**Francisco J. Lima-Cueto \*, Rafael Blanco-Sepúlveda, María L. Gómez-Moreno and Federico B. Galacho-Jiménez**

Geographic Analysis Research Group, Department of Geography, University of Malaga, Campus of Teatinos, s/n. 29071 Malaga, Spain; rblanco@uma.es (R.B.-S.); geolugom@uma.es (M.L.G.-M.); fbgalacho@uma.es (F.B.G.-J.) **\*** Correspondence: lima@uma.es

Received: 25 September 2019; Accepted: 29 October 2019; Published: 1 November 2019

**Abstract:** In olive groves, vegetation ground cover (VGC) plays an important ecological role. The EU Common Agricultural Policy, through cross-compliance, acknowledges the importance of this factor, but, to determine the real impact of VGC, it must first be quantified. Accordingly, in the present study, eleven vegetation indices (VIs) were applied to quantify the density of VGC in olive groves (*Olea europaea* L.), according to high spatial resolution (10–12 cm) multispectral images obtained by an unmanned aerial vehicle (UAV). The fieldwork was conducted in early spring, in a Mediterranean mountain olive grove in southern Spain presenting various VGC densities. A five-step method was applied: (1) generate image mosaics using UAV technology; (2) apply the VIs; (3) quantify VGC density by means of sampling plots (ground-truth); (4) calculate the mean reflectance of the spectral bands and of the VIs in each sampling plot; and (5) quantify VGC density according to the VIs. The most sensitive index was IRVI, which accounted for 82% (*p* < 0.001) of the variability of VGC density. The capability of the VIs to differentiate VGC densities increased in line with the cover interval range. RVI most accurately distinguished VGC densities > 80% in a cover interval range of 10% (*p* < 0.001), while IRVI was most accurate for VGC densities < 30% in a cover interval range of 15% (*p* < 0.01). IRVI, NRVI, NDVI, GNDVI and SAVI differentiated the complete series of VGC densities when the cover interval range was 30% (*p* < 0.001 and *p* < 0.05).

**Keywords:** UAV; vegetation ground cover; multispectral; vegetation indices; agro-environmental measures; olive groves; southern Spain
