Aircraft don't plan to fly at 300' MSD (or 250' MSD if you're any good) across built up areas full stop and they endeavour to stay around 2000' within 15nm of their home base to avoid cheesing off the locals (or at least that's what they do at the secret Oxfordshire Airbase).
250' MSD is measured by flying at either a Radalt or QFE 250' across the airfield and getting the picture in your head then maintaining that enroute. It's a best effort and I like to think that everyone nowadays would rather err on the side of being too high than the opposite in today's complaint and NIMBY-ridden society.
Last edited by kharmael; 15th Sep 2012 at 10:17.
Reason: Spolling erreaur