Agricultural Drainage Tile Surveying Using an Unmanned Aircraft Vehicle Paired with Real-Time Kinematic Positioning—A Case Study
Freeland, R. S., B. A. Allred, N. S. Eash, L. Martinez, and D. Wishart.  2019.  Computers and Electronics in Agriculture.

A 2012 agricultural census reported approximately 218,000 U.S. farms have subsurface tile networks that artificially drain over 19.7 million hectares. These drainage networks present a significant expense to landowners during their installation, inspection, maintenance, and upgrading. The conventional method for locating buried drain tiles is by excavating with heavy machinery and employing hand tile probes, which is expensive and time-consuming. Aerial photography offers promise as a less costly alternative for mapping tile networks over vast acreages. In aerial photography, ground-surface discolorations periodically appear directly atop functioning drains from the concentration of draining moisture. We have noted drainage surface patterns occurring on Midwestern fields having minimal surface residue one to three days following major rainfall. An unmanned aerial vehicle (UAV) equipped with visible or thermal infrared imaging can detect these distortions at opportune times. This case study investigated the pairing of UAVs with Real Time Kinematic (RTK) technology for both mapping and then subsequently locating the subsurface drainage networks. Illustrations compared Wide Area Augmentation System (WAAS) Global Positioning System (GPS) positional accuracy of point objects within the aerial survey. Positional LiDAR elevation contours were compared to digital terrain models (DTMs) generated using RTK-based aerial photogrammetry. This case study found WAAS GPS provided insufficient accuracy and precision for physically locating the buried narrow tiles. Although more expensive and complex, satellite RTK technology increases the vertical and horizontal alignment of UAV surveys, as it projects aerial imagery precisely and accurately onto a geodetic coordinate system. Results demonstrate that pairing RTK with a UAV supplied a necessary method of physically locating buried narrow tile lines in the field when using UAV photogrammetry.