Critical viewing distance
CVD is based on your ability to detect picture structure, pixels and scan lines, within an image. For NTSC this required 8 screen heights to make the picture structure blend into a complete image.
HDTV resolution is directly related to being close enough to perceive it. For HD this requires a CVD of 2.5-3.2 screen heights. HDTV was specifically designed to provide a 30 degree field of vision without picture structure. This specification is also based on the video processing which means you are also not supposed to be able to see digital artifacts from this process such as pixelation, contouring, blocking and mosquito noise.