The authors use the phrase ""ground ""Truth"" with a capital ""T"" (line 50) in order to

To prove LiDAR measurement were better and accurate.

Explanation:

The author made use capital letter T in order to backup the fact that LIDAR measurements are absolutely accurate as compared to measurements gotten from field work.

LIDAR measurement is a surveying method that measures a target distance with laser lights and uses a reflecting sensor to capture the distance covered.