Cliff vegetation monitoring using close range photogrammetry and UAS: technical issues and practical hints

Cliffs are steep slopes rocky formations, often vertical and sometimes overhanging. They are common on coasts, in mountainous areas, as walls of canyons and glacial valleys or along rivers and are usually formed by rocks that are resistant to erosion and weathering. Even if the total extension of cliff surfaces is not available because steep areas are not fully measurable from common maps, cliffs are widely distributed worldwide. Although cliffs and rocky slopes are quite harsh habitats for plants to grow, they are characterized by a widespread phenomenon: the concentration of a number of endemic and rare plant species larger than in the surrounding flat areas [1] [2] [3]. Although so relevant for biodiversity conservation, studies on plant communities of the cliffs are sporadic, mainly because of site inaccessibility [2]. In such conditions conventional field sampling at the community level (e.g. permanent plots or transects) is challenging even for safety issues and sometimes personnel with rock climbing skills is required [4] [5]. To overcome the inaccessibility of such habitats, remote data collection by means of optical tools (i.e.binoculars, telescopes, telephoto lenses) is often used [6]. Goñi et al. [4] have developed two specific estimate methods to carry out censuses and population demography remote monitoring of rupicolous plant species. The first is based on plant individuals remote counts multiplied by an average Correction Factor (CF = real total number / distant total count); the second is based on scaled panoramic photographs, GIS analysis of total area of occupancy of vegetation patches and estimation of average individual density in a patch. In addition to inaccessibility, verticality of cliff and steep rocky faces poses further problems. As highlighted by Goñi et al. [4] and Gigante et al. [7], the orthogonal projection of vertical surfaces used in conventional maps or orthomosaics brings some shortcomings, like the underestimation of the total area. To overtake such difficulties various attempts are reported. In case of long-term monitoring, the Fixed Point Photography (FPP) or Photo Point Monitoring (PPM) technique was used [8]; it consists in taking multitemporal terrestrial photographs of a site, from a fixed point (x,y,z) and angle, using always the same lens focal length. Moreover, an alternative approach based on oblique aerial photographs, derived from video imagery captured using an helicopter, was proposed by Barron et al. [5]. Within this scenario, the overall aim of our research was to test new protocols and technical solutions for long-term vegetation monitoring on inaccessible cliffs and rocky slopes, taking into account that they all must be realistically practicable in terms of time, staff involved and costs. The herein described study represents a proof of concept showing how close range photogrammetry by means of Unmanned Aerial Systems (UAS), low-cost digital cameras and Structure from Motion (SfM) software can be used to derive high-resolution orthomosaics (orthoplanes) of vertical rocky faces to be used for vegetation and species long-term monitoring. More specifically we report about the effects of various factors on image resolving power and in turn on the usability of such images for vegetation patch mapping and species recognition. Our research was carried out in southern Italy coastal sites classifiable as the EU Habitat 1240 (Vegetated sea cliffs of the Mediterranean coasts with endemic Limonium spp.) and Habitat 8210 (Calcareous rocky slopes with chasmophytic vegetation). Some of the species of these two adiacent habitats are: Crithmum maritimum L., Limonium sp.pl., Limbarda crithmoides (L.) Dumort., Dianthus rupicola Biv., Primula palinuri Petagna, Eokochia saxicola (Guss.) Freitag