Not quite true Miraz. 1 NM is the "distance bisected by 1 minute of arc at the earth's surface", so the NM is exactly the same size whatever height you're at. A NM is a NM is a NM, which is why it's ideal for navigation. A DME mile is equal to a NM all the time, but if you're at FL350 you still have to take slant range into account. If you're flying an ILS approach, slant range doesn't really matter because the DME is where you want to be i.e. on the ground!