A flexible approach for woody cover assessment from SPOT HRV XS data in semi-arid West Africa. Application in northern Burkina Faso

Spectral mixture analysis was applied to woody cover appraisal in a semi-arid landscape of West Africa. The spectral response of a SPOT HRV XS image was modelled from signatures and relative abundance of four pure basic components: green tree foliage, associated shadow and two types of bare soil surfaces (clear vs. dark petroferric). Signatures were determined from the image by interpreting a bidimensional scattergram of pixels in the brightness/greenness spectral space. Estimation of subpixel abundances was made possible by prior observation that dense woody cover cannot occur on dark petroferric soils. Consequently, the scattergram had a peculiar shape enabling the distinction between a `soil ridge' consisting of pixels bearing no woody cover, and a `vegetation ridge' expressing the variation of woody cover on clear soils. The relationship between canopy surface and associated shadowing was considered through Li-Strahler geometric-optical canopy reflectance models, with the main parameters estimated from field measurements of individual trees. Subpixel abundance of canopy cover proved highly correlated with an independent estimate obtained from large-scale aerial photographs. The present approach could hence yield relevant estimates of woody cover, even in the presence of varying soil surface conditions, and with no prerequisite of characterizing spatial patterns of trees.