Wiktionary
groundline
n. 1 (context art English) A line drawn to signify the surface of the ground, especially in architectural blueprints and in drawings by children which lack dimension. 2 In equestrian show jumping, a marker such as a row of flowers which signifies the point from which a horse should begin a jump over an obstacle.
Usage examples of "groundline".
The troopers went into an instinctive crouch, and Ali cursed, rolling back into the sandbagged slit and reaching for the groundline com.