Abstract
Green vegetation cover fraction (VCF) is an important indicator of vegetation status in ecology and agronomy. Digital image analysis (DIA) has been widely accepted as a new VCF measurement technique. In this study, we present a novel fully automatic threshold segmentation algorithm for VCF measurements, which is named as upper inflection point plus mean gradient magnitude of edge pixels (UIP-MGMEP). The algorithm performs VCF estimation upon the vegetation index Excess Green (EXG). UIP-MGMEP optimizes the EXG threshold by searching the upper inflection point (UIP) of the M-Et curve (mean gradient magnitude of edge pixels (MGMEP) vs. EXG threshold), based on the assumption that EXG variance of the boundary pixels between vegetation and background is larger than the variance of the background. Five typical sample images are used to illustrate how ground complexity reduces the distinctness of the UIP. Three controlled experiments are illustrated to test the robustness of UIP-MGMEP to resolution, exposure, and ground complexity. The results show that UIP-MGMEP is a promising algorithm for automatic VCF estimation upon digital images. Compared to broad-leaved grass, narrow-leaved grass is more sensitive to resolution and exposure. To reduce ground complexity, smaller footprint size while more images to cover the same area may be better than one image with large footprint size. UIP-MGMEP is fully automatic, making it promising for batch processing of VCF measurements that is very difficult in any wide-range field survey in the past. UIP-MGMEP algorithm can only extract green vegetation and is not suitable for non-green (even grayish-green) vegetation, due to the limits of vegetation index EXG. In addition, UIP-MGMEP is not recommended for images with VCF less than 0.5% or greater than 99.5%.
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Acknowledgments
We would like to thank Dr. Alison Beamish from the University of British Columbia and Prof. Zhang Jian from the East China Normal University for their assistance with the English language and grammatical editing of the manuscript.
Funding
This work was supported by the TIWTE Research and Innovation Fund Project under Grant TKS160112 and TKS170211, Qinghai Provincial Transportation Science and Technology Fund Project under Grant 31118022.
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Hu, J.B., Dai, M.X. & Peng, S.T. An automated (novel) algorithm for estimating green vegetation cover fraction from digital image: UIP-MGMEP. Environ Monit Assess 190, 687 (2018). https://doi.org/10.1007/s10661-018-7075-7
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DOI: https://doi.org/10.1007/s10661-018-7075-7