A Simple Normalized Difference Approach to Burnt Area Mapping Using Multi-Polarisation C-Band SAR

Jeanine Engelbrecht,Jaco Kemp,Lufuno Vhengani,Andre Theron

doi:10.3390/rs9080764

Abstract

In fire-prone ecosystems, periodic fires are vital for ecosystem functioning. Fire managers seek to promote the optimal fire regime by managing fire season and frequency requiring detailed information on the extent and date of previous burns. This paper investigates a Normalised Difference α-Angle (NDαI) approach to burn-scar mapping using C-band data. Polarimetric decompositions are used to derive α-angles from pre-burn and post-burn scenes and NDαI is calculated to identify decreases in vegetation between the scenes. The technique was tested in an area affected by a wildfire in January 2016 in the Western Cape, South Africa. The quad-pol H-A-α decomposition was applied to RADARSAT-2 data and the dual-pol H-α decomposition was applied to Sentinel-1A data. The NDαI results were compared to a burn scar extracted from Sentinel-2A data. High overall accuracies of 97.4% (Kappa = 0.72) and 94.8% (Kappa = 0.57) were obtained for RADARSAT-2 and Sentinel-1A, respectively. However, large omission errors were found and correlated strongly with areas of high local incidence angle for both datasets. The combined use of data from different orbits will likely reduce these errors. Furthermore, commission errors were observed, most notably on Sentinel-1A results. These errors may be due to the inability of the dual-pol H-α decomposition to effectively distinguish between scattering mechanisms. Despite these errors, the results revealed that burnt areas could be extracted and were in good agreement with the results from Sentinel-2A. Therefore, the approach can be considered in areas where persistent cloud cover or smoke prevents the extraction of burnt area information using conventional multispectral approaches.

Highlights

The effects of wildfires are severe and can include damage to infrastructure and the environment [1,2]as well as contributing to land degradation and affecting global warming due to an increase in CO2 emissions [3,4,5,6,7,8]
Due to the success achieved in mapping burnt areas using polarimetric decompositions, we propose a technique for mapping burnt areas using polarimetric decomposition while minimizing the potential for identifying previously bare areas as burnt areas
The results suggest that high values are associated with the burnt areas as would be expected

Summary

Introduction

The effects of wildfires are severe and can include damage to infrastructure and the environment [1,2]as well as contributing to land degradation and affecting global warming due to an increase in CO2 emissions [3,4,5,6,7,8]. Fires can be disastrous events with significant impacts on infrastructure and the environment, in many ecosystems, periodic fires are vital for ecosystem functioning and keeping vegetation species in a healthy condition. In these ecosystems, periodic fires stimulate species diversity, controlling age and influencing nutrient cycles [4]. NDαItotoidentify identify burnt areas and thereby contribute to operational monitoring systems, the algorithm was tested in an area located near Simonsberg. Mountain, situated fire monitoring systems, the algorithm was tested in an area located near Simonsberg. Cape situated the of towns of Stellenbosch and Paarl in the CapeProvince.

Results

Discussion

Conclusion