## 1 Introduction  
Geological landscapes and structures can be easily studied starting from high spatial resolution Digital Elevation Models (DEMs) and a set of useful tools able to extract specific landforms and quantify geomorphological parameters [1; 2; 3]. Moreover, topographical maps of sufficient scale and precision are often not available for many regions in the world. In these areas, the combination of remote sensing data from different sources into Geographic Information Systems (GIS) allows to obtain new information and integrate them with field-based data. The detection of karst morphologies from DEMs processing is a well-known methodology, largely applied for geomorphological studies, risk and geohazard assessment, and land use management [4; 5; 6; 7; 8; 9; 10]. Studies based on the comparison between DEM and field-based data have demonstrated that the use of digital images with an appropriate spatial resolution generally provides reliable results in the morphometric characterization of sinkholes [11; 12]. The elevation model extracted from optical satellite images, however, must guarantee a quality standard. Accurate elevation and morphological details are critical points for geomorphological mapping purposes if satellite data are to be used [2]. Terrain roughness, ground resolution, base-to-height ratio, and image contrast affect the accuracy of the produced DEMs from optical space images [12; 13]. In the last years, several very-high resolution satellites were launched (e.g., QuickBird, OrbView, GeoEye-1, and the series of WorldView platforms, as well as Pleiades), with the capacity of providing images with a ground resolution of up to 0.3 m.  
Conventional techniques for DEM creation, based on terrestrial surveying and aerial images and improved using GPS, have been integrated by the production of elevation models created with satellite stereoscopic images [14; 15; 16]. Optical and Synthetic Aperture Radar (SAR) are consolidated methods to obtain DEMs with different resolution and accuracy at the local, regional, and global scales. Among them, the stereoscopic triplet can be considered an improvement of the simple stereo-couple, allowing for a better retrieval of altitudes over impervious terrains, steep slopes, and shadows [17]. The Pleiades mission was the first satellite system that introduced the stereoscopic triplet, adopting a new acquisition mode, based on the nearly simultaneous acquisition of three images, one backward looking (B), one forward looking (F), plus a third near-nadir image (N) [18]. The Pleiades constellation consists of two satellites: 1A spacecraft was launched on 16 December 2011, and Pleiades 1B in late 2012. Panchromatic and multispectral data are acquired simultaneously at a nominal resolution of 0.5 m and 2 m. The products are mainly used to produce 50-cm ortho-imagery, and the acquisition of synchronous images of the same area with different looking angle, allows for obtaining very high-resolution DEMs. In particular, the use of a near-nadir (N) image improves the signal of the classical forward (F) and backward (B) looking stereo pairs, allowing a better retrieval of heights over terrains with high roughness and steep slopes characterized by large areas with shadows (Figure 1). DEMs extracted from these data proved to be well-suited for the analyses of elevation in different geomorphological environments such as mountain glaciers, volcanic ranges, and in urban areas [13; 19; 20]. Moreover, the use of these data is focused on the multitemporal analysis of DTMs based on multiple acquisitions with the possibility to extract surface and volume changes [13; 17]. The aim of this study is to verify the use of DEMs extracted from Pleiades tri-stereo optical data for the evaluation of morphometric parameters in the Middle Atlas in Morocco, where high resolution topographic maps are not available. In particular, this study addresses the problem of evaluating the depth of sinkhole-like depressions dotting the basaltic plateau of Azrou.