Abstract
Structure-from-motion (SfM) photogrammetry from unmanned aerial system (UAS) imagery is an emerging tool for repeat topographic surveying of dryland erosion. These methods are particularly appealing due to the ability to cover large landscapes compared to field methods and at reduced costs and finer spatial resolution compared to airborne laser scanning. Accuracy and precision of high-resolution digital terrain models (DTMs) derived from UAS imagery have been explored in many studies, typically by comparing image coordinates to surveyed check points or LiDAR datasets. In addition to traditional check points, this study compared 5 cm resolution DTMs derived from fixed-wing UAS imagery with a traditional ground-based method of measuring soil surface change called erosion bridges. We assessed accuracy by comparing the elevation values between DTMs and erosion bridges along thirty topographic transects each 6.1 m long. Comparisons occurred at two points in time (June 2014, February 2015) which enabled us to assess vertical accuracy with 3314 data points and vertical precision (i.e., repeatability) with 1657 data points. We found strong vertical agreement (accuracy) between the methods (RMSE 2.9 and 3.2 cm in June 2014 and February 2015, respectively) and high vertical precision for the DTMs (RMSE 2.8 cm). Our results from comparing SfM-generated DTMs to check points, and strong agreement with erosion bridge measurements suggests repeat UAS imagery and SfM processing could replace erosion bridges for a more synoptic landscape assessment of shifting soil surfaces for some studies. However, while collecting the UAS imagery and generating the SfM DTMs for this study was faster than collecting erosion bridge measurements, technical challenges related to the need for ground control networks and image processing requirements must be addressed before this technique could be applied effectively to large landscapes.
Highlights
Repeat topographic surveys are an important tool for studying and managing dryland ecosystems, for tracking soil erosion and gully formation [1,2,3,4,5]
The easting and northing RMSE for the February 2015 acquisition was nearly identical with RMSE of
The vertical accuracy of our unmanned aerial system (UAS)-based SfM digital terrain models (DTMs) were quite similar to other studies using very fine spatial resolution imagery
Summary
Repeat topographic surveys are an important tool for studying and managing dryland ecosystems, for tracking soil erosion and gully formation [1,2,3,4,5]. SfM differs from traditional photogrammetry in that it can reconstruct 3-dimensional scenes based purely on a large number (typically millions) of automatically detected ground points in many overlapping images independent of ground references [18]. This allows it to be very flexible in terms of accepting images with inconsistent overlap, rotation between successive images, and images from different angles (i.e., nadir and oblique) [19]. Scientists and land managers have the ability to make on-demand imagery products, often at finer spatial and temporal resolutions along with reduced costs compared to imagery from satellite or manned aircraft [21]
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