Multiplatform-SfM and TLS Data Fusion for Monitoring Agricultural Terraces in Complex Topographic and Landcover Conditions

Sara Cucchiaro,Kristof Van Oost,Paolo Tarolli,Daniel J. Fallu,Antony G. Brown,Kevin Walsh,He Zhang

doi:10.3390/rs12121946

Abstract

Agricultural terraced landscapes, which are important historical heritage sites (e.g., UNESCO or Globally Important Agricultural Heritage Systems (GIAHS) sites) are under threat from increased soil degradation due to climate change and land abandonment. Remote sensing can assist in the assessment and monitoring of such cultural ecosystem services. However, due to the limitations imposed by rugged topography and the occurrence of vegetation, the application of a single high-resolution topography (HRT) technique is challenging in these particular agricultural environments. Therefore, data fusion of HRT techniques (terrestrial laser scanning (TLS) and aerial/terrestrial structure from motion (SfM)) was tested for the first time in this context (terraces), to the best of our knowledge, to overcome specific detection problems such as the complex topographic and landcover conditions of the terrace systems. SfM–TLS data fusion methodology was trialed in order to produce very high-resolution digital terrain models (DTMs) of two agricultural terrace areas, both characterized by the presence of vegetation that covers parts of the subvertical surfaces, complex morphology, and inaccessible areas. In the unreachable areas, it was necessary to find effective solutions to carry out HRT surveys; therefore, we tested the direct georeferencing (DG) method, exploiting onboard multifrequency GNSS receivers for unmanned aerial vehicles (UAVs) and postprocessing kinematic (PPK) data. The results showed that the fusion of data based on different methods and acquisition platforms is required to obtain accurate DTMs that reflect the real surface roughness of terrace systems without gaps in data. Moreover, in inaccessible or hazardous terrains, a combination of direct and indirect georeferencing was a useful solution to reduce the substantial inconvenience and cost of ground control point (GCP) placement. We show that in order to obtain a precise data fusion in these complex conditions, it is essential to utilize a complete and specific workflow. This workflow must incorporate all data merging issues and landcover condition problems, encompassing the survey planning step, the coregistration process, and the error analysis of the outputs. The high-resolution DTMs realized can provide a starting point for land degradation process assessment of these agriculture environments and supplies useful information to stakeholders for better management and protection of such important heritage landscapes.

Highlights

Over the last two decades, our ability to monitor and characterize landscapes [1,2] through submetric digital terrain models (DTMs) has greatly improved due to important developments in high-resolution topography (HRT) techniques, methods, sensors, and platforms [3,4]
This was not possible in the past, when the direct georeferencing method could achieve only decimeter to meter accuracies, as indicated in many studies [103,104,105,106]. This was caused by the low accuracy of the navigation-grade onboard global positioning system (GPS) units used for most current consumer unmanned aerial vehicles (UAVs) [49,56,103]
These results were in line with previous studies [21,106,111,112] which highlighted that direct georeferencing allowed the acquisition of robust centimetric HRT data, similar to the ground control point (GCP) solution [57,107,113]

Summary

Introduction

Over the last two decades, our ability to monitor and characterize landscapes [1,2] through submetric digital terrain models (DTMs) has greatly improved due to important developments in high-resolution topography (HRT) techniques, methods, sensors, and platforms [3,4]. Among the most widely used technologies, airborne laser scanning (ALS) is the most suitable system for the acquisition of HRT data over remote and vegetated areas over short time periods across large areas [7,8,9,10,11] This approach is costly for small area surveys. Ground-based platforms such as terrestrial laser scanning (TLS) are used when a higher resolution and more flexibility in the scanning angle is needed [13] Other traditional techniques such as Global Navigation Satellite System (GNSS) and total station survey are not ideal for large areas and can prove costly [14]. Recent developments in digital photogrammetry (e.g., structure from motion (SfM)) paired with multiview stereo (MVS) algorithms (hereafter together referred to as SfM) have increased the possibility of low-cost and high-resolution surveys, both ground- and aerial-based [15,16], opening new opportunities for landscape investigation and management [17]

Objectives

Results

Conclusion