Which spotting indicates that the observer has lost sight of the round's impact?

The term that indicates the observer has lost sight of the round's impact is "Lost." This communicates that the observer is unable to see where the artillery round has landed, which is crucial for making adjustments to subsequent fire missions. When the observer loses sight, it often means they cannot provide feedback to the gun crews about the effectiveness of the round or make necessary corrections based on the round’s impact. This situation can arise due to various factors such as visibility issues, obstructions, or rapid movement of the observer's position.

The other terms serve different purposes in the context of artillery observation. For instance, "Doubtful" usually indicates uncertainty about the location of impact but implies the observer might have seen something. "Unobserved," on the other hand, is used when the observer did not see the impact due to either being overly far away or having visibility obstructed but does not necessarily suggest loss of sight entirely. "Over" refers to a situation where the round has landed beyond the intended target area, indicating the observer has seen the impact but that it was not within the desired parameters. Understanding these distinctions is essential for effective communication during fire support operations.