Detection of rocky outcrops from LiDAR-derived DEM in Dinaric Karst

Karst is a geomorphological system that covers almost 50% of the area of Slovenia and is mainly characterised by circular concave and convex landforms such as conical hills, dolines, uvalas and poljes. These large landforms can be easily detected with a number of already developed (semi-)automatic detection methods. In addition to these large landforms, the karst surface is dissected by smaller scale features consisting mainly of numerous rocks of different shapes and sizes. Due to the different lithology that makes up the Slovenian karst (e.g., limestone, dolomite), rocky outcrops have different morphographic and morphometric characteristics due to the different dynamics of the mechanical weathering of the bedrock. The variety in shapes and sizes of rocky outcrops makes their detection by automatic or semi-automatic methods difficult. In our study area in the Dinaric Karst in Slovenia, they reach heights of up to several metres and lengths of about 10 metres.

Field mapping or digitizing such landforms would be time-consuming, labour-intensive, and costly. The combination of high-resolution LiDAR-derived DEMs (digital elevation models) and (semi-)automatic landform detection and delineation methods in GIS environments enables remote and low-cost mapping, which has an outstanding potential for large-scale spatial analysis and mapping in remote, forested, and difficult-to-access areas such as the Dinaric Karst.

The main objective of this study was to develop an approach for quantitative identification and detection of rocky outcrops. The approach is based on spatial analysis of high-resolution (1 m × 1 m) LiDAR DEM and field analysis of outcrop morphography and morphometry. The study was conducted in the Dinaric Karst area in Slovenia, which consists mainly of Cretaceous and Jurassic limestones and dolomites. First, we calculated the values of TPI (Topographic Position Index) to identify all convex shapes (i.e., ridges) within a search radius of 10 m around each cell. Slope was used as an additional criterion for defining rocky outcrops. Based on field measurements, we found that bedrock in areas with limestone (30°) outcrop at a lower surface slope than in areas with dolomite (50°).

The study has shown that the different spatial distribution, shape and size of the rocky outcrops are related to the geological structure. In the limestones they are much denser and more numerous than in the dolomites. In average, the dimensions of the outcrops are also much larger. This is due to the porosity of the dolomites, which causes greater mechanical weathering. We have also found that rocky outcrops often occur on certain landforms, e.g. on the slopes of dolines and other karst depressions or fluviokarst valleys.

Airborne LiDAR DEMs can be a useful source of information for detecting and studying the spatial patterns and morphometric settings of rocky outcrops. The number of landforms detected indicates that, in addition to dolines, rocky outcrops are one of the most common landforms in the Dinaric Karst.

Key words: Rocky outcrops, karst landform, GIS, LiDAR, (semi-)automatic methods, geomorphology, Dinaric mountains