)).r -
     texture2D(uTexSamp, vec2(aPos.x, aPos.y - d)).r, -d-d);
   aNorm = normalize(aNorm);
   vec4 rotNorm = mvMatrix * vec4(aNorm, .0);

   float i = max( 0., abs(dot(rotNorm, dirDif)) );
   color = vec4(.0, .5*i, i, 1.);
   i = pow( max( 0., abs(dot(rotNorm, dirHalf)) ), 120.);
   color += vec4(i, i, i, 0.);
}
</script> 

   gl.drawArrays(gl.TRIANGLES, 0, 6*(N-1)*(N-1));