A decision from a junior competition a couple of years ago. The general view is on the left, while the position of the ball, half under the sleepers edging the path, is shown in the right hand picture. The middle picture shows the plan view with the green to which they were playing.
While the competition was using the England Golf hard card which at that time stated that Penalty Areas were DEFINED by stakes or lines (this is no longer the case) and the ball was clearly outside of the straight line between yellow stakes the first interpretation of Rule 17.1a (which defines when a ball is in a Penalty Area) explains that areas that are clearly part of the Penalty Area are still part of that Penalty Area even if outside the stakes.
My view was that the markings otherwise followed the edge of the sleepers, which was also the “break in slope” around the pond, so this curved line was the edge of the Penalty Area. Part of the ball was therefore within said Penalty Area and so the player had to proceed under the Penalty Area Rule 17. This was not popular with the player’s father who argued that the ball was outside the line between the stakes and so there should have been free relief from the Immovable Obstruction.
Rule 17 is quite clear that if any part of the ball touches or is above any part of the Penalty Area then it is within that Penalty Area. This is further clarified in Rule 2.2c which gives an order of precedence if a ball lies in more than one Area of the Course. This order is 1. Penalty Area, 2. Bunker, 3. Putting Green, 4. Teeing Area, 5. General Area
If the whole of the ball had been below the sleeper, then it would have been in the General Area and free relief would have been available from the path as an Abnormal Course Condition (i.e. an Immovable Obstruction).
Note that where a line defines the edge, the line itself is inside the Penalty Area. Similarly if stakes are used to define then the stakes are inside the area and a straight line between their outside points at ground level should be used (subject to the interpretation explained above). However it does pay to read the Local Rules – many (most?) clubs will use stakes to INDICATE and not DEFINE Penalty Areas which are then DEFINED by natural boundaries, such as a break in slope.