If you operate HF and do not understand take off angles and that local terrain around you can affect them, your signal strength at the other end of the path is the result of pure luck. It is particularly important to Contesters and DXers. If you already know that, then there likely isn't anything new here for you. If you've never seen what High Frequency Terrain Analysis (HFTA) can tell you, then please join in. The presentation discusses how I made a decision between a 50 and 70 foot tower, but is also applicable in deciding whether it's worthwhile to try and get that 35 foot high dipole further up in a tree or what spacing to use for stacked monobanders. Wouldn't it be nice to have a clue ahead of time before you tried? YT and HFTA were once considered by many to be just a mildly interesting prediction tool. It has become so trusted that some are using it to analyze expected HF performance before buying their dream home. You may never use HFTA, but general knowledge of how things work tends to help success in many subtle ways.