The objective of an impact registration is to get spottings of what along the observer-target line?

The objective of an impact registration is to obtain a clear understanding of the projectile's accuracy with respect to a specific target location. By getting spottings of four rounds—two over and two short—along the observer-target line, the observer can adequately assess the range error involved.

This method allows the observer to determine the correct range to the target with a high degree of confidence, as having multiple impacts provides a more comprehensive data set to analyze. The two rounds that fall over the target help in gauging how far past the target the rounds are landing, while the two rounds that fall short indicate how far the rounds are lacking in their range. This dual information is crucial in making necessary corrections to effectively engage the target, ultimately leading to a successful strike.

Overall, the collection of four rounds (two over and two short) provides the most effective means to measure and adjust for any range discrepancies, making this approach the preferred method for impact registration.