Sensitivity of the Enhanced Vegetation Index (EVI) and Normalized Difference Vegetation Index (NDVI) to Topographic Effects: A Case Study in High-density Cypress Forest

TLDR

Vegetation indices such as EVI and NDVI are widely used for monitoring vegetation, but their accuracy can be compromised by atmospheric, soil, and especially topographic effects in rugged terrain. The study aims to theoretically analyze how topographic conditions differentially affect EVI and NDVI. The authors employ a non‑Lambertian model and two airborne images from a mountainous Japanese cypress plantation to conduct this analysis. The results show that the soil‑adjustment factor L in EVI makes it more sensitive to topography than NDVI, and the authors recommend correcting reflectance for topographic effects before computing EVI and similar indices in rough terrain.

Abstract

Vegetation indices play an important role in monitoring variations in vegetation.The Enhanced Vegetation Index (EVI) proposed by the MODIS Land Discipline Groupand the Normalized Difference Vegetation Index (NDVI) are both global-based vegetationindices aimed at providing consistent spatial and temporal information regarding globalvegetation. However, many environmental factors such as atmospheric conditions and soilbackground may produce errors in these indices. The topographic effect is another veryimportant factor, especially when the indices are used in areas of rough terrain. In thispaper, we theoretically analyzed differences in the topographic effect on the EVI and theNDVI based on a non-Lambertian model and two airborne-based images acquired from amountainous area covered by high-density Japanese cypress plantation were used as a casestudy. The results indicate that the soil adjustment factor “L” in the EVI makes it moresensitive to topographic conditions than is the NDVI. Based on these results, we stronglyrecommend that the topographic effect should be removed in the reflectance data beforethe EVI was calculated—as well as from other vegetation indices that similarly include a term without a band ratio format (e.g., the PVI and SAVI)—when these indices are used in the area of rough terrain, where the topographic effect on the vegetation indices having only a band ratio format (e.g., the NDVI) can usually be ignored.