Correction And Application Of Vegetation Index Based On Terrain Factors

Due to the influence of terrain undulation,elevation altitude,shady slope and sunny slope,NDVI inverts vegetation information bias.In this study,the spatiotemporal variation law and spatial distribution characteristics of NDVI in Yunnan Province were analyzed based on MODIS image data and DEM data,and secondly,the correlation between NDVI and topographic factors was discussed,and according to the correlation conclusion,the vegetation index was greatly affected by topographic undulation,which was used as a prerequisite for constructing vegetation index correction factor.According to the physical characteristics of the visible red band and near-infrared band of vegetation information inversion,the correction factor is constructed,and finally the modified index is applied and the effect is evaluated.(1)Yunnan Province belongs to a high-altitude area,which gradually decreases from north to south.The highest elevation is 6146m,and it is located in Diqing and other prefectures in the northern region.The lowest elevation is 73m,and it is located in the southern region of Wenshan and other prefectures.The overall slope of Yunnan Province is large,the maximum slope is 79.64°,and the average slope is 17.45°.The northern and central western regions have steeper slopes,and the eastern regions have smaller slopes.The distribution of slope in Yunnan Province was relatively balanced,and there was no obvious difference.The interannual variation of vegetation index in Yunnan Province from 2012 to 2021 showed a slow upward trend,and the overall change of vegetation index in four seasons within 10A in Yunnan Province showed an overall upward trend,among which the vegetation index was the lowest in spring and the highest in summer and autumn.The monthly change of 10A vegetation index in Yunnan Province showed a trend of first decreasing and then rising.(2)By analyzing the correlation between NDVI and topographic factors,it was shown that the correlation between altitude and NDVI was the largest,and the vegetation index fluctuated slightly before the altitude of 3000m.After reaching3000m above sea level,the vegetation index continued to decline with the increase of altitude,and flattened out at 6000m.The correlation between NDVI and slope was small,R~2was 0.4228,and NDVI showed a trend of first increasing and then decreasing with the increase of slope.The R~2between NDVI and the aspect is 0.276,the vegetation index of the plane is the lowest.The R~2between NDVI and surface roughness was 0.62,and the vegetation index gradually decreased with the increase of surface roughness.(3)The vegetation index is greatly affected by the undulations of the terrain.The multivariate equation between NDVI and terrain factor is established in the province,and according to the actual terrain distribution in the experimental area,the normalized value of the corresponding terrain gradient is substituted into the equation to obtain the terrain adjustment parameter C,and the correction equation is established,After the correction,the effect of index anti-terrain was well evaluated,and the correlation between NDVI and solar incidence angle cosine cosi was greater than that between TRVI and solar incidence angle cosine cosi.The NDVI index difference between the shade slope and the Yang slope was 0.155,and the TRVI difference was 0.017,that is,the NDVI difference between the Yin slope and the Yang slope was greater than the TRVI difference,indicating that the corrected index had obvious terrain resistance effect.The TRVI was verified by selecting the area with large terrain undulations outside the experimental area,and the results showed that the anti-terrain effect of TRVI was better than that of NDVI,and the effect was well evaluated.