@Override
    public Vector3f getNormal(Vector2f xz) {
        // offset
        float x = ((xz.x - getWorldTranslation().x) / getWorldScale().x) + (totalSize-1) / 2f;
        float z = ((xz.y - getWorldTranslation().z) / getWorldScale().z) + (totalSize-1) / 2f;
        Vector3f normal = getNormal(x, z, xz);

        return normal;
    }
gets an interpolated value at the specified point
jme3-terrain/src/main/java/com/jme3/terrain/geomipmap/TerrainQuad.java