Once a fossil was exposed from the ground, it was necessary to continue or risk losing it. Erosion or a rainstorm could damage or wash away a fossil, meaning that paleontologists excavating a fossil had to work fast. Paleontologists usually utilized tarps over the site, and dug trenches around the perimeter to prevent water runoff or erosion. To determine the size of the trenches needed, they needed to determine how large the fossil was, for which they used CAST.
Jurassic Park by Michael Crichton page 55 (First Edition, Thirty Fourth printing)