How often do you hear (or use) the words “line of flight?” Would it surprise you to know that there is no such thing? The concept is often called upon (incorrectly) to determine a place to drop a ball that has gone into a water hazard or is unplayable in some other way.
If your ball is in the water and you need to put another ball in play, you must take your drop based on where the ball last crossed the margin of the hazard. You have two options if the hazard is a direct hazard and is marked in yellow: 1) replay the shot; or 2) go back as far as you’d like on an imaginary straight line beginning at the hole and going through the point on the margin where your ball last crossed. If the hazard is marked in red, you have two additional options: 1) take two club lengths from the point on the margin where your ball last crossed (not closer to the hole); or 2) find the point equidistant from the hole and not closer to it on the opposite margin of the hazard. All of these options are based on the point at which your ball crossed into the hazard, not the ball’s “line of flight,” and they all require taking a one-stroke penalty.
If you’re in some other kind of awkward situation (i.e. under a large pine tree) and need relief, you have three options: replay the shot; take two club lengths from where the ball lies not closer to the hole; or, keeping the ball between you and the hole, go back as far as you like. These options also come with a one-stroke penalty.